Occupational Noise and Hearing Conservation
Selected Issues
1. NIOSH definition of what constitutes Significant Threshold Shift (STS) and use of age corrections when determining STS
Criteria for Significant Threshold Shift
Since the promulgation of the Hearing Conservation Amendment [46 Fed. Reg.4078 (1981); 48 Fed. Reg. 9776 (1983)], Hearing Conservation Programs (HCPs) have been widely implemented to prevent the development of noise-induced hearing loss among workers exposed to occupational noise. The components of HCPs include sound exposure monitoring to assess the degree of the hazard, engineering and administrative noise controls to reduce the hazard, hearing protectors to reduce the noise entering the wearer's ears, education to motivate personnel to take an active part in protecting their own hearing, and annual audiometric evaluations to detect any significant changes in the hearing thresholds of noise-exposed workers.
Audiometric evaluations play a critical monitoring function in an HCP. Susceptible individuals may develop noise-induced hearing loss if their hearing protectors are fitted poorly, are used inconsistently or improperly, or provide inadequate noise reduction. The worker's baseline audiogram serves as a reference for his or her hearing status at the time of entry into the HCP, and later audiometric results are compared to the baseline to detect hearing changes. Significant hearing changes consistent with noise-induced causation should trigger follow-up actions to increase the degree of protection from noise for the worker. In addition, medical referrals may be made based on audiometric results.
An audiogram is a table or a chart that displays the hearing thresholds measured for the subject in each ear at specified test frequencies. In OSHA-mandated HCPs, 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 the 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 level (dB HL) with 0 dB HL 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) indicate better-than-average hearing.
An individual's audiometric threshold at a given test frequency is not an invariant quantity. There is measurement variability associated with the state of the subject (including the subject's prior audiometric experience, attention, motivation, and 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 larger the difference must be between thresholds measured on two separate audiograms before the difference can be considered potentially indicative of actual hearing change.
A summary of the auditory effects of noise is presented by Ward [1986]. The primary effect is to increase hearing thresholds; thresholds are shifted toward higher values (poorer hearing). Exposure to extremely intense noise may cause an immediate permanent hearing loss known as acoustic trauma. In contrast, 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 when 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 noise-induced permanent threshold shift (NIPTS) begins to develop.
Because TTS serves as a precursor of NIPTS, it is desirable to perform monitoring audiometry on noise-exposed workers at the end of or during their daily work shifts so that any TTS which is present can be detected. Responding to TTS will permit worker protection to be increased so that NIPTS will not develop or progress. If audiometric tests are performed prior to noise exposure on the day of testing, the threshold shifts observed will already be permanent so that the audiometric monitoring will serve only to document the hearing loss, not to prevent it.
Some people consider industrial audiometry too variable to be useful in detecting beginning threshold shifts [Hetu 1979; Atherley and Johnston 1981]. Certainly, if testing procedures are too inconsistent, then measurement variability may not be distinguishable from TTS or NIPTS. The challenge is to select a criterion for significant threshold shift (STS) 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 NIPTS which HCP staff hope to detect is of the same order of magnitude as typical audiometric measurement variability -- about 10 dB. The more consistent audiometric testing procedures are, the better hearing changes can be identified. However, daily TTS would be expected to be larger in magnitude than the developing NIPTS, so testing workers during their work shifts (when TTS may be present) increases the probability of identifying workers who are not adequately protected from noise.
In the 1972 criteria document, the NIOSH STS is defined as any threshold shift (to a higher threshold) that equals or exceeds 10 dB at 500, 1000, 2000, or 3000 Hz, or 15 dB at 4000 or 6000 Hz in either ear. In 1992, the "performance" of this criterion, among 5 other STS criteria, was tested with several audiometric databases by Dr. Julia Royster under contract with NIOSH [Royster 1992]. These databases contain sequential annual audiograms of workers in hearing conservation programs. The following STS criteria were evaluated in the contract report:
- OSHA STS (in this particular criterion only, STS = Standard Threshold
Shift): a change of 10 dB or more in the average of hearing thresholds at
2000, 3000 and 4000 Hz
- AAO-HNS(a) 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
- 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
- 15-dB SHIFT: a change of 15 dB or more at any test frequency from 500 through 6000 Hz
- 15-dB TWICE: a shift of 15 dB or more at any test frequency from 500 through 6000 Hz which is present in one annual audiogram and is persistent at the same frequency in the same ear on the next audiogram
- 10-dB AVG. 3-4 kHz: a shift of 10 dB or more in the average of thresholds at 3000 and 4000 Hz
- AAO-HNS(a) 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
The study methodology, database characteristics, and results are described in detail in Dr. Royster's report [Royster 1992]. None of the shift criteria evaluated is best in every respect. The relative pros and cons of each criterion are tabulated in Table 1. 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 also tag a reasonably high number of workers that includes a high percentage 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, resulting in a higher percentage of true tags. This criterion would be misused, however, if the second test were administered as much as one year later. The second test should be administered as soon as is reasonable. If the 15-dB shift appears again in the same ear and at the same frequency, the worker is tagged as having met the 15-dB TWICE criterion.
