NOISE

TLV® exposures of 80 dBA or greater should be used in

the above calculations. With sound level meters, this
formula should be used for sounds with steady
These TlVs refer to sound pressure levels and
levels of at least 3 seconds. For sounds in which this
durations of exposure that represent conditions
condition is not met, a dosimeter or an integrating
under which it is believed that nearly all workers may
sound level meter must be used. The limit is
be repeatedly exposed without adverse effect on
exceeded when the dose is more than 100% as
their ability to hear and understand normal speech.
indicated on a dosimeter set with a 3 dB exchange
Prior to 1979, the medical profession had defined
rate and an 8-hour criteria level of 85 dBA.
hearing impairment as an average hearing threshold
The TLV is exceeded on an integrating sound
level in excess of 25 decibels (ANSI S3.6-1989i 1>at
level meter when the average sound level exceeds
500, 1000, and 2000 hertz (Hz). The recommended
the values given in Table 1.
values should provide protection against a hearing
loss at higher frequencies, such as 3000 Hz and
TABLE 1. Threshold Limit Values for NoiseA
4000 Hz. The values should be used as guides in
the control of noise exposure and, due to individual Duration Sound Level
susceptibility, should not be regarded as fine lines per Day dBA8
between safe and dangerous levels. Hours 24 80
It should be recognized that the application of 16 82
the TlVs for noise will not protect all workers from 8 85
the adverse effects of noise exposure. The TlVs 4 88
should protect the median of the population against 2 91
a noise-induced hearing loss exceeding 2 dB after 1 94
40 years of occupational exposure for the average of Minutes 30 97
0.5, 1, 2, and 3 kHz. A hearing conservation 15 100
program with all its elements including audiometric 7.50C 103
testing is necessary when workers are exposed to 3.75C 106
noise at or above the TLV levels. 1.88C 109
0.94C 112
Continuous or Intermittent Noise
SecondsC 28.12 115
The sound pressure level should be determined 14.06 118
by a sound level meter or dosimeter conforming, as 7.03 121
a minimum, to the requirements of the American 3.52 124
National Standards Institute (ANSI) Specification for 1.76 127
Sound Level Meters, S1 .4-1983, Type S2A,<2>or 0.88 130
ANSI S1.25-1991 Specification for Personal Noise 0.44 133
Dosimeters.<3> The measurement device should be 0.22 136
set to use the A-weighted network with slow meter 0.11 139
response. The duration of exposure should not
No exposure to continuous, intermittent, or impact noise in
exceed that shown in Table 1. These values apply to
excess of a peak C-weighted level of 140 dB.
total duration of exposure per working day 8 Sound levels in decibels are measured on a sound level meter,
regardless of whether this is one continuous conforming as a minimum to the requirements of the
exposure or a number of short-term exposures. American National Standards Institute Specification for Sound
When the daily noise exposure is composed of two Level Meters, Sl.4 (1983)<2>Type S2A, and set to use the A-
or more periods of noise exposure of different levels, weighted network with slow meter response.
their combined effect should be considered rather cLimited by the noise source - not by administrative control.
than the individual effect of each. If the sum of the It is also recommended that a dosimeter or integrating sound
following fractions: level meter be used for sounds above 120 dB.

C1 C2 Cn
+ + Impulsive or Impact Noise
T1 T2 Tn
By usinc% the instrumentation specified by the
(4)
exceeds unity, then the combined exposure should ANSI S1 .4, S1 .25,<3>or IEC 804, impulsive or
be considered to exceed the TLV. C 1 indicates the impact noise is automatically included in the noise
total duration of exposure at a specific noise level, measurement. The only requirement is a
and T 1 indicates the total duration of exposure measurement range between 80 and 140 dBA and
permitted at that level. All on-the-job noise the pulse range response must be at least 63 dB. No

