DRAFT DOCUMENT

The Occupational Safety and Health Act of 1970 (Public Law 91-596) states that "the Secretary of Health and Human Services shall... produce criteria... enabling the Secretary of Labor to meet his responsibility for the formulation of safety and health standards" [29 USC^* 669(a)(2)]. An occupational safety and health standard is defined as one that is reasonably necessary or appropriate to provide safe or healthful employment or places of employment [29 USC 652]. In promulgating standards dealing with harmful physical agents under both the Occupational Safety and Health Act of 1970 (Public Law 91-596) and the Federal Mine Safety and Health Act of 1977 (Public Law 95-164), the Secretary of Labor shall set the standard which most adequately assures, to the extent feasible, that no employee will suffer material impairment of health or functional capacity even if such employee has regular exposure to the hazard for the period of his working life. In addition to the attainment of the highest degree of health and safety protection for the employee, other considerations shall be the latest available scientific data in the field, the feasibility of the standard, and experience gained under this and other health and safety laws [29 USC 655(b)(5); 30 USC 811(a)(6)(A)]. The National Institute for Occupational Safety and Health (NIOSH) is authorized under 29 USC 671 and 30 USC 811(a)(6)(B) to develop new and improved occupational safety and health standards and to perform all functions of the Secretary of Health and Human Services.

*United States Codes. See USC in references.

In 1972, NIOSH published Criteria for a Recommended Standard: Occupational Exposure to Noise, which provided the basis for a recommended standard to reduce the risk of developing permanent hearing loss as a result of occupational noise exposure [NIOSH 1972]. NIOSH has now evaluated the latest scientific information and is revising some of its previous recommendations.

The NIOSH recommended exposure limit (REL) of 85 dBA for occupational noise exposure was reevaluated using contemporary risk assessment techniques and incorporating the 4000-Hz audiometric frequency in the definition of hearing impairment. The new risk assessment reaffirms support for the 85-dBA REL. The excess risk of developing occupational noise-induced hearing loss (NIHL) for a 40-year lifetime exposure at the 85-dBA REL is 8%, which is considerably lower than the 25% excess risk at the 90-dBA permissible exposure limit currently enforced by the Occupational Safety and Health Administration (OSHA) and the Mine Safety and Health Administration (MSHA).

NIOSH previously recommended an exchange rate of 5 dB for the calculation of time-weighted average exposures to noise, but it is now recommending a 3-dB exchange rate, which is more firmly supported by scientific evidence. The 5-dB exchange rate is still used by OSHA and MSHA, but the 3-dB exchange rate has been increasingly supported by national and international consensus.

NIOSH recommends an improved criterion for significant threshold shift, which is an increase of 15 dB in hearing threshold at 500, 1000, 2000, 3000, 4000, or 6000 Hz that is repeated for the same ear and frequency in back-to-back audiometric tests. The new criterion has the advantages of a high identification rate and a low false-positive rate. In comparison, the criterion recommended in the 1972 criteria document has a high false-positive rate, and the OSHA criterion, called the Standard Threshold Shift, has a relatively low identification rate.

Differing from the 1972 criteria document, NIOSH no longer recommends age correction on individual audiograms. This practice is not scientifically valid, and would delay intervention to prevent further hearing losses in those workers whose hearing threshold levels have increased due to occupational noise exposure. OSHA currently allows age correction only as an option.

The Noise Reduction Rating (NRR) is a single-number, laboratory-derived rating required by the Environmental Protection Agency to be shown on the label of each hearing protector sold in the U.S. In calculating the noise exposure to the wearer of a hearing protector at work, OSHA has implemented the practice of derating the NRR by one-half for all types of hearing protectors. In 1972, NIOSH recommended the use of the full NRR value, but now it recommends derating the NRR by 25%, 50% and 70% for earmuffs, formable earplugs and all other earplugs, respectively. This variable derating scheme, as opposed to OSHA's straight derating scheme, takes into consideration the performances of different types of hearing protectors.

This document also provides recommendations for the management of hearing loss prevention programs for workers whose noise exposures equal or exceed 82 dBA (i.e., 1/2 of the REL). The recommendations include program evaluation, which was not articulated in the 1972 criteria document and is not included in the OSHA and MSHA standards.

Adherence to the revised recommended standard will minimize the risk of developing occupational NIHL.

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This document was prepared by the staff of the Education and Information Division, Paul Schulte, Ph.D., Acting Director, and the Division of Biomedical and Behavioral Science, Janet C. Haartz, Ph.D., Director. The document manager was Henry S. Chan, M.P.H., CIH. The contributions of consultants and other NIOSH personnel are gratefully acknowledged:

[more acknowledgement later]

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TABLES

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Audiogram

A graph or table obtained from an audiometric test showing hearing level as a function of frequency.

Baseline audiogram

The audiogram obtained from an audiometric examination administered prior to employment or within the first 30 days of employment that is preceded by a period of at least 14 hours of quiet. The baseline audiogram is the audiogram against which subsequent audiograms will be compared for the calculation of significant threshold shift.

Continuous noise

Noise levels that vary minimally as a function of time.

Crest factor

Ratio of the peak sound pressure to the RMS (root mean square) sound pressure.

Daily noise dose (D)

A descriptor for noise exposure, in percent, expressed by the following relationship:

D = [C₁/T₁ + C₂/T₂ + .... + C_n/T_n] x 100

where:

C_n = total time of exposure at a specified noise level

T_n = total time of exposure permitted at that noise level

Decibel (dB)

A dimensionless unit used in physics, which is equal to 10 times the logarithm to the base 10 of the ratio of two values:

dB = 10*log(value₁/value₂)

[For the application of this general equation in acoustics, see sound intensity level and sound pressure level in this glossary.]

Decibels, A-weighted (dBA)

Unit representing the sound level measured with the A-weighting network on a sound level meter. [Refer to Table 4-1 for the characteristics of the weighting networks.]

Decibels, C-weighted (dBC)

Unit representing the sound level measured with the C-weighting network on a sound level meter. [Refer to Table 4-1 for the characteristics of the weighting networks.]

Derate

To use a fraction of a hearing protector's noise reduction rating (NRR) to calculate the noise exposure of a worker wearing that hearing protector. [See noise reduction rating in this glossary.]

Equal-energy hypothesis

A hypothesis stating that equal amounts of sound energy will produce equal amounts of hearing impairment, regardless of how the sound energy is distributed in time.

Equivalent continuous noise level (L_eq)

Varying, intermittent or impulsive noise exposure that is equal in energy to a continuous noise level for a certain duration.

Excess risk

Percentage with hearing impairment in an occupational-noise-exposed population after subtracting the percentage who would normally incur such impairment from other causes in a population not exposed to occupational noise.

Exchange rate

An increment of decibels that requires the halving of exposure time, or a decrement of decibels that requires the doubling of exposure time. For examples, a 3-dB exchange rate requires that noise exposure time be halved for each 3-dB increase in noise level; likewise, a 5-dB exchange rate requires that exposure time be halved for each 5-dB increase.

Fence

The hearing threshold level above which a hearing loss is considered to have occurred.

Frequency

The number of times that a function occurs per unit time. [For sound, the unit is cycles per second, or Hertz (Hz).]

Hearing threshold level (HTL)

The amount, in decibels, by which the threshold of audibility for an ear differs from a standard audiometric threshold.

Immission level

A descriptor for noise exposure, in decibels, representing the total sound energy incident on the ear over a specified period of time (e.g., months, years).

Impact noise

A reverberant type of impulsive noise.

Impulse noise

A nonreveberant type of impulsive noise.

Impulsive noise

"Impulsive noise" is characterized by a sharp rise and rapid decay in sound levels and is less than 1 second in duration. For the purpose of this document, it includes impact or impulse noise.

Intermittent noise

Noise levels that are interrupted by intervals of relatively low sound levels.

Noise

Any undesired or unwanted sound, usually of high intensity.

Noise Reduction Rating (NRR)

The NRR, which indicates a hearing protector's noise reduction capabilities, is a single-number rating which is required by law to be shown on the label of each hearing protector sold in the United States.

Permanent threshold shift (PTS)

An irreversible increase in hearing threshold level.

Pulse range

Difference in decibels between the peak level of an impulsive signal and the RMS level of a continuous noise.

Root-mean-square (RMS) sound pressure

The square root of the sum of the square values of sound pressure over a specified period of time.

Significant threshold shift

A shift in hearing threshold, outside the range of audiometric testing variability (+5 dB), that warrants follow-up action to prevent further hearing loss. NIOSH recommends that a change in hearing threshold of 15 dB for the worse at any frequency (0.5, 1, 2, 3, 4, or 6 kHz) that is repeated for the same ear and frequency to meet the criteria for significant threshold shift.

Sound

An auditory sensation evoked by oscillations in pressure in a medium with elasticity and viscosity (e.g., air).

Sound intensity (I)

The average rate at which sound energy is transmitted through a unit area normal to the direction of sound propagation.

Sound intensity level (SIL)

The sound intensity level, in decibels, of a sound is 10 times the logarithm to the base 10 of the ratio of the sound intensity to the reference sound intensity.

SIL = 10*log(I/I₀)

where I = sound intensity
I₀ = reference sound intensity

For sound transmitted in air, the reference sound intensity is 10^-12 watts per square meter.

Sound pressure (P)

The total instantaneous pressure at a point in the presence of a sound wave minus the static pressure at that point.

Sound pressure level (SPL)

The sound pressure level, in decibels, of a sound is 10 times the logarithm to the base 10 of the ratio of the sound pressure squared to the reference sound pressure squared.

SPL = 10*log(P²/P₀²)

SPL = 10*log(P/P₀)²

SPL = 20*log(P/P₀)

where P = sound pressure
P₀ = reference sound pressure

For sound transmitted in air, the reference sound pressure is 20 micropascal (:Pa).

Temporary threshold shift (TTS)

A temporary increase in hearing threshold level, after exposure to noise, that reverts to the pre-exposure hearing threshold level.

Varying noise

Noise levels that fluctuate considerably as a function of time.

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Section 1.1 RECOMMENDED EXPOSURE LIMIT

The recommended exposure limit (REL) for occupational noise exposure encompasses the provisions in Sections 1.1.1 through 1.1.4.

1.1.1 Exposure Levels and Durations

Occupational noise exposure shall be controlled so that workers are not exposed in excess of the combination of exposure level (L) and duration (T) as calculated by the following formula (or as shown in Table 1-1).

1.1.2 Time-Weighted Average

In accordance with Section 1.1.1, the REL is a time-weighted average (TWA) of 85 dBA based on an 8-hour workshift.

1.1.3 Daily Noise Dose

When the daily noise exposure consists of periods of different noise levels, the daily dose (D) shall not exceed 100, as calculated according to the following formula:

D = [C₁/T₁ + C₂/T₂ + .... + C_n/T_n] x 100

where:

C_n = total time of exposure at a specified noise level
T_n = total time of exposure permitted at that noise level

The daily dose can be converted into an 8-hour TWA level according to the following formula (or as shown in Table 1-2):

TWA = 10.0 x Log(D/100) + 85

1.1.4 Ceiling Limit

Occupational exposure to continuous, varying and/or intermittent noise shall not exceed 115 dBA.

Section 1.2 HEARING LOSS PREVENTION PROGRAM

The employer shall institute an effective hearing loss prevention program (HLPP) described in Sections 1.3 through 1.11 when any worker's 8-hour TWA exposure equals or exceeds 82 dBA, except where noted.

Section 1.3 EXPOSURE ASSESSMENT

The employer shall conduct exposure assessment when any worker's 8-hour TWA exposure equals or exceeds 82 dBA.

1.3.1 Initial Monitoring

An initial monitoring shall be conducted to determine the noise exposure levels representative of all workers whose 8-hour TWA noise exposures may equal or exceed 82 dBA.

1.3.2 Periodic Monitoring

Exposure monitoring shall be repeated at least bi-annually when any worker's 8-hour TWA exposure equals or exceeds 82 dBA, or when equipment or process changes may impact on the exposure levels.

1.3.3 Instrumentation

Instruments used to measure workers' noise exposures shall be calibrated to ensure measurement accuracy. In addition, they shall conform to the American National Standard Specification for Sound Level Meters, ANSI S1.4-1983 and S1.4A-1985, Type 2 [ANSI 1983; ANSI 1985] or, with the exception of the operating range, to the American Standard Specification for Personal Noise Dosimeters, ANSI S1.25-1991 [ANSI 1991a].

For the purpose of determining TWA exposures, all continuous, varying, intermittent and impulsive sound levels from 80 dBA to 140 dBA shall be integrated into the noise measurements.

Section 1.4 ENGINEERING AND ADMINISTRATIVE CONTROLS AND WORK PRACTICES

Wherever feasible, engineering controls, administrative controls, and/or work practices shall be used to ensure that workers are not exposed to noise above the REL of 85 dBA, 8-hour TWA.

Section 1.5 HEARING PROTECTORS

Workers shall be required to wear hearing protectors when their noise exposures exceed 85 dBA, regardless of duration. Hearing protectors shall be provided by the employer.

Hearing protectors shall provide such attenuation as to reduce the worker's effective noise exposure (i.e., the noise exposure when hearing protectors are worn) to 82 dBA, 8-hour TWA, or below. Methods for calculating the effective noise exposure are provided in the Appendix, this chapter.

The employer shall provide training in the fitting and wearing of hearing protectors.

Section 1.6 MEDICAL SURVEILLANCE

The employer shall provide audiometry for all workers whose exposures equal or exceed 82 dBA, 8-hour TWA.

1.6.1 Audiometry

Audiometric tests shall be performed by an audiologist, or by an occupational hearing conservationist certified by the Council for Accreditation in Occupational Hearing Conservation (CAOHC) working under the supervision of an audiologist or physician. If the audiometric test is performed by an occupational hearing conservationist, the CAOHC certification number shall be recorded on each worker's audiogram.

Audiometric tests shall be pure tone, air conduction, hearing threshold examinations, with test frequencies including 500, 1000, 2000, 3000, 4000, 6000 and 8000 Hz, and shall be taken separately for the right and left ears.

Audiometric tests shall be conducted with audiometers that meet the specifications of, and are maintained and used in accordance with the American National Standard Specifications for Audiometers, ANSI S3.6-1995 [ANSI 1995]. Audiometers shall be given an annual comprehensive calibration, a bimonthly acoustic calibration check, and a daily functional check whenever the audiometers are used. The date of the last annual calibration shall be recorded on each worker's audiogram.

Audiometric tests shall be conducted in a room where ambient noise levels conform to all requirements of the American National Standard Institute Maximum Permissible Ambient Noise Levels for Audiometric Test Rooms, ANSI S3.1-1991 [ANSI 1991b], when measured by instruments conforming to the American National Standard Specification for Sound Level Meters, ANSI S1.4-1983 and S1.4A-1985, Type 1 [ANSI 1983; ANSI 1985] and the American National Standard Specification for Octave-Band and Fractional-Octave-Band Analog and Digital Filters, ANSI S1.11-1986 [ANSI 1986]. For permanent, on-site testing facilities, ambient noise levels shall be checked at least annually. For mobile testing facilities, ambient noise levels shall be tested daily, or each time the facility is moved, whichever is more often. Ambient noise levels shall be recorded on the worker's audiogram or made otherwise accessible to the professional reviewer of the audiograms.

1.6.2 Baseline Audiogram

For each worker in a HLPP, a baseline audiogram shall be obtained prior to employment or within 30 days of enrollment in the HLPP.

Because the baseline audiogram is intended to be the best estimate of the worker's hearing before any exposure to potentially harmful noise, the worker shall not be exposed to workplace noise for a minimum of 14 hours before the baseline audiometric test. The required quiet period shall not be substituted by the use of hearing protectors.

1.6.3 Monitoring Audiogram, Retest Audiogram, and Significant Threshold Shift

On an annual basis, each worker's hearing thresholds shall be monitored by an audiometric test, which shall be conducted during the worker's normal workshift. For the purpose of this section, the audiogram from this test is called the monitoring audiogram.

At the completion of this test, the worker's monitoring audiogram shall be examined immediately to determine whether it indicates any threshold shift (higher threshold) in either ear that equals or exceeds 15 dB at 500, 1000, 2000, 3000, 4000 or 6000 Hz as evidenced by a comparison of that audiogram with the worker's baseline audiogram.