The 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 more workers due to normal testing variability alone. Hearing conservationists need to spend their time on those workers who most need the follow-up attention, but the NIOSH SHIFT tags so many workers that it loses its usefulness as a problem identifier. This disadvantage of the NIOSH SHIFT is partially overcome by increasing the amount of shift to 15 dB at all frequencies (the 15-dB SHIFT); however, too many workers are still tagged by the 15-dB SHIFT to allow any meaningful follow-up.
The ideal STS criterion should tag workers with TTS before they develop NIPTS. On the basis of the data analysis presented by Royster [1992], NIOSH recommends a modified 15-dB TWICE criterion that requires a test and an immediate retest for use in a hearing conservation program. 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 STS criterion by more than 70%. If a 15-dB shift or more in one ear at any one of the test frequencies (500, 1000, 2000, 3000, 4000 or 6000 Hz) is indicated, 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 should be considered to have been met and the worker should be rescheduled for a confirmation(b) test within 30 days.
TABLE 1 -- Advantages and Disadvantages of Each Criterion for Significant Threshold Shift*
CONSIDERATIONS CRITERION
10-dB AAO OSH 15-dB AVG HNS 15-dB NIOSH A TWICE 3-4K Hz SHIFT SHIFT SHIFT STS
Advantages tags a moderate percentage X X X of workers gives highest percentage true X positive tags tags workers earliest X shows largest differences X X between control databases and non-control databases no calculation of frequency- X X X averages required averages noise-susceptible X X X X X frequencies separately or examines each frequency separately
Disadvantages tags the lowest percentage X of workers tags such a high percentage X X of workers that follow-up would be impractical tags workers early in fewer X cases requires calculations of X X X frequency averages averages low frequencies that X are unlikely to be affected by noise exposure averages together frequencies X which vary in susceptibility to noise uses a shift magnitude X within the range of normal audiometric variability in HCPs
*Adapted from [Royster 1992].
Age Correction on the Audiogram
NIOSH does not recommend that age correction be applied to an individual's audiogram for the STS calculation.
Although many people experience some decrease in hearing sensitivity with age, age correction cannot be accurately applied to audiograms in determining an individual's STS because the data on age-related hearing losses describe only the statistical distributions in populations. Thus, the median hearing loss for an age group will not be the same as the actual hearing loss 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 20-year-old subject, for example, from 1970 will experience as much 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 of each generation is different. Thus, if one wishes to accurately apply age corrections to a worker's audiogram before calculating for STS, one should have available a non-occupational-noise-exposed cohort of the same demographic and generational make-up as the worker to determine what those corrections should be. However, these kind of data are not complete and readily available.
The adjustment of audiometric thresholds for aging has become a common practice in workers' compensation litigation. In this practice, the effort results in a reduced amount of hearing loss which may be attributable to noise exposure, with the consequent reduction in the amount of compensation to be paid for the noise-induced hearing loss. 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. 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 is in the 50th percentile, but in reality the individual may be in the 10th or 90th percentile, or anywhere in between. Thus, one cannot 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 HCP. The purpose of the HCP is to prevent NIPTS. If an audiogram is "age corrected", regardless of the source of the correction values, the time by which STS is noted will be delayed. Delaying the tagging of a worker with STS is completely contrary to the purpose of the HCP.