ACGIH® © 2006 Noise-1

exposures of an unprotected ear in excess of a carbon disulfide, mercury, and trichloroethylene.
C-weighted peak sound pressure level of 140 dB 3. There is evidence to suggest that noise exposure
should be permitted. If instrumentation is not avail- in excess of a C-weighted, 8-hour TWA of 115
able to measure a C-weighted peak, an unweighted dBC or a peak exposure of 155 dBC to the
peak measurement below 140 dB may be used to abdomen of pregnant workers, beyond the fifth
imply that the C-weighted peak is below 140 dB. month of pregnancy, may cause hearing loss in
the fetus.
Notes 4. The sum of the fractions of any one day may
1. For impulses above a C-weighted peak of 140 exceed unity, provided that the sum of the
dB, hearing protection should be worn. The fractions over a 7-day period is 5 or less and no
MIL-STD-1474c<5>provides guidance for those daily fraction is more than 3.
situations in which single protection (plugs or 5. Table 1 is based on daily exposures in which
muffs) or double protection (both muffs and there will be time away from the workplace in
plugs) should be worn. which to relax and sleep. This time away from the
2. Exposure to certain chemicals may also result in workplace will allow any small change to the
hearing loss. In settings where there may be worker's hearing to recover. When the worker, for
exposures to noise and to carbon monoxide, times greater than 24 hours, is restricted to a
lead, manganese, styrene, toluene or xylene, space or series of spaces that serve as both a
periodic audiograms are advised and should be workplace and a place to relax and sleep, then
carefully reviewed. Other substances under the background level of the spaces used for
investigation for ototoxic effects include arsenic, relaxation and sleep should be 70 dBA or below.

DOCUMENTATION

CONTINUOUS OR noise-induced permanent threshold shift (NIPTS)

values for a 40-year exposure to 85 dB. These
INTERMITTENT NOISE values are 0 dB, 0 dB, 1 dB, and 5 dB for the
audiometric frequencies of 0.5 kHz, 1 kHz, 2 kHz,
Airborne sound can be described as propagated and 3 kHz, respectively.
fluctuations in atmospheric pressure capable of Before 1950, overall sound pressure levels, in
causing the sensation of hearing. In occupational decibels, were used to define the noise aspect of
health, the term "noise" is used to denote unwanted damage risk criteria.<7> Following recognition that the
sound. Noise-induced hearing loss has been overall intensity of a noise, by itself, was not
recognized and reported for several hundred years. sufficient to describe the potential for damage and
However, prior to about 1950, reliable dose-effect that the frequency characteristic must also be
data were not available. Before World War II, due to considered, criteria incorporating spectral levels
lack of uniformity in instrumentation and related units (usually octave band levels) were developed. Many
and scales, studies from various parts of the world of these are summarized in the U.S. National
often yielded a significant difference in result. Institute for Occupational Safety and Health (NIOSH)
Present day American, European, and Noise Criteria Document.<8>An octave band analysis
International standards< 1-s> relating to the is a relatively lengthy procedure requiring expensive
instrumentation and methodology of both noise and instrumentation, and there was some concern that
hearing acuity measurement are now in reasonable the layman had difficulty in interpreting the results.
accord. This accord resulted in an international Recognizing the desirability of a single reading and
standard, ISO-R-1999.2 (1990), issued by the the fact that most industrial NIPTS data were
International Organization for Standardization available for single weighted noise levels, the
(IS0).<5>This standard had been available for review lntersociety Committee in 1967 proposed the use of
for over 12 years and was well accepted A-weighted sound levels in the development of
internationally upon its promulgation. criteria.<9>The A-weighted characteristic of a sound
This standard<5>is the basis for the changes to level meter is designed to approximate the
the TLV with respect to the time intensity trading frequency-selective response of the human ear at
relation and for combining continuous noise with moderate intensities. In one report, Botsford< 10>
impulse noise. For example, the statement that the demonstrated that A-weighted levels are as reliable
TLV should protect the median of the population as octave band levels in the prediction of effects on
against a noise-induced hearing loss exceeding 2 hearing in 80% of the occupational noises
dB after 40 years of occupational exposure for the considered and slightly more conservative in 16% of
average of 0.5, 1, 2, and 3 kHz was developed from the cases. Passchier-Vermeer< 11 >and Cohen et a1.< 12>
the ISO-R-1999, Table E-1, which provided median similarly demonstrated that A-weighted levels