If the monitoring audiogram indicates a hearing threshold shift, a retest shall be conducted immediately, following re-instruction of the worker and refitting of the earphones.

If the retest audiogram shows a shift of 15 dB or more at the same frequency in the same ear as in the monitoring audiogram, the worker shall be considered to have met the significant threshold shift criterion for the purpose of this section and shall be given a confirmation audiometric test within 30 days. This confirmation test shall be conducted under the same conditions as in a baseline audiometric test.

1.6.4 Confirmation Audiogram and Fellow-Up Action

If the worker's confirmation audiogram shows a shift of 15 dB or more at the same frequency in the same ear as in the previous retest audiogram, the worker's audiograms and other appropriate records shall be reviewed by an audiologist or a physician.

If this review confirms that the significant threshold shift is persistent, the significant threshold shift shall be recorded in the worker's medical record, and the confirmation audiogram shall be used for the calculation of any subsequent significant threshold shift in future years.

A worker whose significant threshold shift is of any etiology other than noise, as determined in this review, shall be referred to the worker's physician. If the probable etiology is of occupational noise exposure, the employer shall take appropriate action to protect the worker from additional hearing loss due to occupational noise exposure. Examples of appropriate action include, but is not limited to, re-instruction and refitting of hearing protectors, additional training of the worker on hearing loss prevention, and reassignment of the worker to a quieter work area.

1.6.5 Exit Audiogram

An exit audiogram shall be obtained for a worker who is leaving employment or is permanently rotated out of an occupational noise exposure at or above 82 dBA, 8-hour TWA. The audiometric test shall be conducted following a minimum of 14 hours of quiet.

Section 1.7 POSTING

A warning sign shall be located at the entrance to or the periphery of areas where noise exposures exceed 85 dBA, regardless of duration. All warning signs shall be printed both in English and the predominant language of workers who do not read English, if applicable. Workers unable to read the warning signs shall be informed verbally regarding the instructions printed on signs in hazardous work areas of the facility. The warning sign shall consist of the following:

The type size of the word "warning" shall be no less than 36 point, and the rest of the sign shall be no less than 24 point.

Section 1.8 HAZARD COMMUNICATION AND TRAINING

1.8.1 Hazard Communication

All workers who are exposed to noise at or above 82 dBA, 8-hour TWA, shall be informed of the potential consequences of noise exposure and the methods of preventing noise-induced hearing loss.

1.8.2 Training

The employer shall institute a continuing education program conducted by persons qualified by experience or training in occupational hearing loss prevention. The program shall ensure that all workers exposed to noise understand how noise damages the ear and causes hearing loss and how to prevent hearing loss. The program shall have methods of delivery from formal presentation with oral and written materials to informal, on the spot presentations. Allowances shall be made to allow for one-on-one training to be presented at the time of the annual audiogram so that the worker may judge his or her own effectiveness in preventing occupational hearing loss. The scope of educational and motivational materials shall include, at a minimum, what the employer is doing to prevent noise-induced hearing loss and what the worker can do to protect his or her own hearing. The employer shall maintain a record of provision of educational and motivational sessions and shall have documentation of the materials used in these sessions.

Section 1.9 PROGRAM EVALUATION CRITERIA

The effectiveness of the HLPP shall be evaluated at the level of the individual worker and at the programmatic level.

The evaluation at the worker level shall take place at the time of the annual audiometry. If the worker's audiogram meets the condition for significant threshold shift, all possible steps shall be taken to make sure that the change in hearing sensitivity is not caused by something other than exposure to workplace noise. If the significant threshold shift is attributed to workplace noise, then steps shall be taken to prevent further significant threshold shift for that worker.

The evaluation at the programmatic level shall take place no less than annually. If the only data available come from a summary of workers experiencing significant threshold shift, then a significant threshold shift incidence rate based on the total number of workers in the HLPP of 5% or less should be considered evidence of an effective HLPP. If other data, such as the hearing threshold levels (HTLs) of workers not exposed to noise, are available, then a successful program should have a similar incidence rate of significant threshold shift for exposed and non-exposed workers.

Section 1.10 RECORDKEEPING

The employer shall establish and maintain recordkeeping in accordance with the requirements in Sections 1.10.1 through 1.10.5.

1.10.1 Exposure Assessment

The employer shall establish and maintain an accurate record of all exposure measurements required in Section 1.3. These records shall include no less than the name of the worker being monitored; social security number; duties performed and job locations; dates and times of measurements; type, make, model and size of hearing protectors used (if any); the measured exposure levels; and the identification of the person taking the measurements. Copies of a worker's exposure history resulting from this requirement shall also be included in the worker's medical file along with the worker's audiograms.

1.10.2 Medical Surveillance

The employer shall establish and maintain an accurate record for each worker subject to the medical surveillance specified in Section 1.6. These records shall include no less than the name of the worker being tested; social security number; duties performed and job locations; the worker's medical, employment and noise-exposure history; dates, times and types of tests (i.e., baseline, annual, retest, confirmation); hours since last noise exposure before each test; hearing threshold levels at the required audiometric frequencies; tester's identification and assessment of test reliability; the etiology of any significant threshold shift; and the identification of the reviewer.

1.10.3 Record Retention

In accordance with the requirements of 29 CFR 1910.20(d), Preservation of Records, the employer shall retain the records described in Sections 1.3 and 1.6 of this document for at least the following periods:

• 30 years for exposure monitoring records, and
  
• Duration of employment plus 30 years for medical monitoring records.
  

In addition, records of audiometer calibrations and the ambient noise measurements in the audiometric testing room shall be kept for 5 years.

1.10.4 Availability of Records

In accordance with 29 CFR 1910.20, Access to Employee Exposure and Medical Records, the employer shall, upon request, allow examination and copying of exposure monitoring records by the subject worker, the former worker, or anyone having the specific written consent of the subject or former worker.

Any medical records required by this recommended standard shall be provided upon request for examination and copying, to the subject worker, the former worker, or anyone having the specific written consent of the subject or former worker.

1.10.5 Transfer of Records

The employer shall comply with the requirements for the transfer of records as set forth in 29 CFR 1910.20(h), Transfer of Records.

Section 1.11 ANSI STANDARDS

All ANSI standards referenced in this standard shall be superseded with the latest versions as they become available.

Table 1-1
Exposure Level (L) and Duration (T)^*

*Any exposure time above the ceiling limit of 115 dBA shall be included in the calculation of the 8-hour time-weighted-average exposure.

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TABLE 1-2
Noise Dose (D) and Time-Weighted Average (TWA)^*

*TWA = 10 x Log(D/100) + 85

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Appendix - Determination of Effective Noise Level (dBA') for Ear Protectors

This appendix is a summary of Appendix B, Methods for Estimating the Adequacy of Hearing Protector Attenuation, in the Occupational Noise Standard [29 CFR 1910.95] with two exceptions: (1) The 8-hour TWA noise exposure is based on the 3-dB exchange rate. (2) The manufacturer's Noise Reduction Rating (NRR) of a hearing protector is derated. The "NRR" in the equations shown below shall be a derated NRR as follows:

Measure noise exposure levels in dBC or dBA with a sound level meter or noise dosimeter.

1. When the noise exposure level in dBC is known, the effective noise level dBA' is:

dBA' = dBC - NRR

2. When the noise exposure level in dBA is known, the effective noise level DBA' is:

dBA' = dBA - (NRR - 7)

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2.1 RECOGNITION OF NOISE AS A HEALTH HAZARD

The sounds of industry, growing in volume over the years, have heralded not only technical and economic progress, but also the threat of an ever increasing incidence of hearing loss and other noise-related disturbances to exposed workers. Noise, essentially any unwanted or undesirable sound, is not a new hazard. Indeed, noise-induced hearing loss was observed centuries ago. Ramazzini in "De Morbis Artificium Diatriba" in 1700 described how those hammering copper "have their ears so injured by that perpetual din.....that workers of this class became hard of hearing and, if they grow old at this work, completely deaf." Before the Industrial Revolution, however, comparatively few people were exposed to high levels of workplace noise. It was the advent of steam power in connection with the Industrial Revolution that first brought general attention to noise as an occupational hazard. Workers who fabricated steam boilers were found to develop hearing loss in such numbers that such a malady was dubbed "boilermakers disease." Increasing mechanization in all industries and most trades has since proliferated the noise problem.

2.2 NOISE-INDUCED HEARING LOSS

The ear is the organ structure of the body especially adapted and most responsive to the pressure changes in sound production. Anatomically, it is divided into three subdivisions--the outer, middle, and inner ear. The principal functions of the outer and middle ear are to collect and transmit sound pressure to the inner ear where the hair cell receptors for hearing are located. The latter are arranged in several rows along the entire length of the basilar membrane, one of the two partitions which spiral around the bony axis of the cochlea. These hair cells together with their supporting cells comprise the organ of Corti, the auditory sense organ. In the cochlea, mechanical energy from the sound pressure is transformed into electrical energy that is carried by the auditory nerve to the brain. Excessive noise exposure produces a sensorineural hearing loss involving injury to the hair cells.

The most recognized effect of exposure is noise-induced hearing loss (NIHL). Initially, the exposure to high levels of noise causes temporary threshold shift (TTS), which refers to the difference in a person's hearing level measured before and after exposure to noise. This shift is temporary because the individual normally recovers his or her pre-exposure hearing level several hours after cessation of exposure. Repeated exposures over several years can result in a permanent threshold shift (PTS), which is an irreversible, sensorineural hearing loss.

There are other causes of hearing loss besides occupational noise. Hearing loss due to aging occurs naturally and is called presbycusis. Hearing losses caused by non-occupational or recreation noise sources (e.g., loud music, guns, lawnmowers, etc.) effect the ear the same as occupational noise, but are collectively called sociocusis. Conductive hearing losses, as opposed to sensorineural hearing losses caused by noise, are usually traceable to diseases of the outer and middle ear. Concomitant exposures to noise and certain physical or chemical agents (e.g., vibration, organic solvents, carbon monoxide, ototoxic drugs, and certain metals) appear to have synergistic effects on hearing [Jauhianien et al. 1972; Hamernik and Henderson 1976; Brown et al. 1978; Gannon et al. 1979; Brown et al. 1980; Hamernik et al. 1980; Hamernik et al. 1981; Pryor et al. 1983; Rebert et al. 1983; Humes 1984; Boettcher et al. 1987; Young et al. 1987; Byrne et al. 1988; Fechter et al. 1988; Johnson et al. 1988; Morata et al. 1993].

The most obvious effect of noise exposure is the loss of hearing. However, there are other effects that do not involve the auditory system. These nonauditory effects include psychological stress and disruption of job performance [Cohen 1973; EPA 1973; Taylor 1984; Ohrstrom et al. 1988; Suter 1989], and may include hypertension [Parvizpoor 1976; Jonsson and Hansson 1977; Takala et al. 1977; Lees and Roberts 1979; Malchaire and Mullier 1979; Manninen and Aro 1979; Singh et al. 1982; Belli et al. 1984; Delin 1984; Talbott et al. 1985; Verbeek et al. 1987; Wu et al. 1987; Talbott et al. 1990]. Noise may also be a contributory factor in industrial accidents [Cohen 1976; Schmidt et al. 1980; Wilkins and Acton 1982].

2.3 PHYSICAL PROPERTIES OF SOUND

The perception of sound begins when vibration or turbulence causes pressure changes in the air (or in some other medium). These pressure changes produce longitudinal waves that propagate away from the vibrating or turbulent source to the receiver (i.e., human ear) in the form of alternating compression and rarefaction of molecules. The receiver in turn translates these waves into what is known as sound. The effects of sound on the receiver depend on three physical parameters of sound: amplitude, frequency, and duration.

2.3.1 Amplitude

Sound pressure level (SPL), expressed in decibels (dB), is a measure of the amplitude of the pressure change that produces sound. This amplitude is perceived by the listener as loudness. In sound measuring instruments, weighting networks, which will be described in Chapter 4, are used to modify the sound pressure level. The A-weighted network is commonly used in measuring noise to evaluate its effect on people. Exposure limits are expressed in decibels, A-weighted network (dBA). When used without a weighted network suffix, the expression "dB" usually implies "dB SPL" (i.e., no network).

2.3.2 Frequency

Each wave of compression and rarefaction of molecules described above represents a complete cycle. The frequency, in Hertz (Hz), represents the number of cycles occurring in one second, and determines the pitch perceived by the listener. Humans with normal hearing can hear a frequency range of about 20 to 20,000 Hz. An ordinary sound generally contains a mixture of frequencies.

2.3.3 Duration

Duration refers to the pattern of sound levels over time. Industrial noise can be arbitrarily classified as continuous, varying, intermittent and impulsive. Lacking quantitative definitions, continuous, varying and intermittent noises together denote a continuum of relatively constant sound levels to noticeably different sound levels occurring within an exposure period (usually an 8-hour workday). For simplicity, when compared with impulsive noise, the term "continuous noise" would normally include continuous, varying and intermittent noises. Impulsive noise is distinguished from continuous noise by a steep rise in sound pressure level to a high peak, followed by a rapid decay, and can be either reverberant (impact noise) or nonreverberant (impulse noise). In many workplaces, the exposures are often a mixture of continuous and impulsive sounds.

2.4 NUMBER OF NOISE-EXPOSED WORKERS IN THE UNITED STATES

From 1981 to 1983, NIOSH conducted the National Occupational Exposure Survey (NOES). The NOES was designed to provide data descriptive of the occupational safety and health conditions in the United States. The surveyors visited and gathered information at various workplaces throughout the country. For the purposes of the NOES, workers were considered to be noise-exposed if the noise levels were 85 dBA or greater, regardless of the exposure duration. On the basis of the information collected, the percentages of noise-exposed workers in different economic sectors were estimated. When these percentages are applied to the more recent Bureau of Census [1993] data, the estimated number of noise-exposed workers in the United States is approximately 30 million. Table 2-1 provides a breakdown of the estimated numbers of noise-exposed workers by economic sector.

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Table 2-1
Estimated numbers of workers exposed to noise levels at 85 dBA or greater by economic sector, based on data collected in the National Occupational Exposure Survey (NOES) and
on data collected by the Bureau of Census.

¹Standard Industry Classification (SIC) (Source: Anonymous [1987])

²Numbers not available from NOES are extracted from unpublished data from the following sources: NIOSH Farm Family Health Hazard Survey (agricultural services), Mine Safety and Health Administration (mining), and U.S. Army Center for Health Promotion and Preventive Medicine (government civilian work force).

³Source: Bureau of Census [1993]

⁴(NOES % of Noise-Exposed Worker) x (Number of Production Workers in 1992)

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2.5 LEGISLATIVE HISTORY

Efforts to effectively regulate occupational noise in the United States began about 1955. The military was first to establish such regulations for members of the armed forces [U.S. Air Force 1956]. Under the Walsh-Healey Public Contracts Act of 1936, as amended, safety and health standards had been issued that contained references to excessive noise, but they prescribed neither limits nor acknowledged the occupational hearing-loss problem. A later regulation under this act [41 CFR 50-204.10] promulgated in 1969, defined noise limits for occupational exposure for purposes of hearing conservation. These limits were applicable to only those firms having supply contracts with the government in excess of $10,000; similar limits were made applicable to work under Federal service contracts of $2,500 or more under the Service Contract Act. The noise rule in the Walsh-Healey Act regulations was adopted under the Coal Mine Health and Safety Act of 1969 (Public Law 91―173) and thereby became applicable to underground and surface coal mine operations.

In 1970, the Occupational Safety and Health Act (Public Law 95-164) was enacted, which established the Occupational Safety and Health Administration (OSHA) within the Department of Labor as the law enforcement agency responsible for protecting the safety and health of a large segment of the American work force. The Act also stipulated that the Secretary of Health, Education, and Welfare (now the Secretary of Health and Human Services) would, on the basis of available data, develop criteria for harmful physical agents that describe exposure levels safe for various periods of employment. In compliance with this provision, NIOSH published Criteria for a Recommended Standard.... Occupational Exposure to Noise in 1972 [NIOSH 1972]. The document provided the basis for a recommended standard to reduce the risk of developing permanent hearing loss as a result of occupational noise exposure. The criteria document presented a recommended exposure limit (REL) of 85 dBA, 8-hour time-weighted average (TWA); and methods for measuring noise, calculating noise exposure, and providing a hearing conservation program. However, the criteria document acknowledged that NIOSH was not able to determine the technological feasibility of the REL and that approximately 15% of the population exposed to occupational noise at the 85-dBA level for a working lifetime would still be at risk of developing noise-induced hearing loss (NIHL).