2. NIOSH methodology for determining the effective sound level reaching the
ear when personal hearing protectors are worn
NIOSH's methodology as it is known today was published in a compendium of
hearing protection devices (HPDs) in 1984 [Lempert 1984], and although the
method can be traced back to the 1972 criteria document [NIOSH 1972], the term
"noise reduction rating" (NRR) for the HPD was first used by NIOSH in this
compendium. The methodology remains the same today [NIOSH 1994], and has been
adapted for use by OSHA in its noise standard [29 CFR 1910.95, Appendix B].
However, the issue really concerns the NRR. The NRR, which is assumed to
reflect the attenuation achieved by 98% of the wearers, does not reflect
actual field experience, and HPDs provide only a fraction of the attenuation
in the field that they do in the laboratory (see Table 2).
The NRR is a single-number rating required by the Environmental Protection
Agency to be shown on the label of each hearing protector sold in the United
States [40 CFR 211]. The values of sound attenuation used for calculation of
the NRR are determined in accordance with ANSI S3.19-1974, "American National
Standard for the Measurement of Real-Ear Hearing Protector Attenuation and
Physical Attenuation of Earmuffs" [ANSI 1974].
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 HPDs have been conducted [Abel et al. 1982; Behar 1985;
Berger and Kieper 1991; Casali and Parks 1991; Chung et al. 1983; Crawford and
Nozza 1981; Edwards et al. 1983; Edwards and Green 1987; Fleming 1980; Goff
and Blank 1984; Hachey and Roberts 1983; Hempstock and Hill 1990; Mendez et
al. 1986; Padilla 1976; Pekkarinen 1987; Pfeiffer et al. 1989; Regan 1975;
Smoorenburg et al. 1986]. In general, these studies involved testing the
hearing thresholds of occluded and unoccluded ears of subjects who wore the
HPDs 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 2, the NRRs derived from these
real-world attenuation data were compared with the manufacturers' labeled 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 HPDs, and as an interim measure, NIOSH recommends that the labeled
NRRs be derated by 25%, 50% and 70% for earmuffs, formable earplugs and all
other earplugs, respectively. These percentages are estimates based on Table
2.
TABLE 2. Summary of Real-World NRRs Achieved by 84% of the Wearers
(NRR84)
of Hearing Protector (HPs) in 20 Independent Studies*
3. NIOSH comments on including a 115 dBA ceiling level
Damage risk criteria for noise employ the concept of dose that has an inherent
assumption that an exchange rate exists between the intensity of the noise and
the time of exposure to the noise. The 1972 criteria document [NIOSH 1972]
proposed that a 100% dose would be 85 dBA for 8 eight hours. The exchange
rate was 5 dB; that is, for each change in noise level of 5 dB the allowable
exposure duration was changed by a factor of 2. Thus, a 100% dose could also
be achieved by exposure to 90 dBA for 4 hours, 95 dBA for 2 hours, 100 dBA for
1 hour, 105 dBA for 30 minutes, 110 dBA for 15 minutes, and 115 dBA for 7.5
minutes.
The American Conference of Governmental Industrial Hygienists (ACGIH) [1994]
has adopted a revised recommended Threshold Limit Value (TLV) for noise. The
exchange rate has been changed to 3 dB per doubling/halving of time with an
8-hour exposure limit set at 85 dBA. Thus, allowable exposure time to 115
dBA would be 28.8 seconds and exposure to 140 dBA would be 0.1 seconds, about
the duration of a rifle shot or hammer blow.
While limiting exposure to a noise level of 90 dBA for 8 hours as the
permissible exposure limit, the OSHA Occupational Noise Standard [29 CFR
1910.95] sets a ceiling of 115 dBA for allowable exposures of 15 minutes or
less. Thus, exposures to noise levels of greater than 115 dBA would not be
permitted regardless of the duration of the exposure. The ceiling 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. Put simply, above some critical
level the ear is subject to instant structural damage by the noise that
results in instant NIPTS.
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 the critical level the amount of NIPTS is related to the
intensity of the noise and the daily duration of exposure. 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 beyond which no exposure should be permitted.
4. NIOSH criteria for evaluating the effectiveness of a Hearing
Conservation Program (HCP)
The effectiveness of a hearing conservation program should be evaluated in
terms of the hearing losses prevented individually and programmatically. This
evaluation should be a continual process.