2-Noise ACGIH® © 2006

provide a reasonable estimate of the hazard to impairment definition is particularly important when
hearing in most industrial environments. The one considers that, in almost all cases, noise-
abbreviation dBA is used to denote decibels A- induced hearing loss first appears in the frequency
weighted and can be described as a unit of range from 3000 to 6000 Hz. With continued
measurement of sound level corrected to the A- exposure, the loss in hearing eventually spreads to
weighted scale as defined in ANSI S1.41983.<2> the lower frequencies of 500, 1000, and 2000 Hz.
Today, A-weighted sound levels are in general use Clearly, it is necessary to define the beginning of
in hearing risk criteria. impairment before one can propose a damage risk
Permanent noise-induced hearing loss is related criterion, either in terms of a zero-risk criterion such
to the intensity and frequency distribution of the that the prevalence of impairment in an exposed
noise, the time pattern and duration of exposure, group is no greater than the prevalence in a control
and individual susceptibility. The ability to hear and or a no-noise group, or in terms of a percentage-risk
understand everyday speech under normal criterion that would provide an estimate of the
conditions is regarded as the most important increase in the number of impaired subjects in a
function of the hearing mechanism. Thus, most group exposed to noise levels in a stated excess of
present-day studies focus on the resultant or the zero-risk criterion.
predicted hearing loss in the speech frequency For these reasons, the TLV is based on a
range. However, research has clearly shown that the formula that includes 3000 Hz. Using ISO-R-
speech range does not stop at 2000 Hz and should 1999.2,<5> the median amount of NIPTS after 40
include frequencies from 2000 to 4000 Hz. For this years of exposure to 90 dB is 2 dB for the average
reason, the American Academy of Ophthalmology of 500, 1000, and 2000 Hz. The same 40-year
and Otolaryngology (MOO) included 3000 Hz in exposure at 85 dB for the average of 500, 1000,
their hearing impairment formula in 1979_<13>This 2000, and 3000 Hz is 1. 75 dB. Thus, everything else
inclusion also impacts the selection of the TLV. being equal, inclusion of 3000 Hz will drop the 8-
Audiometric testing is carried out to determine hour criterion level from 90 dB to 85 dB.
an individual's hearing threshold for pure tones. In
this testing, a series of tones is presented to the History
subject through earphones. Each ear is tested in
turn. The test tones normally used are 500, 1000, In 1954, the ANSI Z.24.X.2 Committee reported
2000, 3000, 4000, 6000, and 8000 Hz. The intensity the need to define hearing impairment and
of each tone is adjusted until the subject indicates protection goals. There was some indication that
that he/she can just hear the signal. The threshold of exposure to octave band levels in excess of 80 dB
hearing for each test tone is then recorded in could cause some loss in hearing.<17>
decibels. The amount in decibels by which the After experimenting with and proposing the 3-dB
subject's threshold exceeds the zero setting on the rule and after extensive study by the National
audiometer is then called the hearing threshold level, Research Council Committee on Hearin_fi Bio-
or loss, at that particular frequency. The zero acoustics, and Biomechanics (CHABAi i of the
settings on the audiometer are based on response National Academy of Sciences, the U.S. Air Force
levels derived from testing large groups of young introduced the equal energy rule in its re~ulation on
people. There is general agreement that progression Hazardous Noise Exposure in 1956.<19·20
in hearing loss at 500, 1000, 2000, and 3000 Hz In 1961, the ISO published a series of noise
eventually will result in impaired hearing, i.e., ability rating curves and proposed that the noise rating
to hear and understand speech. The MOO curve N85 be used as the limit for habitual workday
Subcommittee on Noise<1<iJ and the American exposure to broadband noise. A permissible
Medical Association< 15>defined estimated hearing temporary threshold shift (TTS) was considered, and
level for speech as the simple average of hearing a series of curves based on the average level of the
levels at the three frequencies 500, 1000, and 2000 300 to 600, 600 to 1200, and 1200 to 2400 Hz
Hz. The point at which impairment begins was then octave bands was suggested to provide an estimate
set at the 25-decibel average hearing level. This was of permissible exposure to intermittent noise.<21 >
referred to as the hearing loss index (0.5, 1,2) or HLI In 1963, Kryter<22>proposed a family of 1/3
(0.5, 1,2) = 25 dB. In an investigation by A.H. octave and octave band curves based on predicted
Suter< 15f it was reported that, "Correlation tests TTS in the speech frequencies. The slope of the
revealed that frequency combinations that included curves varied significantly from those of ISO. The
frequencies above 2000 Hz were significantly better criteria provided limits for daily exposure times
predictors of speech discrimination scores than the ranging from 1.5 minutes or less (ceiling) up to 8
combination of 500, 1000, and 2000 Hz." By 1980, hours. Kryter's criteria were the same as those
there was also general agreement that the hearing contained in the draft of the Technical Report of the
level at 3000 Hz is related to the hearing and Armed Forces< 19>and CHABA.< 15>
understanding of speech, particularly in the In 1964, the MOO Subcommittee on Noise
presence of noise. reported that, " ... If the sound energy of the noise is
The inclusion (or exclusion) of 3000 Hz in an distributed more or less evenly throughout the eight