Initially, OSHA adopted the Walsh-Healey exposure limit of 90 dBA, 8-hour TWA with a 5-dB exchange rate, as its permissible exposure limit (PEL) [29 CFR 1910.95] for general industry. In 1974, responding to the NIOSH criteria document, OSHA proposed a revised noise standard [39 Fed. Reg. 37,773 (1974a)], but left the PEL unchanged. The proposed standard was not promulgated, but it articulated the requirement for a hearing conservation program. In 1981, and again in 1983, OSHA amended its noise standard to include specific provisions of a hearing conservation program for occupational exposures at 85 dBA or above [46 Fed. Reg. 4,078 (1981); 48 Fed. Reg. 9,738 (1983)]. The OSHA noise standard as amended does not cover all industries. Specifically, the Hearing Conservation Amendments do not cover noise-exposed workers in transportation, oil/gas well drilling and servicing, agriculture, construction, and mining. For examples, the construction industry is covered by another OSHA noise standard [29 CFR 1926.52], and the mining industry is regulated by four separate standards [30 CFR 56; 30 CFR 57; 30 CFR 70; 30 CFR 71] which are enforced by the Mine Safety and Health Administration (MSHA). These standards vary in specific requirements regarding exposure monitoring and hearing conservation, but all maintain an 8-hour TWA exposure limit of 90 dBA. Although they are required to comply with OSHA regulations by Executive Order 12196, the U.S. Air Force [1993] and the U.S. Army [1994] have chosen a more stringent exposure limit of 85 dBA, 8-hour TWA with a 3-dB exchange rate. Thus, the American working population is given various degrees of protection from occupational noise.

Thus far, the exposure limits discussed above apply only to continuous noise. The generally-accepted exposure limit for impulsive noise, 140 dB peak SPL, the level not to be exceeded at any time, is dealt with differently among the regulatory standards. It is either enforceable or non-enforceable, indicated by the word "shall" or "should," respectively. In the MSHA standards [30 CFR 56; 30 CFR 57; 30 CFR 70; 30 CFR 71], this exposure limit is enforceable, but in the OSHA standards [29 CFR 1910.95; 29 CFR 1926.52], it is non-enforceable. The NIOSH REL of 85 dBA, in principle, includes impulsive noise, but for practical purposes it is an exposure limit for continuous noise only.

2.6 SCOPE

This document evaluates and presents the criteria for recommending the exposure limit, the exchange rate, control methods and occupational hearing loss prevention programs. Where the information is incomplete to support definitive recommendations, research needs are suggested for future criteria development. Nonauditory effects of noise, and hearing losses due to causes other than noise, are beyond the scope of this document.

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3.1 QUANTITATIVE RISK ASSESSMENT

The selection of an exposure limit depends on defining two parameters: 1) the maximum hearing threshold level (HTL) that is acceptable, above which there is material impairment and below which hearing is considered to be within normal limits; and 2) the proportion of the noise-exposed population to be protected from material hearing impairment. Also referred to as the fence, the HTL that divides normal hearing from impaired hearing is often calculated as the average of the HTLs for two, three, or four contiguous audiometric frequencies. The proportion of the population to be protected from material hearing impairment is related to the excess risk, which is the difference in the risk of developing material hearing impairment between two populations: one exposed to occupational noise and the other whose members are from a similar population but not exposed to occupational noise. The risk of developing material hearing impairment in these populations is denoted as the expected proportion of individuals developing material hearing impairment. Mathematical models are used to estimate the risk of developing material hearing impairment among workers in these populations.

The most common protection goal has been one aimed at the preservation of hearing for speech discrimination. Using this protection goal, NIOSH [1972] defined hearing impairment as an average of HTLs at the audiometric frequencies 1000, 2000 and 3000 Hz that exceeds 25 dB (the 1-2-3-kHz definition), and the U.S. Public Health Service (PHS) [1991] also reaffirmed this definition as "significant hearing impairment." With this definition, NIOSH [1972] assessed the excess risk of hearing impairment as a function of levels and durations (e.g., 40-year working lifetime) of occupational noise exposure. For a 40-year lifetime exposure to average daily noise levels of 80, 85 and 90 dBA in the workplace, the excess risk estimates were 3%, 16% and 29%, respectively, and on the basis of this risk assessment, NIOSH recommended an 8-hour TWA exposure limit of 85 dBA [NIOSH 1972].

In order to compare the NIOSH excess risk estimates with those developed by other organizations, the NIOSH data were also analyzed using the same 25-dB fence but averaging the HTLs at 500, 1000 and 2000 Hz (the 0.5-1-2-kHz definition) [NIOSH 1972]. Table 3-1 presents the excess risk estimates developed by NIOSH [1972], EPA [1973], and the International Organization for Standardization (ISO) [1971] for hearing impairment caused by occupational noise exposure. OSHA used these estimates as the basis for requiring hearing conservation programs for occupational noise exposures at or above 85 dBA, 8-hour TWA [46 Fed. Reg. 4,078 (1981)].

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TABLE 3-1

Estimated excess risk of incurring hearing impairment^* as a function of noise exposure over a 40-year working lifetime^**

*For purposes of comparison in this table, hearing impairment is defined as an average of the hearing threshold levels at 500, 1000, and 2000 Hz that exceeds 25 dB.

**Adapted from 39 Fed. Reg. 43,802 [1974b].

***Percentage with hearing impairment in an occupational-noise-exposed population after subtracting the percentage who would normally incur such impairment from other causes in an unexposed population.

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The data used for the NIOSH risk assessment were collected by NIOSH in 13 noise and hearing surveys (collectively known as the Occupational Noise and Hearing Survey, or ONHS) from 1968 to 1971. The industries that were involved in the surveys included steel making, paper bag processing, aluminum processing, quarrying, printing, tunnel traffic controlling, woodworking and trucking. Questionnaires and audiometric examinations were given to noise-exposed and non-noise-exposed employees who had consented to participate in the surveys. More than 4,000 audiograms were collected, but excluded from the sample were audiograms of employees not exposed to a specified continuous noise level over their working lifetime and those with abnormal hearing levels as a result of their medical history and a variety of otological problems. Thus, 1,172 audiograms were used, which represented 792 noise-exposed and 380 non-noise-exposed workers (controls) [NIOSH 1972; Lempert and Henderson 1973].

Because of the prolific use of hearing protectors in the U.S. workplace since the early 1980's, which would confound the determination of dose-response relationships, new data that can be used for risk assessment of occupational NIHL in the U.S. are not known to exist. Prince et al. [1996] derived a new set of excess risk estimates using the ONHS data with a model (referred to as the "1996-NIOSH" model) which differed from the model used in 1972 [NIOSH 1972] (referred to as the "1972-NIOSH" model). A noteworthy difference between the two models is that Prince et al. [1996] considered more flexible shapes for the effects of noise in the 1996-NIOSH model, whereas the 1972-NIOSH model was based on a linear assumption for the effects of noise. Prince et al. [1996] found that linear models similar to the 1972-NIOSH model did not fit the data as well. In addition to using the 0.5-1-2-kHz and the 1-2-3-kHz definitions of hearing impairment to assess the risk of occupational NIHL, Prince et al. [1996] also used the definition of hearing handicap¹ proposed by the American Speech-Language-Hearing Association (ASHA) Task Force on the Definition of Hearing Handicap, which is an average of the HTLs at 1000, 2000, 3000 and 4000 Hz that exceeds 25-dB (the 1-2-3-4-kHz definition) [ASHA 1981].

¹The American Speech-Language-Hearing Association makes a distribution between the terms "hearing impairment" and "hearing handicap," but for the purpose of the subsequent discussion in this criteria document, only the term "hearing impairment" is used.

In 1971, ISO issued the first edition of ISO 1999, Assessment of Occupational Noise Exposure for Hearing Conservation Purposes [ISO 1971] (referred to as the 1971-ISO model). This edition included estimates of the risk of hearing impairment from occupational noise exposures. In 1990, the ISO issued a second edition of ISO 1999, Acoustics - Determination of Occupational Noise Exposure and Estimation of Noise-Induced Hearing Impairment [ISO 1990] (referred to as the 1990-ISO model). Both ISO models are based on broadband, steady noise exposures for 8-hour workshifts during a working lifetime of up to 40 years. For comparison among the risk analyses by different organizations over the years, Prince et al. [1996] also calculated the excess risk estimates using the 1990-ISO model and the information in Annex A of ISO 1999 [ISO 1990]. The comparison is summarized in Table 3-2.

Table 3-2

Excess risk estimates for hearing impairment at age 60, based on 40-year lifetime occupational exposures to 80, 85 and 90 dBA - Comparison of the 1971-ISO, 1972-NIOSH, 1973-EPA, 1990-ISO and 1996-NIOSH models against the 0.5-1-2-kHz, the 1-2-3-kHz and the 1-2-3-4-kHz definitions of hearing impairment.

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The excess risk estimates derived from the 1971-ISO, 1972-NIOSH, 1973-EPA and 1996-NIOSH² models are reasonably similar. However, except for the 1-2-3-4-kHz definition, the excess risk estimates derived from the 1990-ISO model are considerably lower than those derived from the other models. These disparities may be due to differences in the ISO 1999 [1990] statistical methodology or in the underlying data used. However, regardless of which set of excess risk estimates is used, an excess risk of hearing impairment remains at and below 85 dBA.

²Prince et al. [1996] found that the excess risk estimates at exposure levels below 85 dBA were not well defined. Insufficient data for workers with average daily exposures below 85dBA led to considerable variability in the estimation, depending on the statistical assumptions used in the modeling.

As mentioned earlier in this section, the protection goal incorporated in the definitions of hearing impairment has been to preserve hearing for speech discrimination. The 4000-Hz audiometric frequency has been recognized as not only being sensitive to noise but also extremely important for hearing and understanding speech in unfavorable or noisy listening conditions [Kuzniarz 1973; Aniansson 1974; Suter 1978]. Because listening conditions are not always ideal in everyday life, and on the basis of the ASHA [1981] Task Force's proposal, NIOSH has modified its definition of hearing impairment to include the 4000-Hz audiometric frequency for use in assessing the risk of occupational NIHL. Hence, with this modification, NIOSH defines hearing impairment as an average of the HTLs at 1000, 2000, 3000 and 4000 Hz that exceeds 25 dB. Based on this definition of hearing impairment, Prince et al. [1996] have estimated that the excess risk is 8% for a 40-year lifetime occupational exposure to an average daily noise level of 85 dBA. This excess risk estimate is lower than the 16% derived from the 1972-NIOSH model with the 1-2-3-kHz definition for the same exposure parameters; thus, NIOSH continues to recommend the exposure limit of 85 dBA, 8-hour TWA.

3.2 CEILING LIMIT

NIOSH [1972] also recommended a ceiling limit of 115 dBA. Exposures to noise levels greater than 115 dBA would not be permitted regardless of the duration of the exposure. This ceiling limit is based on the assumption that above a critical intensity level the ear's response to energy no longer has a relation to the duration of the exposure, but is only related to the intensity of the exposure. Recent research with animals indicates that the critical level is between 115 and 120 dBA [Price and Kalb 1991; Henderson et al. 1991; Danielson et al. 1991]. Below this critical level, the amount of PTS is related to the intensity and duration of exposure; but above this critical level, the relationship does not hold. For a noise standard to be protective, there should be a noise ceiling level above which no unprotected exposure is permitted. Given the recent data, 115 dBA is a reasonable ceiling limit beyond which no unprotected exposure should be permitted.

3.3 EXCHANGE RATE

The following discussion is summarized from a NIOSH contract report [Suter 1992a], of which the conclusion is supported by NIOSH.

Health effect outcomes are dependent on exposure level and duration. For some time, scientists have attempted to identify the relationship between noise level and duration that will best predict hearing impairment. Currently, this relationship is called the "exchange rate," although other terms have been used to describe it, including the "doubling rate," "trading ratio," and "time-intensity tradeoff." The most commonly used exchange rates incorporate either 3 dB or 5 dB per doubling or halving of exposure duration.

The 3-dB exchange rate, which is used by the EPA, Great Britain, and many European countries, is also known as the equal-energy rule or hypothesis. First proposed by Eldred et al. [1955], it was later supported and expanded by Burns and Robinson [1970]. This hypothesis maintains that equal amounts of sound energy will produce equal amounts of hearing impairment, regardless of how the sound energy is distributed in time. Theoretically, this principle could apply to exposures ranging from a few minutes to many years. Ward and Turner [1982], however, suggest restricting its use to the sound energy accumulated in one day only. They make a distinction between an interpretation of the "total energy" theory that would allow a whole lifetime's exposure to be condensed into a few hours, and a restricted "equal-A-weighted-daily-energy" interpretation of the theory. Burns [1976] also cautions against the misuse of the equal-energy hypothesis, noting that it was based on data gathered from individuals who experienced daily 8-hour occupational exposures for periods of months to years, and thus, extrapolation to very different conditions would be inappropriate.

On an energy basis, the 3-dB exchange rate provides for the calculation of a mathematically-valid TWA exposure to noise. This is true when the general definition of "decibel" is examined. A decibel (dB) is a dimensionless unit used in physics, which is equal to 10 times the logarithm to the base 10 of the ratio of two values:

dB = 10*log(value₁/value₂)

The "value" in the above equation can be one of several types of measures, including sound intensity (a measure of energy). Based on the above equation, the relationship between sound intensity and sound intensity level is defined by the following equation:

Sound Intensity Level (dB) = 10*log(I/I_o)

where

I = sound intensity
I_o = reference sound intensity

By this mathematical relationship, every doubling of energy is represented by an increase of 3 dB in the following calculations:

Let X = the exchange rate

10*log(I/I_o) + X = 10*log(2I/I_o)

X = 10*log(2I/I_o) - 10*log(I/I_o)

= 10*log(2)

= 10(0.301)

= 3.01 dB

This same relationship does not hold true for the 5-dB exchange rate. To derive X = 5 dB, the sound intensity would have to be more than doubled in this equation. Thus, the 5-dB exchange rate does not provide for the doubling or halving of energy per 5-dB increment.

The 5-dB exchange rate is sometimes called the OSHA rule, and it is less protective than the equal-energy hypothesis. It attempts to account for the interruptions in noise exposures that commonly occur during the work day [40 Fed. Reg. 12,336 (1975)], presuming that some recovery from TTS occurs during these intermittencies, and the hearing loss is not as great as it would be if the noise were continuous. The rule itself makes no distinction between continuous and non-continuous noise, and it will permit comparatively long exposures to continuous noise at higher sound levels than would be allowed by the 3-dB rule. Based on the limited data that existed in the early 1970's, NIOSH [1972] recommended the 5-dB exchange rate, but after reviewing the more recent scientific evidence, NIOSH now recommends the 3-dB exchange rate.

The evolution of the 5-dB exchange rate began in 1965 when the National Academy of Sciences-National Research Council, Committee on Hearing, Bioacoustics, and Biomechanics (CHABA) issued criteria for assessing allowable exposures to continuous, fluctuating, and intermittent noise [Kryter et al. 1966]. The CHABA criteria was an attempt to predict the hazard from nearly every conceivable noise exposure pattern, based on TTS experimentation. In the development of its criteria, CHABA used the following postulates:

1. TTS₂ (temporary threshold shift measured 2 minutes after a period of noise exposure) is a consistent measure of the effects of a single day's exposure to noise.

2. All noise exposures that produce a given TTS₂ will be equally hazardous (the "equal temporary effect" theory).

3. PTS produced after many years of habitual noise exposures, 8 hours per day, is about the same as the TTS₂ produced in normal ears by an 8-hour exposure to the same noise.