The effectiveness of the HCP in preserving the hearing sensitivity of
individual workers is best evaluated through audiometric monitoring. Each
audiogram is a marker of the effectiveness of the hearing loss prevention
effort for the individual worker. For the audiogram to be of value, its
reliability must be verified. The best way to verify audiogram reliability is
to immediately compare hearing thresholds to those already collected for the
same worker, including the baseline audiogram. Any apparent changes in
hearing can be verified while the worker is available for retesting.
To assess the effectiveness of the HCP from a programmatic level, it is
necessary to have an evaluation method that can monitor trends in the
population of workers enrolled in the HCP and thus indicate program
effectiveness before many individual shifts occur. Thus, there is the need
for an overall program evaluation. This evaluation has two components. The
first component 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 1991]. 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 differences, 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 component 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
calculation that compliance with the current noise regulation would result in
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 would have
otherwise been expected by retirement (later OSHA calculations have revised
this estimate to be 10-15%). This method has the obvious disadvantage of not
permitting evaluation of the HCP until enough workers have reached retirement
age when modifications to the HCP will be unable to prevent their loss of
hearing.
Another method involved evaluating the effectiveness of the HCP on the basis
of the percentage of workers showing STS. Ideally, the criterion percentage
of STS could be based on a control group (i.e., non-noise-exposed) within the
same company. Others who have investigated the possibility of per cent STS as
an evaluation criterion have reported that 3-6% [Morrill 1981], or 5% [Franks
et al. 1989; Simpson et al. 1994] are reasonable rates which can be met by
effective programs. STS rates exceeding these percentages might then be
considered evidence of a deficient program. A disadvantage of this system is
the lack of consideration for the effects of other variables (e.g., age,
gender, race, previous noise exposure history) that might differentially
affect the STS rates noted in different programs whose populations differ
substantially along these variables. Another disadvantage is that the system
does not permit establishment of the cause of the deficiency; the problems
could be as likely due to poor audiometry as to excessive noise exposure
[Melnick 1984; Simpson et al. 1994].
The third method, used by Pell [1972] in evaluating the effectiveness of the
HCP 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 HCP 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, it would be preferable to define the "quiet" group as those exposed to less than 80 dBA instead of 85 dBA.
The U.S. Army Center for Health Promotion and Preventive Medicine (CHPPM)
(formerly the U.S. Army Environmental Hygiene Agency) evaluates its HCPs 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; the program evaluator is also given some
flexibility is assigning ratings. Such a system is helpful in that it defines
strict criteria for every aspect of the HCP 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 HCPs to have unacceptably
high incidences of STS [Byrne and Monk 1993].
The success of a smaller program 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 the
individual worker's results and get an adequate picture of the HCP's efficacy.
At the present time, there is not one generally accepted method for the
overall evaluation of HCPs. Furthermore, there is not one method which stands
out as being superior to the rest. Therefore, at this time, an occupational
STS rate of 5% or less is considered evidence of an effective HCP [Morrill
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. NIOSH comments on a reasonable time frame for conducting audiometric
testing once an employee is enrolled in an HCP
6. NIOSH comments on using Type 1 sound level meters to measure the
background sound levels in audiometric test rooms
Although ANSI S3.1-1991 [ANSI 1991], Maximum Permissible Ambient Noise Levels
for Audiometric Test Rooms, calls for the use of Type 1 sound level meters
(SLMs), the important requirement is for a SLM to be capable of measuring
sound levels in the range specified in ANSI S3.1-1991. Many Type 2 SLMs (with
the appropriate microphone) designed since 1989 are capable of measuring sound
levels in this range. Each manufacturer can advise whether its SLMs are
electrically quiet enough to be used for monitoring test room background noise
levels.
On the other hand, the requirement for a Type 1 SLM to measure the background
sound levels in audiometric test rooms would not be an extra burden on the
hearing conservation program. For a non-mobile facility, the background noise
measurements can be performed by the same persons performing the audiometer
calibration, for which Type 1 SLMs are required by ANSI S3.6-1989 [ANSI 1989],
the audiometer calibration standard to which OSHA presently holds HCPs.
Mobile facilities should have background noise levels checked at each new set
up site, so an appropriate SLM should be part of the standard equipment for
the facility.