ACGIH® © 2006 Noise-3

octave bands and if a person is to be exposed to this NIOSH published its Criteria Document on
noise regularly for many hours a day, five days a occupational exposure to noise in 1972_<5>This
week for many years, then if the noise level in either included audiometric and noise exposure data
the 300 to 600 Hz or the 600 to 1200 Hz band is 85 NIOSH obtained from 792 noise-exposed workers
dB, the initiation of noise exposure control and tests from various industries and 380 workers from the
of hearing is advisable. <23>
11
same industries without noise exposure, who served
In 1967, the lntersociety Committee on as controls. An analysis of the data indicates
Guidelines for Noise Exposure Contro1<9>reviewed approximately 19% risk of impairment (HL[0.5, 1,2] >
published data and some private communications 25 dB) for workers exposed for more than 30 years
and provided an estimate that at 85 dBA, in terms of to 85 dBA. The NIOSH document contains a
percentage risk, some 3% of a population exposed comprehensive review of published data from many
throughout a working lifetime would suffer some studies. The NIOSH percent risk values for long-
hearing impairment (HLI [0.5, 1.2] > 25 dB). In view term exposures to various noise levels were
of the data scatter, however, the Committee compared with those derived from three other
indicated that this was not significant. At 90 dBA, the studies: 1) the lntersociety Study, <9>2) the early ISO
percentage risk was approximately 10% and this Recommendation R-1999,<25>and 3) the Burns and
was felt to be significant. Again, frequencies above 2 Robinson study.<25>The ISO risk values, which were
kHz were not used in making this determination. based on Baughn's data,<24>were similar to those of
In May 1967, the ACGIH Threshold Limit Values NIOSH. When the Burns and Robinson<25>
for Physical Agents (TLv®-PA) Committee, which audiometric data were adjusted to conform with the
had representation on the lntersociety Committee, audiometric baseline normally present in United
recommended a limit of 90 dBA for an 8-hour day, States studies, the risk values were comparable.
40-hour week exposure, on the basis that this The lntersociety Study risk values were significantly
should protect 90% of the long-term exposure group. lower. This difference was attributed to several
For exposures less than 8 hours per day, the TLv®- factors including a) use of results from one ear only,
PA Committee recommended that, for each halving b) nonseparation of experience groups, c) use of
of the exposure time, the limit be increased by 5 dBA speech interference levels and subsequent
(5 dBA slope). Even at that time, there was some conversion into approximate dBA values, and d) use
evidence that damage to the hearing mechanism of a dissimilar composite population in the
may be directly related to the acoustical energy lntersociety Study. The importance of 3000 Hz in the
involved. If this is true, the limit should be increased hearing and understanding of speech under
by only 3 dBA (3 dBA slope) for each halving of everyday conditions was reviewed. It was concluded
exposure time. that HL1(1,2,3) > 25 dB should be accepted as the
While even at the time there was some beginning of impairment.
substantiation for the 3-dBA slope for uninterrupted In 1974, the U.S. Environmental Protection
exposure to steady-state noise, the TLv®-PA Agency (EPA) published the "Levels" document.<21>
Committee felt that, based on the available TTS and In this document, an 8-hour level of 75 dBA was
animal studies, the ear could tolerate more established as the level that would protect "public
acoustical energy briefly than it could for continuous health and welfare with an adequate margin of
8-hour exposure. Furthermore, the Committee felt safety." Much of this result was based on the work of
that, in most plant situations, rest periods plus Johnson,<25>who combined the works of Passchier-
equipment shutdowns contributed to interruptions in Vermeer,<29> Baughn,<24>and Burns and Robinson.<25>
exposure that significantly increased the ear's These were the same data available to the TLv®-PA
tolerance. This first TLV (90 dBA) was subsequently Committee. A major difference, however, was the
adopted in 1970. use of 4000 Hz as the most sensitive indicator of
In 1967, Baughn first [>resented data, later hearing loss. Johnson<25>observed that the
published in a final report,<24>based on the difference between protecting 4000 Hz and the
evaluation of audiograms of 6835 noise-exposed average of 0.5, 1, and 2 kHz is about 15 dB.
workers. This was one of the largest groups studied In 1974, having reviewed the published data, the
at that time. There was some stated lack of controls TLv®-PA Committee believed that 85 dBA provided
in the study. Acknowledging this constraint, the an appropriate limit for an 8-hour day, 5-day week,
indication was that there was an 8% risk of hearing long-term exposure to occupational noise. On the
impairment at 85 dBA and that the risk factor was basis of HLl(0.5, 1,2) > 25 dB, the 85-dBA limit
increased to 18% at 90 dBA. should ensure the protection of approximately 90%
A study by Burns and Robinson<25>in 1970 of exposed workers.
included such factors as the use of dBA, variability in Concerning "uninterrupted" exposures to steady-