The CHABA postulates were not validated, and also because TTS proved not to be a good predictor of permanent hearing damage, criteria based on TTS patterns could not be relied upon for predicting the long-term adverse effects of noise exposure. TTS₂ is not a consistent measure of the effects of a single day's exposure to noise, and the PTS after many years may be quite different from the TTS₂ produced at the end of an 8-hour day. Research has failed to show a significant correlation between TTS and PTS [Burns and Robinson 1970; Ward 1980], and the relationships among TTS, PTS, and cochlear damage are equally unpredictable [Ward 1970; Ward and Turner 1982; Hetu 1982; Clark and Bohne 1978; Clark and Bohne 1986].

CHABA's assumption of the equal temporary effect theory is also questionable in that some of the CHABA-permitted intermittent exposures can produce delayed recovery patterns even though the magnitude of the TTS was within "acceptable" limits and chronic incomplete recovery will hasten the advent of PTS. The CHABA criteria also assume regularly spaced noise bursts, interspersed with periods that are sufficiently quiet to permit the necessary amount of recovery from TTS. Both of these assumptions fail to characterize noise exposures in the manufacturing industries.

Botsford [1967] published a simplified set of criteria based on the CHABA criteria, having observed that the CHABA method had proved too complicated for general use. The Botsford [1967] method assumes that interruptions will be of "equal length and spacing so that a number of identical exposure cycles are distributed uniformly throughout the day." These interruptions would occur during coffee breaks, trips to the washroom, lunch, and periods when machines are temporarily shut down.

During the same period, there was another parallel, but related, development that led to the 5-dB exchange rate. Simplifying the criteria developed by Glorig et al. [1961] and adopted by the International Organization for Standardization (ISO) [1961], the Intersociety Committee [1970] published its criteria that consisted of a table showing permissible exposure levels (starting at 90 dBA) as a function of duration and the number of occurrences per day. The exchange rates varied considerably depending on noise level and frequency of occurrence. For continuous noise with durations less than 8 hours, the Committee recommended maximum exposure levels based on a 5-dB exchange rate.

In 1968, the Department of Labor proposed a noise standard under the authority of the Walsh-Healey Public Contracts Act [33 Fed. Reg. 14,258 (1968)]. The proposal contained a permissible exposure limit of 85 dBA for continuous noise. Exposure to non-continuous noise was to be assessed over a weekly period according to a large table of exposure indices. The exchange rate varied according to level and duration; a rate of 2 to 3 dB was used for long-duration noises of moderate level, and 6 to 7 dB for short-duration, high-level bursts. This standard was promulgated early in 1969 [34 Fed. Reg. 790 (1969a)], but was withdrawn after a short period. Later in that same year the Walsh-Healey noise standard that is in effect today was issued [34 Fed. Reg. 7,948 (1969b)]. In this version, any special criteria for non-continuous noise had disappeared and the 5-dB exchange rate became official. Thus, the 5-dB exchange rate appears to have been the outgrowth of the many simplifying processes that preceded it.

Although the exact history of the 3-dB rule is not certain, the study of Burns and Robinson [1970] adds to the credibility of the 3-dB rule, which has been increasingly supported by national and international consensus [EPA 1973; EPA 1974; 39 Fed. Reg. 43,802 (1974b); IS0 1971; von Gierke at al. 1981; IS0 1990; U.S. Air Force 1993; U.S. Army 1994; ACGIH 1995]. The only field study that has been repeatedly cited as supporting the 5-dB rule is the study of miners by Sataloff et al. [1969].

Data from animal experiments support the use of the 3-dB exchange rate for single exposures of various levels within an 8-hour day [Ward and Nelson 1971; Ward and Turner 1982; Ward et al. 1983], but there is increasing evidence that intermittency can be beneficial, especially in the laboratory [Bohne and Pearse 1982; Ward and Turner 1982; Ward et al. 1982; Bohne et al. 1985; Bohne et al. 1987; Clark et al. 1987]. However, these benefits are likely to be smaller or even nonexistent in the industrial environment, where sound levels during intermittent periods are considerably higher and where interruptions are not evenly spaced.

Data from a number of field studies correspond well to the equal-energy hypothesis, as Passchier-Vermeer [1971 and 1973] and Shaw [1985] have demonstrated. In Passchier-Vermeer's portrayal of the data [Passchier-Vermeer 1973], the Passchier-Vermeer [1968] and the Burns and Robinson [1970] prediction models for hearing losses as a function of continuous-noise exposure level fit the data on hearing losses from intermittent or varying noise exposures quite well. The fact that comparisons using the newer ISO standard [ISO 1990] corroborate Passchier-Vermeer's findings lend additional support to the equal-energy hypothesis.

Some field data from certain occupations, such as forestry and mining, show less hearing loss than expected when compared with continuous noise data [Sataloff et al. 1969; Holmgren et al. 1971; Johansson 1973; and INRS 1978], although these findings have not been supported by the two NIOSH [1976 and 1982] studies of intermittently exposed workers or the analyses conducted by Passchier-Vermeer [1973] and Shaw [1985]. If such a trend exists, it is further supported by the evidence with experimental animals that laboratory intermittencies produce a savings over continuous noise exposure.

However, the ameliorative effect of intermittency does not support the use of the 5-dB exchange rate. For example, although Ward [1970] noted that some industrial studies have shown lower PTS from intermittent noise exposure than would be predicted by the 3-dB rule, he did not favor selection of the 5-dB exchange rate as a compromise to compensate for the effects of intermittency because it would allow single exposures at excessively high levels. In his opinion, "this compromise was futile and perhaps even dangerous." [Ward 1970]

One response to the evidence from the animal studies and certain field studies would be to select the 3-dB exchange rate, but to allow an adjustment (increase) to the maximum permissible exposure limit for certain intermittent noise exposures, as suggested by EPA [1974] and Johansson et al. [1973]. This would be in contrast to a 5-dB exchange rate, for which there is little scientific justification. Ideally, if an adjustment is needed, the amount of such an adjustment should be determined by the temporal pattern of the noise and the levels of quiet between noise bursts. At this time, however, there is little quantitative information about these parameters in industrial environments. Therefore, the need for an adjustment should await clarification by further research. While the 3-dB rule may be somewhat conservative in truly intermittent conditions, the 5-dB rule will be under-protective in most others. Whether or not an adjustment is used for certain intermittent exposures, the 3-dB exchange rate is the method most firmly supported by the scientific evidence for assessing hearing impairment as a function of noise level and duration.

3.4 IMPULSIVE NOISE

Impulsive noise is a short burst of acoustic energy characterized by a rapid rise to a peak sound level followed by a rapid decay. With peak levels ranging from less than 100 dB to well above 140 dB, burst durations may vary from several microseconds to several hundred milliseconds. As a general rule, the burst duration of an impulsive noise is less than 1 second. Coles et al. [1968] defined two basic types of impulsive noise: a nonreverberant A-type, also known as impulse noise (e.g., gunfire) and a reverberant B-type, also known as impact noise (e.g., a hammer striking a metal surface).

The OSHA occupational noise standard [29 CFR 1910.95] states: "Exposure to impulsive or impact noise should not exceed 140 dB peak sound pressure." Thus, in this context, the 140-dB limit is advisory rather than mandatory. This number was first proposed by Kryter et al. [1966], and later acknowledged by Ward [1986] as "little more than a guess." NIOSH [1972] did not address the hazard of impulsive noise although it stated that the provisions of the recommended standard in the criteria document were "intended to apply for all noise." To date, there is not yet consensus as to what criteria best describe the relationship between NIHL and exposure to impulsive noise, either by itself or in the presence of continuous noise.

In many industrial operations, impulsive noise occurs in a background of continuous noise. There are at least two approaches to evaluating exposures to impulsive and continuous noises. One approach, based on the equal-energy hypothesis, is to integrate impulsive noise with continuous noise in determining the TWA exposure level. Another approach favors the evaluation of impulsive noise exposure separate from that of continuous noise exposure.

3.4.1 Impulsive Noise Conforming to the Equal-Energy Hypothesis

In 1968, CHABA published damage risk criteria for impulsive noise based on the equal-energy hypothesis [CHABA 1968]. Over the years, there has been individual and organizational support for treating impulsive noise on an equal-energy basis [Coles et al. 1973; EPA 1974; Coles 1980; ISO 1990].

Burns and Robinson [1970] proposed the concept of "immission," which is based on the equal-energy hypothesis, to describe the total energy from a worker's exposure to continuous noise over a period of time (i.e., months or years). Atherley and Martin [1971] modified this concept to include impulsive noise in the calculation of the equivalent continuous noise level (L_eq), which is the A-weighted TWA level of continuous noise equivalent to the same amount of energy contributed by all noises, including impulsive noise, in an 8-hour workday.

Studying 76 men who were exposed to impact noise in two drop-forging factories, Atherley and Martin [1971] calculated each man's noise exposure (immission level) during his employment period, and plotted it against his age-corrected HTLs over 6 audiometric frequencies. They found that the observed HTLs of the population came close to the predicted HTLs according to Robinson [1968], and concluded that the equal-energy hypothesis was applicable to impact noise. Similarly, Atherley [1973] examined the HTLs of 50 men exposed to impact noise produced by pneumatic chisels used on metal castings, and found good agreement between observed and predicted HTLs.

Guberan et al. [1971] compared the HTLs of 70 workers exposed to impact noise in drop-forging workshops with the predicted HTLs according to Robinson [1968] at the 3, 4 and 6 kHz audiometric frequencies. Again, the observed HTLs were in close agreement with the predicted.

A study of 716 hammer and press operators in seven drop forges by Taylor et al. [1984] indicates that hearing losses resulting from impact and continuous noises in the drop-forging industry are as great or greater than those resulting from equivalent continuous noise. Using noise dosimetry, Taylor et al. [1984] found that the hammer operators were exposed to average L_eq of 108 dBA, and the press operators, to 99 dBA. The investigators also conducted audiometry for the operators. The median HTLs of hammer operators of all age groups approximated those predicted by the Robinson [1968] immission model. The median HTLs of younger press operators (15 to 34 years of age) also corresponded closely with the predicted values, but those of older press operators (34 to 54 years of age) were significantly higher than predicted. The results indicate that, up to certain limits, the equal-energy hypothesis can be applied to combined exposure to impact and continuous noises.

3.4.2 Impulsive Noise Not Conforming to the Equal-Energy Hypothesis

Despite its simplicity, the equal-energy hypothesis as applied to exposure to combined impulsive and continuous noises is not universally accepted. In her evaluation of the effects of continuous and varying noises on hearing, Passchier-Vermeer [1971] found that the HTLs of workers in steel construction works did not conform to the equal-energy hypothesis; that is, the hearing losses in these workers, who were exposed to noise levels with impulsive components, were higher than predicted. Later studies reported by Ceypek et al. [1973], Hamernik and Henderson [1976] and Nilsson et al. [1977] also indicated that continuous and impulsive noises have a synergistic, rather than additive, effect on hearing.

Comparing the studies of Passchier-Vermeer [1973] and of Burns and Robinson [1970], Henderson and Hamernik [1986] suggested that the steeper slope of Passchier-Vermeer's exposure-response curve at the 4000-Hz audiometric frequency might have been due to noise exposures which contained impulsive components, a characteristic not present in the Burns and Robinson data. Citing the similarity of Passchier-Vermeer's data to those collected by Taylor and Pelmear [1976] and Kuzniarz et al. [1976] on workers exposed to impulsive noise environments containing impulses, Henderson and Hamernik [1986] indicated that exposure to continuous and impulsive noises in combination may be more hazardous than exposure to continuous noise alone.

Voight et al. [1980] studied noise exposure patterns in the building construction industry and related the L_eq to audiometric records of over 81,000 construction workers in Sweden. They found differences in hearing loss among groups exposed to noise of the same L_eq but with different temporal characteristics. Groups exposed to impulsive noise had more hearing loss than those exposed to continuous noise of the same L_eq.

Sulkowski and Lipowczan [1982] conducted noise measurement and audiometric testing in a drop-forge factory. The HTLs of 424 workers in the factory were compared to the predicted values according to the Burns and Robinson equation [1970]. The observed and the predicted values differed in that there was a smaller observed hearing loss in the lower audiometric frequencies but a greater observed hearing loss at the higher audiometric frequencies. In their study of hearing loss in weavers, who were exposed to continuous noise, and drop-forge hammermen, who were exposed to impact noise of equivalent energy to the continuous noise level, Sulkowski et al. [1983] found that the latter group of workers had substantially worse hearing than did the former group.

Thiery and Meyer-Bisch [1988] conducted a cross-sectional epidemiological study at an automobile manufacturing plant. The automotive workers were exposed to continuous and impulsive noises at L_eq levels ranging from 87 to 90 dBA. When their HTLs were compared to those of workers exposed to continuous noise at L_eq of 95 dBA for the same exposure time, the automotive workers showed greater hearing losses at the 6000-Hz audiometric frequency than the reference population after nine years of exposure.

Starck et al. [1988] compared the HTLs at the 4000-Hz audiometric frequency of forest workers using chain saws and shipyard workers using hammers and chippers. The former group was exposed to continuous noise, and the latter group was exposed to impact noise. Starck et al. [1988] also used the immission model developed by Burns and Robinson [1970] to predict the HTLs for both groups. They found that Burns and Robinson's model was accurate at 4000 Hz for the forest workers, but that it substantially underestimated the HTLs at 4000 Hz for the shipyard workers.

The studies described above provide evidence to indicate that the effects of combined exposure to impulse and continuous noises are synergistic rather than additive as the equal-energy hypothesis would support. One measure for protecting a worker from such synergistic effects would be to require that a correction factor be added to a measured TWA noise exposure level when impulsive components are present in the noise. The magnitude of such a correction has not been quantified. The matter becomes more complicated when other parameters of impulsive noise are considered. It appears that noise energy is not the only factor that affects hearing. The amplitude, duration, rise time, number of impulses, repetition rate and crest factor appear to be involved [Henderson and Hamernik 1976; Starck and Pekkarinen 1987; Pekkarinen 1989]. The criteria for exposure to impulsive noise based on the inter-relationships of these parameters await the results of further research.

3.4.3 Combined Exposure to Continuous and Impulsive Noises

Whether the effects of combined exposure to continuous and impulsive noises are additive or synergistic, exposure to these noises does cause hearing loss. In many industrial environments, impulsive noise is often present with continuous noise. It is also a matter of practicality to measure all noise levels when different types of noises are present. Therefore, the criteria for limiting exposure to continuous noise should be extended to include impulsive noise.

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While a variety of instruments for measuring noise is available today, the choice of a particular instrument depends on the measurement need. This discussion is limited to the two most commonly used instruments for measuring noise exposures, the sound level meter and the noise dosimeter. More detailed discussions on instrumentation can be found in available reference sources [NIOSH 1973; Earshen 1986; Johnson et al. 1991].

4.1 SOUND LEVEL METER

The sound level meter is the basic measuring instrument for noise exposures. It consists of a microphone, a frequency selective amplifier, and an indicator. When it is calibrated, it measures sound pressure level in decibels.

4.1.1 Frequency Weighting Networks

The definition of sound pressure level makes no reference to sound frequency. In actuality, the ear does not show equal response to all frequencies. Most sound level meters have A and C weighting networks or scales, and some include the B scale also. The frequency characteristics of these scales are shown in Table 4-1. These scales, which modify sound pressure level detected by the sound level meter to approximate the ear's response, are empirically derived. The A scale is commonly used in measuring noise to evaluate its effect on people and has been incorporated in many occupational noise standards.

TABLE 4-1

Relative Response of Sound Level Meter Weighting Networks^*

*Adapted from ANSI [1983].

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4.1.2 Meter Response

The meter response can be set to either SLOW or FAST, corresponding to the integration periods 1 and 0.125 second, respectively. The indicator reflects the average sound pressure level measured by the meter during the period selected. In general, continuous sounds can be measured easily using either SLOW or FAST response, but fluctuating sounds should be measured using SLOW response to reduce indicator fluctuations. The indicator fluctuates less with the longer 1-second integration time of SLOW response relative to the 0.125-second integration time of FAST response.

4.2 NOISE DOSIMETER

Measuring noise exposure with a sound level meter is relatively simple when the noise levels are continuous and non-varying and where the worker remains essentially stationary during the workshift. When the noise levels are varying or intermittent, or contain impulsive components, and where the worker moves around frequently during the workshift, the use of a noise dosimeter to measure a worker's noise exposure is preferred.