7. NIOSH position on the Exchange Rates, 3 dB vs. 5 dB
NIOSH supports the adoption of the 3-dB exchange rate. The following
discussion is an abbreviated version of a NIOSH contract report [Suter 1992],
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 Environmental Protection Agency
(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 rule, 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
true time-weighted-average exposure to noise. The relationship between sound
power level and sound power (a measure of energy) is defined by the following
equation:
This same relationship does not hold true for the 5-dB exchange rate.
The 5-dB exchange rate is sometimes called the OSHA rule, and it is less
conservative than the equal energy rule. 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 temporary
threshold shift (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.
In 1965 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:
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 (1974); IS0 1971; von Gierke at al. 1981; IS0 1990;
U.S. Air Force 1993; ACGIH 1994]. 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
[Bohne and Pearse 1982; Ward and Turner 1982; Ward et al. 1982; Bohne et al.,
1985; Bohne et al. 1987; Clark et al. 1987] that intermittency can be
beneficial, especially in the laboratory. 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 rule,
as Passchier-Vermeer [1971 and 1973] and Shaw [1985] have demonstrated. The
fact that in Passchier-Vermeer's portrayal of the data fewer points fall below
the Burns and Robinson curve than below the Passchier-Vermeer curve seems to
demonstrate the effect of Burns' and Robinson's rigorous screening procedures
rather than support for any particular exchange rate. The fact that
comparisons using the newer ISO standard [ISO 1990] corroborate
Passchier-Vermeer's findings lend additional support to the equal-energy rule.
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 has 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.
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----------------
HP Type Model Reference N NRR NRR NRR NRR
84 84 84
Foam E-A-R/Decidamp Crawford and Nozza [1981] 58 29 19
Foam E-A-R/Decidamp Hachey and Roberts [1983] 31 29 9
Foam E-A-R/Decidamp Lempert and Edwards [1983] 56 29 12
Foam E-A-R/Decidamp Edwards and Green [1987] 28 29 19
Foam E-A-R/Decidamp Edwards and Green [1987] 28 29 14
Foam E-A-R/Decidamp Lempert and Edwards [1983] 56 29 5
Foam E-A-R/Decidamp Abel et al. [1978] 55 29 9
Foam E-A-R/Decidamp Abel et al. [1978] 24 29 9
Foam E-A-R/Decidamp Behar [1985] 42 29 14
Foam E-A-R/Decidamp Behar [1985] 24 29 16
Foam E-A-R/Decidamp Pfeiffer et al. [1989] 69 29 10
Foam E-A-R/Decidamp Casali and Park [1991] 10 29 6
Foam E-A-R/Decidamp Casali and Park [1991] 10 29 23
Foam E-A-R/Decidamp Hempstock and Hill [1990] 72 29 13
Foam E-A-R/Decidamp Berger and Kieper [1985] 22 29 20 12.5 13.2
Premolded Ultra-Fit Casali and Park [1991] 10 21 4
Premolded Ultra-Fit Casali and Park [1991] 10 21 17
Premolded Ultra-Fit Royster et al. [1984] 19 21 5
Premolded Ultra-Fit Berger and Kieper [1985] 29 21 3 5.8 7.3
Premolded V-51R Royster et al. [1984] 12 23 3
Premolded V-51R Abel et al. [1978] 20 23 2
Premolded V-51R Edwards et al. [1978] 84 23 1
Premolded V-51R Fleming [1980] 9 23 6
Premolded V-51R Padilla [1976] 183 23 -1 0.1 2.2