The noise dosimeter may be thought of as a sound level meter with an additional storage and computational function. It measures and stores the sound levels during an exposure period and computes the readout as the % dose or TWA. Many dosimeters available today can measure the noise dose or time-weighted average using various exchange rates (e.g., 3-, 4-, and 5-dB), 8-hour criterion levels (e.g., 80, 84, 85 and 90 dBA) and sound measurement ranges (e.g., 80 to 130 dBA). The wide selections exist to accommodate different noise standards. It should be noted that the choice of FAST or SLOW meter response on the dosimeter does not affect the computed noise dose or time-weighted average when the 3-dB exchange rate is used. However, this does not apply when other exchange rates are used.

OSHA requires that, for the purposes of the Hearing Conservation Amendment, all sound levels from 80 to 130 dBA must be included in the noise measurements [29 CFR 1910.95 (d)(2)(i)]. This range was specified on the basis of instrument capabilities available at that time [ANSI 1978], and OSHA had intended to increase the upper limit of the range to 140 or 150 dB as improved dosimeters became readily available [46 Fed. Reg. 4,135 (1981)].

To measure all sound levels from 80 to 140 dBA, a noise dosimeter should have an operating range of at least 60 dB and a pulse range of the same magnitude. In contrast, the ANSI S1.25-1991 standard specifies that dosimeters should have an operating range of at least 50 dB and a pulse range of at least 53 dB [ANSI 1991a]. Today, noise dosimeters that have operating and pulse ranges in excess of 65 dB are quite common. Therefore, NIOSH considers that measuring all sound levels from 80 to 140 dBA with a noise dosimeter is technically feasible.

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Whenever potentially hazardous noise exists in the workplace, measures should be taken to reduce those levels as much as possible, to protect workers who are exposed to the noise, and to monitor the effectiveness of these intervention processes. Employers have an obligation to protect their workers from this debilitating occupational hazard [46 Fed. Reg. 4,078 (1981); 48 Fed. Reg. 9,738 (1983)]. Additionally, research has shown that there are numerous other benefits of implementing effective hearing loss prevention programs (HLPPs)(also known as hearing conservation programs) in the workplace. For example, Cohen [1976] found reduced employee absenteeism following the establishment of a hearing conservation program. Similarly, Schmidt et al. [1980] reported a reduction in workplace injuries following the introduction of a hearing conservation program. Alternatively, other reports have documented detrimental non-auditory effects of noise such as decreased productivity in high noise environments [Noweir 1984; Suter 1992b]. It has also been suggested that the employer who effectively protects his workers' hearing may also reap the economic benefits of lower worker's compensation rates because of fewer claims for noise-induced hearing loss.

NIOSH recommends that HLPPs be implemented for all workers whose unprotected (i.e., without the use of hearing protectors) 8-hour TWA exposures equal or exceed 82 dBA (i.e., 1/2 of the REL)³ and that the programs include at least the following components [NIOSH 1995]:

³The selection of one-half the exposure limit as the trigger level for the HLPP has precedence in other occupational health standards, and is necessary to ensure the coverage of those workers whose time-weighted-average exposure levels may exceed the exposure limit on any given day but not everyday [NIOSH 1977; NIOSH 1978].

1. Initial and annual audits of procedures

2. Assessment of noise exposures

3. Engineering or administrative control of noise exposures

4. Audiometric evaluation and monitoring of workers' hearing

5. Use of hearing protectors for exposures greater than 85 dBA, regardless of exposure duration

6. Education and motivation of workers

7. Recordkeeping

8. Program evaluation for effectiveness

Implementation of a HLPP must hinge on the fact that occupational noise-induced hearing loss is 100% preventable. The key to developing and implementing an effective program lies in a commitment by both management and workers to truly prevent hearing loss [Helmkamp et al. 1984]. Achievement of this end is facilitated by integrating the HLPP into the company's overall health and safety program [Berger 1981; NIOSH 1995]. This gives the prevention of hearing loss the same weight as the prevention of other work-related illnesses and injuries, thus indicating to workers and management that occupational hearing loss is something to be taken seriously. Other factors which facilitate an effective HLPP include encouraging workers to carry over their good hearing conservation practices to off-the-job situations; using simple, clearly-defined procedures; making compliance with the HLPP a condition of employment; and incorporating safety requirements into written company policy. Today, there is no legitimate reason for any worker to incur an occupational hearing loss [NIOSH 1995].

5.1 PERSONNEL REQUIREMENTS

Responsibility for developing and implementing a HLPP usually resides with a team of professionals. The American Occupational Medical Association (AOMA) [1987] identifies the team approach to hearing conservation as necessary for its success. The number of team members and their professional disciplines may vary with the kind of company and the number of noise-exposed workers; however, members frequently include audiologists, physicians, occupational health nurses, occupational hearing conservationists, engineers, industrial hygienists, safety professionals, management representatives, and employee/union safety representatives.

Regardless of whether program responsibility resides in a team or a single individual, one person should act as champion for the program and must hold overall responsibility for its implementation [NIOSH 1995; Royster and Royster 1990]. This individual will be referred to in this document as the program implementor. The program implementor should ensure that all aspects of the program are fully and properly administered, and should enlist the support of management and workers to actively prevent hearing loss. Royster and Royster [1990] recommend that the primary qualification of the program implementor should be a genuine interest in preserving workers' hearing. AOMA [1987] recommends that the program implementor be a physician. NIOSH [1995] maintains that the professional discipline of the program implementor is not as important as his/her ability to act as the "champion" of the HLPP by focusing management and worker attention on hearing conservation issues. Additionally, the program implementor's stature in the organization should allow him/her to make decisions, correct deficiencies, enforce compliance, and supervise other team members with regard to the program.

In addition to the program implementor, there should be an individual specifically responsible for the audiometric aspects of the HLPP; this individual will be referred to in this document as the audiometric manager. The professional qualifications of this individual are critical. The audiometric manager should be an audiologist or a physician specializing in otological or occupational medicine. The overall program implementor and the audiometric manager may be the same person, provided that the individual meets the qualifications for both positions. If the overall program implementor and the audiometric manager are not the same person, the audiometric manager should report to the overall manager, regardless of the professional credentials of either party.

5.2 INITIAL AND ANNUAL AUDITS (Component 1)

Ideally, an initial audit should be conducted before a HLPP is implemented or any changes are made to an existing program. This audit will serve as a basis for comparison against which the effectiveness of an improved program can be assessed. The audit should begin by examining administrative issues such as corporate responses to health and safety regulations, official policies promoting good safety and health practices, assurance of adequate resources to conduct the program, and the status within the company of the program implementor. Current engineering and administrative controls should be evaluated, and the systems for monitoring noise exposures and conducting audiometry should be critically examined. Employee and management training should be noted, with past successes and failures analyzed so that improvements can be made. In particular, if engineering and administrative controls are insufficient, it should be noted whether effective training is provided in the selection, fitting, and daily use of hearing protectors. Recordkeeping procedures should receive a meticulous inspection, since how the records concerning audiometry, noise exposure, and other aspects of the overall program are maintained can greatly influence the success or failure of a program. NIOSH recommends that a HLPP audit should be conducted annually as a part of an overall program evaluation so that the strengths of the program may be clearly identified, and any weakness promptly addressed [NIOSH 1995].

5.3 EXPOSURE ASSESSMENT (Component 2)

Section 6(b)(7) of the Occupational Safety and Health Act of 1970 [29 USC 651 et seq.] directs that, where appropriate, occupational health standards shall provide for monitoring or measuring employee exposure at such locations and intervals in such a manner as may be necessary for the protection of employees. Accurate characterization of the noise hazard present in the workplace and the subsequent identification of affected workers are both extremely important. These two elements form the basis for all subsequent actions within the HLPP [NIOSH 1995]. Monitoring procedures should be specifically defined to ensure consistency. Instrumentation, calibration, measurement parameters, and methods for linking results to worker records should be clearly delineated. Exposure assessment should be done during typical production cycles; however, if noise levels vary significantly during different phases of production, then exposures should be assessed separately for each phase [Royster and Royster 1990; NIOSH 1995].

Exposure assessment should be conducted by an industrial hygienist, an audiologist, or other professional with appropriate training [NIOSH 1995]. Workers should be permitted and encouraged to observe and participate in monitoring activities, insofar as such observation or participation does not interfere with the monitoring procedure. Their participation will help ensure valid results, as the workers frequently have the experience to identify the prevailing noise sources, to indicate periods when noise exposure may differ, and to recognize whether given noise levels are typical or atypical. They can explain how different operating modes affect equipment sound levels, and describe worker tasks and positions. Having the cooperation of workers is also critical in order to ensure that workers do not advertently or inadvertently interfere with obtaining valid measurements. The initial exposure monitoring can serve as an introduction to the HLPP by raising the awareness of workers and management regarding noise as a hazard. The monitoring survey, if conducted cooperatively, can help establish a rapport that will assist in obtaining the cooperation of both employees and essential management in later phases of the program [Royster and Royster 1990; NIOSH 1995].

The frequency with which noise exposure assessments are updated depends upon a number of variables. These might include the intensity of the noise, potential changes in exposure due to changes in equipment or production, the rate of significant threshold shift noted among workers, other changes noted in additional measures of program effectiveness, the requirements of various governmental regulations, directives from the Bureau of Workers' Compensation, union contract stipulations, and specific company policies [Royster et al. 1986].

In general, NIOSH [1995] recommends that exposure monitoring be repeated at least bi-annually. Monitoring should be repeated sooner if there is a change in production, process, equipment, or personnel which might affect exposure levels [Royster et al. 1986; Royster and Royster 1990; NIOSH 1995].

Workers should be notified of the noise exposure level determined for their particular job, and the relative risk that such an exposure poses to their hearing. This information should also be cross-referenced to individual worker records. Notification should include a description of the specific hazardous noise sources in the worker's area, the purpose and proper use of any noise control devices, and requirements for hearing protectors, if appropriate. This notification can be incorporated into the worker training program [Royster and Royster 1990; NIOSH 1995].

Noise contour maps should be posted and readily available for the entire facility, so that workers may be aware of the noise levels in other areas. Also, warning signs should be posted on the periphery of noise areas [Royster and Royster 1990; NIOSH 1995]. The warning signs should include a requirement that hearing protectors be worn in the area, and a supply of several types of hearing protectors should be readily accessible. Signs should be printed in English and in the predominant language of the workers who do not read English.

5.4 ENGINEERING AND ADMINISTRATIVE CONTROLS (Component 3)

For occupational hearing loss prevention, NIOSH defines engineering control as "any modification or replacement of equipment, or related physical change at the noise source or along the transmission path (with the exception of hearing protectors) that reduces the noise level at the employee's ear" [NIOSH 1995]. Typical mechanisms for engineering noise controls include reducing noise at the source (installing a muffler), altering the noise path (building an acoustic enclosure or barrier), reducing reverberation (covering walls with sound-absorbing materials), and reducing equipment vibration (installing vibration mounts). Engineering controls should be the first order of protection from excessive noise exposure [Suter 1986; AOMA 1987]. When the noise can be reduced or eliminated through engineering controls, so also is the danger to hearing. Any reduction in noise level - even if only a few decibels - serves to make the noise hazard more manageable, reduces the risk of hearing loss, improves communication, and lowers annoyance and related extra-auditory problems associated with high noise levels [NIOSH 1995]. Furthermore, when the noise can be reduced to acceptable levels through engineering controls, employers may forego some of the additional difficulties and expenses related to providing hearing protectors, education and motivation programs, and program evaluation [Royster and Royster 1990].

To reduce noise in an existing facility, it is generally necessary to retrofit engineering controls. Development of these controls should involve engineers, safety and/or industrial hygiene personnel, and the workers who operate, service, and/or maintain the equipment. Development of special noise control measures must be predicated upon a thorough assessment of the noise source and individual worker exposure. Consideration should be given to the relative contribution of each noise source to the overall sound levels. Various noise control options should be evaluated on the basis of their effectiveness, cost, technical feasibility, and implications for the equipment's use, service, and maintenance. Other potential complications of new noise control measures (such as effects on lighting, heat production, ventilation, and ergonomics) should be considered [NIOSH 1995]. Engineering controls must always take into consideration proper maintenance of equipment. Additionally, the function and purpose of any planned or existing engineering controls should be fully discussed with the workers, so that they support the controls and do not inadvertently interfere with them [NIOSH 1995].

Management should also consider noise reduction in planning for new or remodeled facilities. Engineering controls can be most effective when they are incorporated into the design and purchase of equipment from the start. Additionally, the cost of incorporating engineering controls during the design phase is generally much lower than retrofitting them at a later date. By substituting more sound-absorbent materials, modifying equipment structure or mechanical process, and/or isolating sources within the equipment, substantial reductions can be made in the ultimate noise level [Haag 1988a].

A "buy-quiet" policy for new equipment acquisitions should be adopted by management [Royster and Royster 1990; Brogan and Anderson 1994; NIOSH 1995]. Haag [1988b] describes a four-part process which management can implement in order to have an effective buy-quiet policy. The process includes selection of specific products or operations to be targeted for noise reduction through new purchases, setting criteria for new equipment noise levels, requesting noise level specifications from manufacturers, and including this noise level data in bid evaluation. Again, input from workers should be incorporated in the buying process.

When engineering controls are inadequate, supplemental administrative controls may be utilized to help limit exposures. Administrative controls are defined as changes in the work schedule or operations which reduce worker noise exposures. For example, sometimes workers can be scheduled so that their time in a noisy environment is minimized. When extremely noisy operations are unavoidable, they might be scheduled during the shift with the fewest number of protected employees. So administrative controls are best used for workers for whom hearing protectors alone may not be sufficient because workers still are exposed to hazardous noise (protected level greater than 85 dBA). Finally, a quiet, clean, and conveniently located lunch and break area should be provided to facilitate employees periodically gaining relief from workplace noise.

5.5 AUDIOMETRIC EVALUATION AND MONITORING (Component 4)

Audiometric evaluation of workers' hearing is crucial to the success of a HLPP, since it is the only way to actually determine whether occupational hearing loss is being prevented. Because occupational hearing loss occurs gradually, affected employees often will not notice a change in hearing ability until a relatively large change in their hearing sensitivity has occurred. The annual comparison of audiometric tests can trigger prompt hearing loss program interventions, initiating protective measures and motivating employees to prevent further hearing loss.

5.5.1 Audiometry

Audiometry should be conducted by an audiologist or by an occupational hearing conservationist certified by the Council for Accreditation in Occupational Hearing Conservation (CAOHC). All testing should be supervised by an audiologist, an otologist, or an occupational physician. Occupational hearing conservationists should follow the training guidelines proposed by the National Hearing Conservation Association (NHCA) [1987]. Use of microprocessor-based or self-recording audiometers should not waive the qualification requirements for the tester.

For audiometric testing to be beneficial, management must allocate sufficient time and resources to allow for timely and accurate testing. The testing must be conducted carefully to ensure the integrity of the audiometric data. Effective communication and coordination are critical among company personnel, health service providers, and employees.

Audiometry should minimally consist of pure-tone air-conduction threshold testing of each ear at 500, 1000, 2000, 3000, 4000, 6000, and 8000 Hz. While this entire frequency range is not utilized in the assessment of OSHA=s Astandard threshold shift@, all of these frequencies are important in deciding the probable etiology of a hearing loss. Sufficient time should be taken to conduct the test accurately. Testing too quickly sacrifices accuracy and gives the worker the impression that audiometry and the HLPP are unimportant [NIOSH 1995].

Audiograms are displayed and stored as tables or charts of hearing thresholds measured in each ear at specified test frequencies. In OSHA-mandated hearing conservation programs, thresholds must be measured for pure tone signals at the test frequencies of 500, 1000, 2000, 3000, 4000, and 6000 Hz [29 CFR 1910.95(h)(1)]. At each frequency, the threshold recorded for an ear is the lowest signal output level of the audiometer at which the individual responds in a specified percentage of trials (such as 50%) or in 2 of 3 trials. Thresholds are measured in decibels hearing threshold level (dB HTL) with 0 dB HTL representing average hearing ability for young people without any otological pathology. Larger threshold values indicate poorer-than-average hearing, while smaller threshold values (negative thresholds such as -5 or -10 dB HTL) indicate better-than-average hearing.

An individual's audiometric threshold at a given test frequency is not an invariant quantity. Measurement variability is associated with the state of the subject (including the subject's prior audiometric experience, attention, motivation, the influence of upper respiratory problems, drugs, and other factors) and with the testing equipment and methodology [Morrill 1986]. The higher the measurement variability, the more difficult it is to distinguish actual changes in hearing threshold.

Noise exposure increases hearing thresholds, resulting in threshold shifts toward higher values (poorer hearing). Occasionally, exposure to extremely intense noise may cause an immediate permanent hearing loss known as acoustic trauma. Most often, exposure to less intense noise causes the gradual development of hearing damage over months and years. During each overexposure to noise the ear develops a temporary reduction in sensitivity called temporary threshold shift (TTS). This TTS recovers over a period of hours or days if the ear is allowed to rest in a quieter environment. However, if the exposure is high enough or if exposures are repeated, the TTS may not recover completely, and a permanent threshold shift (PTS) begins to develop.

It is because TTS serves as a precursor of PTS, that NIOSH recommends conducting monitoring audiometry on noise-exposed workers at the end of or late in their daily work shifts. Discovering and taking action to prevent further TTS will result in more thorough worker protection from permanent hearing damage. If the annual monitoring audiometry is performed at the beginning of work shifts or before the work day begins, temporary threshold shifts which might have been present from the previous day=s noise exposure will have resolved - any threshold shifts observed will represent permanent shifts in hearing. This type of audiometric monitoring will serve only to document the development of permanent hearing loss, not to prevent it.

Some reports have indicated that industrial audiometry is too variable to be useful in detecting initial threshold shifts [Hetu 1979; Atherley and Johnston 1981]. Certainly, if testing procedures are too inconsistent, then TTS or PTS may not be distinguishable from measurement variability. The challenge is to select a criterion for significant threshold shift which is stringent enough to detect incipient hearing loss, yet not so stringent as to identify large numbers of workers whose thresholds are simply showing normal variability.

This challenge is compounded by the fact that the incipient PTS may manifest itself with the same order of magnitude as typical audiometric measurement variability - about 10 dB change in hearing thresholds. However, daily TTS could be larger in magnitude than the developing PTS, so testing workers near the end of their work shifts (when TTS may be present) increases the probability of identifying workers who are not adequately protected from noise.

In 1972, a significant threshold shift criterion was initially recommended by NIOSH [NIOSH 1972]. In 1992, the performance of this criterion [NIOSH 1972] against 5 other "significant threshold shift" criteria, was examined by Dr. Julia Royster under contract with NIOSH [Royster 1992]. The following threshold shift criteria were evaluated:

1. OSHA STS (Standard Threshold Shift): a change of 10 dB or more in the average of hearing thresholds at 2000, 3000 and 4000 Hz in either ear

2. AAO-HNS SHIFT: a change of 10 dB or more in the average of hearing thresholds at 500, 1000 and 2000 Hz, or at 3000, 4000 and 6000 Hz in either ear

3. 1972 NIOSH SHIFT: a change of 10 dB or more at 500, 1000, 2000 or 3000 Hz; or 15 dB or more at 4000 or 6000 Hz in either ear

4. 15-dB SHIFT: a change of 15 dB or more at any test frequency from 500 through 6000 Hz in either ear

5. 15-dB TWICE: a shift of 15 dB or more at any test frequency from 500 through 6000 Hz in either ear, which is present in one annual audiogram and is persistent at the same frequency in the same ear on the next audiogram

6. 10-dB AVG. 3-4 kHz: a shift of 10 dB or more in the average of thresholds at 3000 and 4000 Hz in either ear

The study methodology, database characteristics, and results are described in detail in the Royster [1992] report. This study compared each of the above six criteria for threshold shifts by applying each criterion to 15 different industrial hearing conservation databases which were contributed to ANSI S12 Working Group 12 (S12/WG12).

Within each database, analyses were restricted to the first eight audiograms for male workers who had at least eight tests. The numbers of workers included from each database ranged from 39 to 1056. Data were analyzed for a total of 2903 workers.

A significant threshold shift for a worker, according to the three non-averaging, any frequency shift criteria (1972 NIOSH shift, 15-dB shift, and 15-dB twice shift), was considered a true positive if the shift was confirmed by the succeeding audiogram, but only if the worker's next test showed persistence of the shift for at least one of the same frequencies in the same ear. For example, if a worker's test 3 showed a 1972 NIOSH SHIFT at 2000, 4000 and 6000 Hz in the left ear, then the shift would be confirmed as a true positive if test 4 results showed the shift to be persistent in the left ear at one or more of these same frequencies. For the three criteria calculated on frequency averages (i.e., OSHA STS, AAO-HNS SHIFT, and 10-dB AVG. 3-4 kHz), a shift was considered a true positive if the worker's next audiogram showed a change by that same criterion whether or not the confirming shift occurred in the same ear and/or the same frequency range (applicable to AAO-HNS). In other words, the original shift could be counted as confirmed not only by a persistent shift in the same ear at the same frequency average, but also by a new shift in the other ear at any frequency average.

The data for classifying true positives from all 15 databases are presented in Table 5-1. The 15-dB TWICE criterion yields 70.9% true positive tags, while the other criteria all yield between 44.4% and 46.1% true positive tags.

Table 5-1

Number of classifiable first shifts (those occurring in comparisons of tests 2 through 7 back to test 1) across 15 databases (N=2903), and numbers and percents of first shifts classified as true positive, for each of the 6 shift criteria^*

*Adapted from Royster [1992].

None of the shift criteria evaluated are best in every respect. The relative merits of each criterion are tabulated in Table 5-2. An acceptable criterion should be able to promptly identify a worker with any measurable threshold shift at the most noise-sensitive audiometric frequencies, and should tag a reasonably high number of true positives. Relative to the "any one frequency" criteria, those criteria that average thresholds at two or more audiometric frequencies (i.e., OSHA STS, AAO-HNS SHIFT, and 10-dB AVG. 3-4 kHz) yield lower numbers of tags with lower percentages of true positives [Royster 1992].

The 15-dB TWICE criterion requires that a threshold shift persist on two tests before the worker is identified or "tagged" for meeting the criterion of significant threshold shift, resulting in a high percentage of true positives. However, this criterion could be misused if the second test was not administered until a year later.

The 1972 NIOSH SHIFT, which shares with 15-dB TWICE the advantage of not requiring any frequency-averaging, uses such a small amount of shift at 500-3000 Hz (only 10 dB) that it tags many audiograms that reflect normal testing variability. Thus, the 1972 NIOSH SHIFT tags so many workers that it loses its usefulness as a problem identifier. This disadvantage can be partially overcome by increasing the amount of required shift to 15 dB (the 15-dB SHIFT); however, too many workers are still tagged by a one-time 15-dB SHIFT to allow any meaningful follow-up.

TABLE 5-2

Advantages and Disadvantages of Each Criterion for Significant Threshold Shift^*

*Adapted from Royster [1992].

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The ideal significant threshold shift criterion should tag workers with temporary threshold shifts before they develop into permanent hearing impairment. On the basis of the data analysis presented by Royster [1992], NIOSH now recommends a modified 15-dB TWICE criterion requiring that a test be followed by an immediate retest if a 15 dB change in threshold is noted at any frequency. The value of two back-to-back tests was observed by Rink [1989], who reported that performing an immediate retest reduced the proportion of workers meeting the OSHA Standard Threshold Shift criterion by more than 70%. Thus, if an annual audiogram indicates a 15-dB shift or more in either ear at any one of the test frequencies (500, 1000, 2000, 3000, 4000 or 6000 Hz), the worker should be reinstructed, the earphones should be refitted, and a retest should be administered. If the retest shows the same results (i.e., 15-dB shift or more in the same ear and at the same frequency), the 15-dB TWICE criterion for a significant threshold shift has been met and the worker should be rescheduled for a confirmation test within 30 days.

With this recommendation in mind, and to provide maximum protection for employees and maximum documentation for employers, NIOSH advocates that audiograms be performed on the following occasions:

1. Pre-employment (baseline).

2. Prior to initial assignment into a hearing hazard work area.

3. Annually as long as the employee is assigned to a noisy job with an exposure level equal to or greater than 85 dBA (monitoring audiometry). Annual testing may lead to a number of retests if a significant threshold shift occurs. Additionally, it may be a good practice to provide audiometry twice a year to workers exposed to more than 100 dBA, because the most susceptible ten percent of a population who are exposed to daily average noise levels of 100 dBA with inadequate hearing protectors could be expected to develop significant hearing loss well before the end of one year [NIOSH 1995].

4. At the time of reassignment out of a hearing hazard job.

5. At the termination of employment.

Baseline audiogram: The baseline audiogram should be obtained within 30 days of enrollment in the HLPP [NIOSH 1972]. It should be preceded by a minimum of 14 hours of unprotected quiet. Use of hearing protectors should not be considered as a substitute for an actual 14 hour quiet period. Use of a mobile testing service should not waive these requirements. It is unacceptable to wait up to a year, as permitted by OSHA [29 CFR 1910.95], for a mobile service to conduct a baseline audiogram because permanent hearing loss can occur within relatively short periods of time (months or even days in susceptible individuals), especially when high levels of noise are involved [ISO 1990]. If a mobile service cannot meet these time constraints, other arrangements should be made to obtain the baseline audiograms before or promptly after employment.

Monitoring audiograms: Monitoring audiometry should be conducted no less than annually. Unlike baseline audiometry, these annual tests should be scheduled at the end of, or well into, the work shift so that temporary changes in hearing due to insufficient noise controls and/or inadequate use of hearing protector will be noted. The results should be compared immediately to the baseline audiogram to check for any change in hearing sensitivity. The collection of audiograms for batch comparison to baseline audiograms at a later date in another location is an unacceptable practice, because it does not afford the opportunity to conduct retests or to discuss the findings with employees in a timely manner.

Retest audiograms: Audiometry should be repeated immediately after any monitoring audiogram which indicates a threshold shift of 15 dB or more at 500, 1000, 2000, 3000, 4000 or 6000 Hz in either ear. The worker must be re-instructed and the headphones refitted before conducting the retest.

Confirmation audiograms: Audiometry should be conducted again within 30 days of any retest audiogram which continues to show a significant threshold shift. A minimum of 14 hours of quiet should precede the confirmation audiogram in order to determine whether the shift is temporary (TTS) or indicative of a permanent change in hearing sensitivity (PTS). The use of hearing protectors as a substitute for an actual quiet environment is not acceptable. Confirmation audiograms indicating persistent threshold shifts should trigger written notification to the worker and a referral to the audiometric manager for review and determination of probable etiology. In this review, all possible etiologies in addition to occupational noise should be explored, including age-related hearing loss, familial hearing loss, medical history, non-occupational noise exposure, etc. [Franks et al. 1989; Stepkin 1993]. Workers showing a threshold shift determined by the audiometric manager to have any etiology other than noise should be counseled by the manager and referred to their physician for evaluation and treatment. Workers should also be referred if they meet any of the otologic or medical criteria recommended by the American Academy of Otolaryngology-Head and Neck Surgery [AAO-HNS 1983]. Appropriate action should be triggered for workers showing a threshold shift determined by the audiometric manager to have a probable etiology of occupational noise exposure. Actions should minimally include re-instruction and refitting of hearing protectors, additional training in worker responsibilities for effective hearing loss prevention, and/or reassignment to quieter work areas. The audiometric manager should be responsible for making whatever recommendations he/she feels are necessary, and for seeing that they are carried out.

Exit audiogram: Audiometry should be conducted when a worker leaves employment or is permanently rotated out of an occupational noise exposure at or above 82 dBA, 8-hour TWA. This exit audiogram, like the baseline, should be performed after a minimum of 14 hours of quiet. The use of hearing protectors as a substitute for quiet is not acceptable.

It is also suggested that hearing testing be offered as a health benefit to workers who are not exposed to hazardous noise levels. In addition to providing a valuable internal control group for comparison to the noise-exposed workers, this policy elevates the perceived importance of the HLPP for management and workers [NIOSH 1995].

5.5.2 Audiometers

Audiometers should minimally conform to the specifications of the appropriate ANSI standard [ANSI S3.6-1995] for Type 4 audiometers, with the additional stipulation that they have the capacity for testing at 8000 Hz. Type 5 audiometers, which only test to 70 dB HTL, are unacceptable for threshold testing within an occupational HLPP.

Audiometers must be kept in calibration for the audiograms to have any value. An audiometer must receive a functional check (sometimes called a biologic check) each day the instrument is used [Morrill 1986; NIOSH 1995]. This type of calibration check involves obtaining an audiogram from an individual with known, stable thresholds and verifying that there have been no changes in HTL that exceed 10 dB. A bioacoustic simulator check may be substituted for this procedure. Additionally, the audiometer attenuator and frequency selection dials should be cycled through while carefully listening for any extraneous noise or distortion which might interfere with testing. The earphone cords should be manipulated to check for any unwanted static or noise. A check for "crosstalk" (presence of the test signal in the non-test earphone) should be made, using an attenuator setting of 60 dB HTL [Morrill 1986].

An acoustic calibration check should be performed whenever the functional check indicates a threshold difference exceeding 10 dB in either earphone at any frequency. An acoustic calibration includes checks of output levels, attenuator linearity, and frequency. If the sound pressure levels differ by more than the allowable variances specified by ANSI 3.6-1995 (or its successor), or if the attenuator linearity differs by more than 1 dB, or if frequency drift exceeds 3%, an exhaustive calibration is necessary [Morrill 1986].

An exhaustive calibration check should be conducted annually or whenever an acoustic calibration indicates the need for such. An exhaustive calibration includes adjusting the audiometer so that it is in compliance with all specifications of ANSI S3.6-1995 (or its successor) and must be done by an audiometer service technician. It is best to have exhaustive calibrations performed on-site. If the audiometer must be shipped out for this service, then an acoustic calibration should be conducted upon its return to ensure that calibration changes did not occur during shipping [Morrill 1986].

The audiometric test area should conform to the ambient noise requirements of ANSI S3.1-1991 [ANSI 1991b]. For permanent, on-site test areas, ambient noise levels should be checked at least annually. For mobile test areas, ambient noise levels should be checked daily or at each new site, whichever is more frequent. Ambient noise levels should be checked with a calibrated sound level meter placed in the test environment at the approximate position the worker's head will occupy during the test procedure. Some bio-acoustic simulators have the capability of measuring ambient noise levels; this is acceptable provided that the unit is placed near the area of the worker's head. All audiometric test equipment, as well as lights, heaters, air conditioners, etc., should be set as they would be during actual testing. The ambient noise levels should also be measured during audiometric testing, and they should be recorded in a log through which they can be traced for each audiogram obtained.

5.6 USE OF HEARING PROTECTORS (Component 5)

NIOSH [1995] defines a hearing protector as "anything that can be worn to reduce the level of sound entering the ear." Hearing protectors are discussed more fully in Chapter 6, but a few brief points should be made here. Hearing protectors are subject to many problems, and should be considered the last resort against hazardous noise. Berger [1980] identified seven reasons why hearing protectors can fail to provide adequate protection in real-world situations: discomfort, poor utilization, poor fit, incompatibility with other safety equipment, dislodging, deterioration, and abuse. Additionally, hearing protectors generally provide greatest protection at high frequencies and significantly less protection from low frequency noise [Berger 1986]. Nevertheless, hearing protectors can work as a short term solution to prevent noise-induced hearing loss if their use is carefully planned, evaluated, and supervised [Berger 1986; Royster and Royster 1990; NIOSH 1995; Franks and Berger 1997].

5.7 EDUCATION AND MOTIVATION (Component 6)

On November 21, 1983, OSHA promulgated an occupational safety and health standard entitled "Hazard Communication" [29 CFR 1910.1200]. Under the provisions of this standard, employers in the manufacturing sector must establish a comprehensive hazard communication program that includes, at a minimum, container labeling, material safety data sheets, and a worker training program. The hazard communication program is to be written and made available to workers and their designated representatives. Although the Hazard Communication standard does not specifically address occupational noise exposure, the intent of the standard to inform workers of health hazards should apply. Annual training should be provided to employees exposed to noise levels of 82 dBA TWA or above. Workers must be informed of the possible consequences of noise exposure and of the various control methods available to protect their hearing. When a HLPP is implemented, workers should be informed of the provisions of the program and of the benefits of their full participation in the program.