Premolded Accu-Fit Fleming [1980] 13 26 2
Premolded Com-Fit Abel et al. [1978] 18 26 7 4.9 4.5
Premolded EP100 Crawford and Nozza [1981] 22 26 0
Premolded EP100 Edwards et al. [1978] 28 26 -2
Premolded EP100 Abel et al. [1978] 45 26 10
Premolded EP100 Smoorenburg et al. [1986] 46 26 -2 2.1 1.5
Premolded NA Mendez et. al. [1986] 30 NA 1 1.0 1.0
Fiberglass Down Lempert and Edwards [1983] 28 15 4
Fiberglass Down Edwards et al. [1978] 56 15 3 3.3 3.5
Fiberglass POP Lempert and Edwards [1983] 28 22 4
Fiberglass POP Behar [1985] 28 22 10
Fiberglass POP Pfeiffer et al. [1989] 51 22 7
Fiberglass POP Mendez et al. [1986] 30 22 10
Fiberglass POP Hempstock and Hill [1990] 39 22 8 7.7 7.8
Fiberglass Soft Hachey and Roberts [1983] 36 26 1
Fiberglass Soft Pfeiffer et al. [1989] 12 26 9
Fiberglass Soft Hempstock and Hill [1990] 32 26 4 3.4 4.7
Custom Adcosil Hachey and Roberts [1983] 44 24 4
Custom NA Crawford and Nozza [1981] 7 NA 7
Custom Prtctear/vent Lempert and Edwards [1983] 56 11 8
Custom Peackeeper Lempert and Edwards [1983] 56 15 4
Custom NA Abel et al. [1978] 48 NA 3
Custom NA Regan [1975] 6 NA 4
Custom NA Padilla [1976] 230 NA 8 6.5 5.4
Semi-aural Sound-Ban #10 Behar [1985] 32 17 10
Semi-aural Sound-Ban #20 Casali and Park [1991] 10 19 6
Semi-aural Sound-Ban #20 Casali and Park [1991] 10 19 12 9.6 9.3
Earmuffs Bilsom UF-1 Hachey and Roberts [1983] 31 25 13
Earmuffs Bilsom UF-1 Casali and Park [1991] 10 25 16
Earmuffs Bilsom UF-1 Casali and Park [1991] 10 25 20
Earmuffs MSA Mk IV Abel et al. [1978] 47 23 11
Earmuffs MSA Mk IV Goff and Blank [1984] 15 23 4
Earmuffs Optac 4000 Pfeiffer et al. [1989] 33 NA 14
Earmuffs Peltor H9A Pfeiffer et al. [1989] 34 22 14
Earmuffs Rcal Agd III Hempstock and Hill [1990] 42 NA 19
Earmuffs Norseg Mendez et al. [1986] 30 NA 8
Earmuffs AO 1720 Goff and Blank [1984] 11 21 6
Earmuffs Bilsom 2450 Pfeiffer et al. [1989] 11 NA 13
Earmuffs Clark E805 Abel et al. [1978] 17 23 15
Earmuffs Glendale 900 Goff and Blank [1984] 10 21 10
Earmuffs Optac 4000S Pfeiffer et al. [1989] 10 NA 14
Earmuffs Safety 208 Abel et al. [1978] 15 22 12
Earmuffs Safety 204 Behar [1985] 9 21 22
Earmuffs Welsh 4530 Regan [1975] 5 25 20
Earmuffs Misc. Pekkarinen [1987] 71 NA 13
Earmuffs Safir E/ISF Hempstock and Hill [1990] 20 NA 14
Earmuffs Misc. Chung et al. [1983] 64 24 18 13.8 13.8
Cap Muffs Bilsom 2313 Hempstock and Hill [1990] 37 23 16
Cap Muffs Hlbrg No Nse Abel et al. [1978] 58 23 11
Cap Muffs Peltor H7P3E Behar [1985] 36 24 13
Cap Muffs AO 1776K Behar [1985] 26 21 14
Cap Muffs Hlbrg 26007 Hempstock and Hill [1990] 20 NA 18
Cap Muffs Misc. Chung et al. [1983] 37 23 17 14.3 14.8
Plug+Muff E-A-R + UF-1 Hachey and Roberts [1983] 10 25 25.0 25.0
*Adapted from [Berger et al. 1994]
Abbreviations:
N = Size of Test Population
NRR = Labeled Noise Reduction Rating
Wght = weighted on the basis of test population size
Sound Power Level, decibels (dB) = 10*log(W/Wo)
where
W = sound power
Wo = reference sound power
By this mathematical relationship, every doubling of energy results in an
increase of 3 dB:
10*log(W/Wo) + X = 10*log(2W/Wo)
X = 10*log(2W/Wo) - 10*log(W/Wo)
= 10*log[(2W/Wo)/(W/Wo)]
= 10*log(2)
= 10(0.301)
= 3.01 dB
REFERENCES
(a) American Academy of Otolaryngology Head and Neck Surgery
(b) audiometric test preceded by 14 hours of quiet
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