The success of a HLPP depends in large measure on effective worker education regarding all aspects of the program. In his review of the hearing conservation literature, Berger [1981] suggests that there are several keys to a successful program: support from management, enforcement of safety policies, education and motivation of the workers, and comfortable and effective hearing protectors. All of these issues depend to some degree on a well-constructed, thorough program of educating and training everyone who is involved in the HLPP.

Obviously, the primary focus of the training component of the HLPP is on the workers. Workers need to be informed about the reasons for and the requirements of the HLPP at the time that they are enrolled in it. The education process should be ongoing, and highlighted by periodic programs focusing on one or more particular aspects of the program. Furthermore, to be optimally effective, education should be tailored to the specific exposure and prevention needs of each worker or group of workers, rather than generic. Education and training will be easily dismissed unless it can be related to each worker's day-to-day functions [Berger 1981]. Worker education should cover all relevant aspects of the hearing conservation program. At a minimum, the following topics should be included [AOMA 1987; Royster and Royster 1990; NIOSH 1995]:

Requirements of and rationale for the occupational noise standard.

Effects of noise on hearing. This should cover both the audiometric effects (i.e., how noise effects show up on an audiogram) and the functional effects (i.e., the impact of noise-induced hearing loss on everyday life).

Company policy for the elimination of noise as a hazard, including noise controls already implemented or planned for the future. This is very important and helps ensure that workers do not accidentally interfere with control measures.

Specific hazardous noise sources at the worksite. The discussion should include monitoring procedures, noise maps of the work environment, and use of warning signs as they apply at the specific site for the workers receiving training.

Training in the use of hearing protectors. This should include the purpose of hearing protectors; the types of protectors available and the advantages and disadvantages of each; selection, fitting, use, and care of hearing protectors; and methods for solving common problems associated with hearing protector usage. This training must include supervised, hands-on practice in the proper fitting of hearing protectors.

Audiometry. Instruction should include a discussion of audiometry's role in preventing hearing loss, a description of the actual test procedure, and interpretation and implications of test results. It should be stressed that temporary or permanent threshold shifts in HTL indicate failure of the HLPP. Workers and managers need to know that often, threshold shifts may be traced to inadequate protection resulting from ineffective noise controls and inconsistent use of hearing protectors.

Individual responsibilities for preventing hearing loss. A discussion of common non-occupational noise sources and suggested ways of controlling these exposures will further increase the effectiveness of an occupational HLPP [Royster and Royster 1990]. Additionally, behavioral research has suggested that it is important to encourage worker's feelings of self-efficacy, control, and personal responsibility for health and safety behavior [Schwarzer, 1992].

Despite the emphasis on employee training, management also needs to be educated about the need for and elements of the HLPP. Strong management support is critical to an effective HLPP [AOMA 1987]. This support must be more than just implicit approval of company hearing loss prevention policies; it must be an outward, active show of approval and compliance with the established policies. It must be clearly evident to lower management, foremen, and the workers. Management needs to know the basics of the legal and professional requirements for effective hearing loss prevention, as well as the administrative requirements for compliance and the liability consequences of non-compliance. Motivation of upper management may be heightened by emphasizing the possible financial benefits of an effective HLPP on worker's compensation costs, improved productivity, and worker retention [Royster and Royster 1990].

In addition to the workers and managers, members of the hearing loss prevention team need to be educated regarding company policy for the program and their role within it. They must receive appropriate training to enable them to successfully fulfill their duties. This is especially important for those who will be responsible for fitting hearing protectors and training workers in their proper use [Royster and Royster 1990]. If there is a hierarchy of responsibility within the program's team, each member should know his/her place in it. Consultants, including physicians or audiologists who conduct follow-up examinations, should also be well-informed about the company's hearing loss prevention policies. This will help prevent recommendations or decisions which might conflict with established company policy [Royster and Royster 1986].

Choice of educational and motivational strategies is critical to the success of the training phase of the HLPP. The techniques used as well as the content selected for presentation must be tailored to the particular needs of the audience [Royster and Royster 1990].

For all groups involved, an effective training program requires that there be both episodic and ongoing educational opportunities. The most useful opportunity for episodic training of the workers occurs at the time of each worker's annual monitoring audiogram. During this time, the worker is most interested in his/her hearing status, and recommendations will have the most relevance. Time should be taken immediately after testing to explain the results of the hearing test, its relationship to the worker's baseline audiogram, and its implications for the adequacy of the worker's hearing protector usage. Stable hearing should be praised in order to reinforce the worker's proper use of noise controls and hearing protectors, and hearing shifts should result in a sincere warning about the need for more consistent use of appropriate hearing protectors. The worker must be given the opportunity to ask questions about his/her role in the HLPP, and should be encouraged to discuss hearing protector difficulties, etc. [Royster and Royster 1986].

Other opportunities for episodic training also exist. Information about specific company policies, results of bi-annual noise exposure monitoring, overviews of the effect of noise on hearing, and related topics should be periodically addressed in special training sessions or regularly planned safety meetings. These training sessions should not be limited to showing a film, but should be personally presented by an educator who is knowledgeable about hearing conservation and has an interesting presentation style. Group size should be small enough to permit interaction with the speaker and among the workers. Content should be varied and continually updated [Royster and Royster 1986; NIOSH 1995].

In addition to these episodic training sessions, there should be an ongoing educational process. HLPP personnel, especially the program implementor, should take the time to go out to the workers' job sites and see how they are doing. They should talk to workers about the program when they meet them in the halls, at lunch, etc. Posters, bulletin boards, informational pamphlets, etc. can be used as a constant reminder of the importance the company places on hearing conservation. Contests or awards for effective hearing conservation practices can be used to promote safety behavior [Royster and Royster 1986; Royster and Royster 1990]; however, incentive programs should be planned and implemented with full worker participation or they may be perceived by the workers as manipulative attempts by management to control workers' behavior [Merry 1995].

5.8 RECORDKEEPING (Component 7)

Recordkeeping involves creating and maintaining documents on each aspect of the HLPP. This documentation is more than just an exercise in paperwork or computer data entry. It provides the only compelling evidence that the HLPP components were properly, consistently, and thoroughly conducted. Program records are often needed many years after they are collected; and if it cannot be established that they are valid, then the records are useless. Clearly, documentation needs to be viewed as one of the most critical aspects of a HLPP [Gasaway 1985].

HLPP records are medical records and should be treated with the same degree of integrity and confidentiality. The recordkeeping system should be compatible with the company's general health and safety record system. The company should keep copies of all records, even if a contractor is used to collect the data [NIOSH 1995]. Additionally, each worker's noise exposure records, audiometric records, hearing protection records, and training participation records should be cross-referenced so that information regarding one program component can be readily linked with information regarding all other program components for that worker. Such cross-referencing is critical to building a total hearing history and establishing the probable etiology of any hearing loss should a claim ever be filed [Gasaway 1985; NIOSH 1995].

5.8.1 Noise Exposure Records

Noise exposure records should include the worker's name, identification number, job code, job description, department, and similar related information such as the current noise exposure level, the date of the last exposure assessment, the monitoring method used, and the name of the person who did the monitoring [NIOSH 1995]. The employee's record should also include the previous noise exposure history. It is useful to include both calculated exposure levels and the raw data from which the calculations were made [Royster et al. 1986].

Noise exposure records should be kept for a minimum of 30 years. This is the length of time OSHA requires employers to keep other industrial hygiene records [29 CFR 1910.20]. However, it may be prudent to keep noise exposure records even longer. Royster et al. [1986] recommend that exposure records be kept for the length of employment plus thirty years. Employers might also consult the Bureau of Workers' Compensation for their state. Most states have a statute of limitations for filing a claim for occupational hearing loss; however, some states do not [ASHA 1992]. Prudence dictates a check with state regulations to be certain that records are kept until it is determined that there will be no further use for them [Royster et al. 1986].

5.8.2 Audiometric Records

Audiometric records should include the worker's name, identification number, gender, date of birth, and a self-reported worker history. The history should include medical information which may have an impact on hearing status, history of past occupational or military noise exposure, and types of non-occupational noise exposure [Helmkamp 1984; NIOSH 1995]. Occupational exposure to potentially ototoxic chemicals should also be recorded [Rybak 1992]. Morrill recommends a brief "high risk" history, which can be readily taken by a technician; this history can then be used as a framework for a more detailed history, as necessary, if the worker is ever referred to an audiologist or physician for further evaluation [Morrill 1986]. The more detailed the history, of course, the more accurately the audiometric manager will be able to determine the actual etiology of any threshold shifts.

For each audiometric examination, the test date, time, and hours since the worker's last noise exposure should be recorded. Audiometric thresholds at all required frequencies should be recorded. The audiometer's make, model, and serial number should be noted; as well as the dates of the last exhaustive calibration, the last acoustic calibration, the last functional check, and the last check of room ambient noise levels. Additionally, the identity of the tester, and the tester's subjective assessment of test reliability should be recorded [NIOSH 1995].

Any time that a significant threshold shift is documented, the etiology determined by the audiometric manager should be recorded. Also, all follow-up actions that were taken should be documented [Gasaway 1985].

Audiometric test results and etiologies of any confirmed shifts should be kept for the length of employment plus thirty years, which is the OSHA requirement for worker health records [29 CFR 1910.20]. Other related records - calibration records, ambient noise level checks, etc. - should be minimally kept for five years. However, remembering that audiometric records are only as valid as documentation indicates, it may be prudent to keep all supporting records for as long as the thresholds themselves are maintained [Gasaway 1985].

5.8.3 Hearing Protection Records

Hearing protection records should include the types of hearing protectors used, including make, model, and size as relevant. Records should also be kept to document training the workers have received in the proper fitting and use of protectors, and any records that may have been made documenting the consistency of compliance with requirements for wearing hearing protectors [NIOSH 1995]. Hearing protection records should be kept for a minimum of 30 years.

5.8.4 Education Records

Education records should include date and type of training provided, who conducted the training, and attendance (if training was a group program) [NIOSH 1995]. Education and training records should also be kept for a minimum of thirty years.

5.8.5 Other Records

Other necessary records might include documentation of periodic audits, exposure assessments, plans for engineering and administrative controls, and results of overall program evaluations [NIOSH 1995]. These records, and any other documentation relevant to the HLPP, should be kept a minimum of thirty years.

5.9 EVALUATION OF PROGRAM EFFECTIVENESS (Component 8)

The effectiveness of a HLPP should be evaluated in terms of the hearing losses prevented for each worker and the overall rate of hearing loss in the population of workers. This evaluation should occur on a continual process.

5.9.1 Individual Effectiveness

The effectiveness of the HLPP in preserving workers' hearing is best evaluated through audiometric monitoring of each noise exposed worker. All workers whose noise exposure meets or exceeds 82 dBA should receive audiometric testing at no cost to the worker at the intervals noted previously under audiometric evaluation. Comparison of a current audiogram with the baseline audiogram will permit the audiometric manager to assess the adequacy of the program elements for that particular worker. Thus, each audiogram serves as a marker of the effectiveness of the hearing loss prevention effort for that individual worker. Any apparent changes in hearing signal a possible failure in the program.

5.9.2 Overall Program effectiveness

To assess the effectiveness of the HLPP from an overall programmatic level, it is necessary to have an evaluation method that can monitor trends in the population of workers enrolled in the program and thus indicate program effectiveness before many individual shifts occur. This evaluation has two parts. The first part evaluates the internal integrity of the audiometric data. Currently, there is a draft ANSI standard which details a method for such an evaluation - Draft ANSI S12.13-1991, "Evaluating the Effectiveness of Hearing Conservation Programs" [ANSI 1991c]. This standard is based on an assumption that year-to-year variability in a population's hearing thresholds reflects the adequacy of the audiometric monitoring program. High variability in sequential thresholds is viewed as indicative of inadequate control of audiometric test procedures, audiometric calibration problems, or poor recordkeeping. Low variability in sequential thresholds is viewed as indicative of a well-controlled program producing results which may be relied upon for accuracy and reliability.

The second part of the program evaluation involves comparing the rate of threshold shift among noise-exposed workers to that of persons not exposed to occupational noise. Toward this end, Melnick [1984] evaluated the efficacy of several different methods. The first method was based on the OSHA estimation that a noise exposed population that was in compliance with the current noise regulations would still demonstrate a prevalence of hearing loss (defined as thresholds exceeding 25 dB at the frequencies of 500, 1000, and 2000 Hz) up to 10% greater than a non-noise exposed population by the time workers reached retirement (later OSHA calculations have revised this estimate to be 10-15%). This method has the obvious disadvantage of delaying evaluation of the HLPP until a number of workers have reached retirement age, and by then, improvements to the HLPP will be too late to prevent their loss of hearing.

Another method involves evaluating the effectiveness of the overall program on the basis of the percentage of workers showing significant threshold shifts. Ideally, the criterion percentage of significant threshold shifts could be based on a control group (i.e., non-noise-exposed) within the same company. Others who have investigated the possibility of percent significant threshold shift as an evaluation criterion have reported that 3-6% [Morrill and Sterrett 1981], or 5% significant threshold shifts [Franks et al. 1989; Simpson et al. 1994] are reasonable incidence rates which can be met by effective programs. Significant threshold shift incidence rates exceeding these percentages might then be considered evidence of a deficient program. A disadvantage of this technique is that it does not account for the effects of other variables (e.g., age, gender, race, previous noise exposure history) that might differentially affect the significant threshold shift incidence rates if the noise and non-noise populations differ substantially along these variables. Another disadvantage is that this technique does not differentiate possible causes of program deficiencies. Problems could be as likely due to poor audiometry as to excessive noise exposure [Melnick 1984; Simpson et al. 1994].

An alternative evaluation method, used by Pell [1972] in evaluating the effectiveness of the hearing conservation program at DuPont, involves a longitudinal analysis of the rate of increased hearing loss (10th, 50th, and 90th percentiles) as a function of age for three classes of worker noise exposure: quiet (<85 dBA), low noise (85-94 dBA), and high noise (95 dBA or greater). Pell judged his hearing conservation program effective by demonstrating that the rate of increase of hearing loss with respect to age did not significantly differ among the three noise categories. This system, however, requires that all workers receive annual audiometric evaluations, regardless of whether or not they are noise exposed. Also, because some individuals are susceptible to hearing loss at the action level of 85 dBA, it would be preferable to define the "quiet" group as those exposed to less than 80 dBA.

The U.S. Army Center for Health Promotion and Preventive Medicine (CHPPM) (formerly the U.S. Army Environmental Hygiene Agency) evaluates its HLPPs by rating each element and sub-element of the program on a 5 point scale ranging from maximally compliant to non-compliant. Total points are added across the sub-elements to achieve a score for that program element; and then a total score is computed for the overall program. There are well-defined criteria for scoring the sub-elements, but the program evaluator is also given some flexibility in assigning ratings. Such a system is helpful in that it defines strict criteria for every aspect of the program which must be met in order to have a fully successful program. However, some of the currently-used criteria are not perfect, as CHPPM has found several highly rated HLPPs to have unacceptably high incidences of significant threshold shifts [Byrne and Monk 1993].

In general, NIOSH suggests that the success of a smaller HLPP should be judged by the audiometric results of individual workers. An overall program evaluation becomes critical when the number of workers grows so large that one cannot simply look at each worker's results and get an adequate picture of the program's efficacy. At the present time, there is not one generally accepted method for the overall evaluation of HLPPs. Furthermore, there is not one method which stands out as being superior to the rest. Therefore, at this time, NIOSH recommends considering a significant threshold shift incidence rate of 5% or less as evidence of an effective HLPP [Morrill and Sterrett 1981; Franks et al. 1989; Simpson et al. 1994]. The 5% criterion method is currently the simplest procedure available, and has no more disadvantages than other potential evaluation methods.

5.10 AGE CORRECTION

NIOSH does not recommend that age correction be applied to an individual's audiogram for significant threshold shift calculations. Although many people experience some decrease in hearing sensitivity with age, age correction cannot be accurately applied to audiograms in determining an individual's significant threshold shift because the data on age-related hearing losses describe only the statistical distributions in populations. Thus, the median hearing loss attributable to presbycusis for a given age group will not be generalizable to the presbycusis experienced by an individual in that age group. Furthermore, the age-correction tables developed in the 1972 criteria document [NIOSH 1972], and subsequently included in the 1983 OSHA Hearing Conservation Amendment to the Occupational Noise Standard [48 Fed. Reg. 9,738 (1983)], were based on a cross-sectional study. Thus, the age corrections were estimated by calculating trends as a function of the age of each member of the sample. When data from a cross-sectional study are used, the inherent assumption is that a subject who was 20-years-old in 1970 can be expected to experience the same hearing loss due to age by 2000 as a 50 year-old subject had experienced due to age in 1970. This assumption may not be valid as the general health and societal noise exposures of each generation are likely to be different.

The adjustment of audiometric thresholds for aging has become a common practice in workers' compensation litigation. In this application, age corrections result in a reduction in the amount of hearing loss which is considered attributable to noise exposure, with a consequent reduction in the amount of compensation paid to workers for their hearing losses. However common and regardless of the extent to which "age correcting" has been and is applied, it is technically inappropriate to apply population statistics to an individual in this manner. Each age-correction number is nothing more than a median value from a population distribution. In age correcting an audiogram, the underlying assumption is that the individual value is given the 50th percentile, when in fact a different value such as the 10th percentile or the 90th percentile may be correct. Thus, one cannot apply age correction formulas to determine with certainty how much of an individual's hearing loss is due to age and how much of it is due to noise exposure.

It is even less appropriate to "age correct" audiograms obtained as part of an occupational HLPP. The purpose of the program is to prevent hearing loss. If an audiogram is "age corrected", regardless of the source of the correction values, the time it takes for a significant threshold shift to be noted will be prolonged. Delaying the tagging of a worker with a significant threshold shift is completely contrary to the purpose of a HLPP.

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The hearing protector is a device which covers or fills the ears so that the sound reaching the ear drum is attenuated. How much attenuation a hearing protector provides is dependent upon the characteristics of the hearing protector and how the worker wears it. Thus, the selected hearing protector should be capable of reducing the noise exposure at the ear to below 82 dBA, 8-hour TWA.

Hearing protectors should not be the sole or primary means by which worker noise exposures are reduced. Hearing protectors should be used only when engineering controls and work practices are not feasible for reducing noise exposures, or during the implementation of engineering controls. Hearing protectors are a temporary solution to the problem of preventing NIHL. The permanent solution is to remove the hazardous noise from the workplace or to remove the worker from the hazardous noise.

In general, hearing protectors should be used by workers who are exposed to noise exceeding 85 dBA, regardless of duration, and should be an integral part of a HLPP. The hearing protector is a form of noise control although it is the least preferable of the noise control methods. It alters the sound path by attenuating the noise at the ear of the worker. It should be recognized that the effectiveness of hearing protectors may vary widely from worker to worker. As such, each worker wearing hearing protectors needs to be monitored to see that the hearing protectors are being worn correctly, are fitted properly and are appropriate for the noise in which they are worn [Helmkamp et al. 1984; Gasaway 1985; Berger 1986; Royster and Royster 1990; NIOSH 1995].

Hearing protectors should be selected for use in the HLPP with considerations made for the noises in which they will be worn, the workers who will be wearing them, the need for compatibility with other safety equipment, and work environmental conditions such as temperature, humidity, and atmospheric pressure [Gasaway 1985; Berger 1986].

Hearing protectors should have adequate attenuation to provide protection from the noise to which workers will be exposed using the methods recommended later in this chapter. Hearing protectors should reduce the noise level at the ear to below 82 dBA, 8-hour TWA, thus reducing the likelihood of occupational NIHL [Gasaway 1985; Berger 1986; NIOSH 1995].

A variety of styles should be provided so that workers may select the hearing protector on the basis of comfort, ease of use, ease of handling, and impact on communication [NIOSH 1995; Royster and Royster 1990]. The best hearing protector is the one the worker will wear all of the time because it is comfortable, effective, and has minimal impact on communication. What is the best hearing protector for some workers may not be the best for others [Casali 1990]. The most common reasons why workers won't use hearing protectors correctly are discomfort and interference with speech communication [Berger 1980; Helmkamp 1986].

Hearing protectors should not be relied upon when the noise exposure level is such that noise under the protector cannot be estimated to be less than 82 dBA, 8-hour TWA. If the condition exists that use of hearing protectors cannot reduce noise adequately, then workers should not be sent into that type of environment. The use of double protection may also be considered [Gasaway 1985; Berger 1986].

Each worker should receive individual training in the selection, fitting, use, repair and replacement of the hearing protector [Gasaway 1985; Royster and Royster 1990; NIOSH 1995]. Workers should not be allowed to provide their own hearing protectors. Devices such as hearing aid earmolds, swim molds, and personal stereo earphones should never be considered as being hearing protective.

One consideration for selecting a hearing protector would be its noise reduction capabilities, which are expressed in terms of a noise reduction rating (NRR). The NRR is a single-number rating which is required by law to be shown on the label of each hearing protector sold in the United States [40 CFR 211].

The NRR is intended to be used to calculate the exposure under the hearing protector by subtracting it from the environmental noise exposure level. OSHA has prescribed six methods⁴ with which the NRR can be used [29 CFR 1910.95, Appendix B]. These methods vary according to the instrumentation used to determine the noise exposure levels, and can be summarized in two basic formulas:

⁴The OSHA methods are a simplication of "NIOSH Method #2" [NIOSH 1975; Lempert 1984; NIOSH 1994].

When dBA level is known:

1. When using a sound level meter set to the A-weighting network: Obtain a representative sample of the A-weighted exposures to calculate the employee's TWA. Subtract 7 dB from the NRR and subtract the remainder from the A-weighted TWA to obtain the estimated A-weighted TWA under the hearing protector.
   
2. When using area monitoring procedures and a sound level meter set to the A-weighting network: Obtain the representative sound level for the area in question. Subtract 7 dB from the NRR and subtract the remainder from the A-weighted sound level for that area.
   
3. When using a dosimeter that is not capable of C-weighted measures: Convert the A-weighted dose to an 8-hour TWA. Subtract 7 dB from the NRR. Subtract the remainder from the A-weighted TWA to obtain the estimated A-weighted TWA under the hearing protector.

When dBC level is known:

1. When using a dosimeter that is capable of C-weighted measurements: Obtain the employee's C-weighted dose for the entire work shift, and convert to an 8-hour TWA. Subtract the NRR from the C-weighted TWA to obtain the estimated A-weighted TWA under the hearing protector.
   
2. When using a sound level meter set to the C-weighting network: Obtain a representative sample of the C-weighted sound levels in the employee's environment. Subtract the NRR from the C-weighted average sound level to obtain the estimated A-weighted TWA.
   
3. When using area monitoring procedures and a sound level meter set to the C-weighting network: Obtain a representative sound level for the area in question. Subtract the NRR from the C-weighted sound level for that area.
   

NRRs and other hearing protector ratings are based on data from listeners in psychophysical experimental conditions. As such, there are many variables which may impact upon the ratings, making the predicted noise reduction different from what the worker would actually experience.

Use of statistical descriptors to select protectors for one employee: In an effort to account for the variance in attenuation for a given protector from person to person, the decision has been made to use the mean attenuation with a correction for variance (some constant times the standard deviation of the mean). In the case of the NRR, the mean attenuation adjusted by two standard deviations is used. However, there is no practical way of knowing how much attenuation a worker may experience with the protector or how his or her experience will vary from day to day and from fitting to fitting.

• Rating creep - the battle for higher rates and higher sales: A single number rating for a hearing protector (be it a rating of noise reduction or a rating of the protector itself) leads consumers and manufacturers to always strive for the higher number. The result is that manufacturers strive to make protectors with high ratings and that consumers tend to purchase protectors with high ratings whether a highly rated protector is necessary for the noise or not.
• Over protection: The most common reasons workers do not use hearing protectors correctly are discomfort and impaired speech communication [Berger 1980; Helmkamp 1986]. Both of these are related to hearing protectors which over protect. As long as the hearing protectors are selected based on the highest rating, they will provide too much noise reduction for some employees and defeat their efforts to speak with fellow employees. The most natural response for the worker is to remove or misuse the protector.

In the late 1970's and early 1980's, two NIOSH field studies found that insert-type hearing protectors in the field provided less than one-half the attenuation measured in the laboratory [Edwards et al. 1978; Lempert and Edwards 1983]. Since the 1970's, additional studies of "real-world" attenuation with hearing protectors have been conducted [Regan 1975; Padilla 1976; Abel et al. 1978; Fleming 1980; Crawford and Nozza 1981; Chung et al. 1983; Edwards et al. 1983; Hachey and Roberts 1983; Goff and Blank 1984; Behar 1985; Mendez et al. 1986; Smoorenburg et al. 1986; Edwards and Green 1987; Pekkarinen 1987; Pfeiffer et al. 1989; Hempstock and Hill 1990; Berger and Kieper 1991; Casali and Parks 1991]. In general, these studies involved testing the hearing thresholds of occluded and unoccluded ears of subjects who wore the hearing protectors for the test in the same manner as on the job. The tests were performed as an attempt to simulate the actual conditions in which hearing protectors are normally used in the workplaces. In Table 6-1, the NRRs derived from these real-world attenuation data were compared with the manufacturers' labelled or laboratory NRRs. The laboratory NRRs consistently overestimated the real-world NRRs by 140% to 2000%. In general, the data show that earmuffs provide the highest real-world attenuation values, followed by foam earplugs, and all other insert-type devices provide the least attenuation. From these results, it can also be concluded that ideally workers should be fit tested for hearing protectors individually. Currently, NIOSH is developing feasible methods for this type of fit testing.

OSHA [1989] has instructed its compliance officers to derate the NRR by 50% in enforcing the engineering control provision of the OSHA noise standard. However, because of the wide variation of real-world NRRs among different types of hearing protectors, and as an interim measure, NIOSH recommends that the labelled NRRs be derated by 25%, 50% and 70% for earmuffs, formable earplugs and all other earplugs, respectively. These percentages are estimates based on Table 6-1.

TABLE 6-1

Summary of Real-World NRRs Achieved by 84% of the Wearers (NRR84) of Hearing Protectors (HPs) in 20 Independent Studies^*

*Adapted from Berger et al. [1996]

Abbreviations:
N = size of test population
NRR = labelled Noise Reduction Rating
Wght = weighted on the basis of test population size

The best hearing protection for any worker is removal of hazardous noise from the workplace. Until that happens, the best hearing protector for a worker is the one he or she will wear willingly and consistently. The following factors are extremely important determinants of the workers' acceptance of hearing protectors: comfort; adequate noise reduction; ease of fit; communication enhancement; durability and repairability; and proportion of time worn.

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The following research is needed to further reduce the risk of NIHL in the workplace.

7.1 NOISE CONTROL

Research is needed to reduce noise exposures through engineering controls in situations where the noise exposures are still being controlled primarily by hearing protectors. A hearing loss prevention program is complex and difficult to manage effectively, the need for which can be obviated by noise control procedures that reduce noise levels to less than 82 dBA. As important as applying new technologies such as active noise reduction are, it is equally important to apply traditional noise control engineering concepts to the building of new facilities and equipment. Research also is needed to improve retrofitting noise control to existing operations. A database of effective solutions should be created and made publicly accessible.

7.2 IMPULSIVE NOISE

Research is needed to define the hazardous parameters of impulsive noise and their interrelationships. These parameters should include amplitude, duration, rise time, number of impulses, repetition rate and crest factor. In the absence of any other option, impulsive noise is integrated with continuous noise to determine hazard. While laboratory research with animals and some retrospective studies of workers exposed to impulsive noise in the presence or absence of other noises indicate that impulsive noise is more hazardous to hearing than continuous noise of the same spectrum and intensity, there are not sufficient data to support the development of damage risk criteria for impulsive noises.

7.3 NONAUDITORY EFFECTS

Research is needed to define dose―response relationships between noise and nonauditory effects, such as hypertension and psychological stress. Studies of hypertension conducted to date on noise exposed workers have established a relationship between hypertension and NIHL, but have not established a relationship between noise exposure and hypertension. Workplace accidents need to be analyzed to determine whether noise interference with oral communication or audio alarms was a contributing factor. Technologies need to be developed that allow easy identification of warning signals and efficient communication in noisy environments but that also provide effective protection of hearing.

7.4 AUDITORY EFFECTS OF OTOTOXIC CHEMICAL EXPOSURES

The ototoxic properties of industrial chemicals and the interaction between them and noise have been investigated only for a few substances. Research with laboratory animals to investigate the range of chemicals known to be ototoxic or neurotoxic is needed to appraise the risk of hearing loss from exposures to these chemicals, alone or in combination with noise. Research to support damage risk criteria for combined exposure is needed.

7.5 EXPOSURE MONITORING

NIOSH was a pioneer in developing an exposure monitoring strategy for air contaminants based on the application of statistical methods [NIOSH 1977]. However, the appropriateness of the strategy for occupational noise exposure was not determined, and not much research has been done on this question since 1977. Limited studies have indicated that a different strategy for monitoring occupational noise exposure may be required [Behar and Plenar 1984; Henry 1992]. It is important that workers' exposures to noise be accurately monitored and that appropriate control measures be implemented when necessary. Several individuals and organizations have proposed different approaches to monitoring noise exposures [Behar and Plenar 1984; CSA 1986; Royster et al. 1986; AIHA 1991; Henry 1992; Simpson and Berninger 1992; Stephenson 1995]. NIOSH acknowledges the contributions of these individuals and organizations to this important subject, and encourages continued effort in the development of exposure monitoring strategies applicable to occupational noise exposure. An important component of HearSf 2000 that is being co―developed by NIOSH, the United Auto Workers―Ford National Joint Committee on Health and Safety, Hawkwa Group, and James, Anderson and Associates is noise monitoring with emphasis on noise exposure characterizations based on the principles of task―based exposure assessment. Research is needed to determine the most important factors for determining the hazard to hearing.

7.6 HEARING PROTECTORS

The attenuation of hearing protectors as they are worn in the occupational environment is usually quite different from that realized in the laboratory. The manufacturer's labeled NRRs, which are currently used by OSHA in determining compliance with the permissible exposure limit when engineering controls are being implemented or not feasible, usually do not reflect actual experiences. Thus, there is a continued need to research and develop a laboratory method that will best estimate the noise attenuation obtained with hearing protectors worn in the field. Research should also lead to the development of hearing protectors that provide increased comfort and improved communication. Just as important as laboratory methods to produce results with predictive value are the development of worksite test methods to determine the amount of protection each worker receives from his or her hearing protection. Additionally, as new technologies such as active level dependancy and active noise reduction are introduced into personal hearing protection, methods need to be developed to describe the effectiveness of these methods alone and when built into passive hearing protectors.

7.7 TRAINING AND MOTIVATION

Research is needed in using behavioral survey tools as resources for developing training and educational programs that address worker's beliefs, attitudes and intentions regarding hearing loss prevention with the purpose of enhancing the effectiveness of training and education. To date, research in training and motivation, when conducted at all, has been focused on materials and their delivery with the worker considered as the passive receptacle. Research is needed to develop materials and programs that involve the worker in the process and give the worker ownership of the information.

7.8 PROGRAM EVALUATION

Several methods for evaluating the effectiveness of a HLPP was discussed in Chapter 5 of this document. There is currently not one generally accepted method which stands out as being superior to the rest. Further research and development of methods for evaluating the effectiveness of HLPPs is needed, and the method deemed to have the best balance between accuracy and ease of use should be adopted. All existing methods rely upon the results of audiometric testing for evaluating effectiveness of the hearing loss prevention program. While audiometric data are crucial for managing the program for each worker, such that if no workers develop occupational hearing loss the program is considered to be 100% effective, too much time must pass for a database of audiograms to be built that can support queries about overall program effectiveness. Methods that do not rely on audiograms, such as observed behaviors that effect the success of a program or questionnaire―type survey instruments that evaluate workers' beliefs and intents, need to be considered as vehicles for immediate assessment of program effectiveness.

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A - E / F - J / K - O / P - R / S - V / W - Z

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