audiometric measurement, the relationship of state noise of less than 8 hours per day, the TLv®-
temporary threshold shift to permanent loss, and the PA Committee favored the retention of the 5-dB
use of the equal-energy approach. Their analysis slope. Concerning intermittent exposures throughout
supported the use of the equal energy approach and a work day, the Committee proposed a simple
A-weighting of sound pressure measurements. summation of such episodes over the work day.
4-Noise ACGIH® © 2006
While the 5-dB doubling rate was thought to be kHz."<32 >
overly permissive in some plant situations for While not all researchers have supported the 3-
continuous exposure, for the case of intermittency, it dB rule,<33 •34>a general consensus favors its use.
would tend to make some allowance for the recovery In a special meeting in 1982, many of the world's
periods between exposures. The Committee leading investigators of noise-induced hearing loss
recognized the need for more field study in this area. reviewed the available literature with respect to the
As will be shown later, additional studies have use of equal-energy.<35>The group endorsed the use
shown that the 3-dB slope (equal-energy) is more of equal-energy as the most practical and reason-
likely the better concept. able method of measuring both intermittent and
A "Notice of Intended Change," incorporating the impacUimpulse noise between 80 dBA and 140 dBA.
above limits, was made in 1974 and subsequently This meeting produced the international consensus
adopted in 1976. that was the basis of ISO-R-1999.2.<5>
Suter<35l reviewed the history of the various
The Equal-Energy Rule trading relations as well as the research that would
best support these relations. She concluded that
Three studies on two data sets are of inter- " ... While the 3-dB rule may be somewhat
est. <30--32> The Inter-Industry Noise Study, reported by conservative in truly intermittent conditions, the 5-dB
Yerg et a1.,<30>collected data on male and female rule will be under-protective in most others." Suter<35>
workers exposed to steady-state noise in the range also concluded that the 3-dB exchange rate was the
of 82 to 92 dBA. The protocol was designed to en- method most firmly supported by the scientific
sure rigorous control of both exposed and control evidence now available. Some of the key arguments
groups. The exposed workers were divided into she summarized are as follows:
groups according to sex and to plant noise level,
• TTS measured 2 minutes after exposure (TTS2)
e.g., low intensity group 82 to 85 dBA and high
"is not a consistent measure of the effects of a
intensity group 86 to 92 dBA. However, subsequent
single day's exposure to noise, and the NIPTS
to the start of the study, repeat checks on the plant
after many years may be quite different from the
noise levels revealed that in some cases there was a
TTS 2 produced at the end of an 8-hour day.
marked change in these levels. Thus, there was
Research has failed to show a significant
some movement of subjects between the high and
correlation between TTS and PTS; 11<25,37 l the
low groups during, and presumably before, the study
relationships between PTS, TTS, and cochlear
period. The conclusions drawn from this study were
damage are equally unpredictable.<38-42J
that a) the hearing levels in the high-intensity group
were not observably different from the hearing levels • Data from animal experiments presented by
Ward and associates<39•40 •43 l "... support the use
in the low-intensity group, b) differences between
females in the exposed and control groups were not of the 3-dB exchange rate for single exposures of
various levels within an 8-hour day." However,
statistically significant, and c) differences in hearing
evidence gathered by Ward and Turner,<39>Ward
levels between males exposed to 82 to 92 dBA and
et a1.,<40>Bohne and Pearse,<44>Bohne et a1.,<45,45>
their controls were small and were not statistically
significant at 500, 1000, and 2000 Hz. However, at and Clark et a1.<47>indicates" ... that
3000, 4000, and 6000 Hz, the levels of hearing loss intermittency can be beneficial, especially in the
in the noise-exposed group significantly exceeded laboratory." Nevertheless," ...these benefits are
those in the control group by approximately 6 to 9 likely to be smaller or even nonexistent in the
dB. industrial environment, where sound levels during
Schori and Johnson<31 >independently analyzed intermittent periods are considerably higher and
the same database. One conclusion of their study where interruptions are not evenly spaced."
was that the hearing changes found in the Inter- • Data from a number of field studies correspond
Industry Noise Study were consistent with the ISO well to the e~ual-energy rule, as Passchier-
R-1999.2 revised standard.<5> Vermeer<45•49 and Shaw<50l have demonstrated.
The Berger et al. report<32>is notable for the • The assumption by CHABA of the equal
precise measurement and steady-state character- temporary effect theory is also questionable in
istic of the plant noise. Following careful screening that some of the CHABA-permitted intermittent
during the study period, only 100 exposed subjects exposures can produce delayed recovery
(42 males and 58 females) were retained from an patterns even though the magnitude of the TTS
original group of 459 employees. The conclusion was within 'acceptable' limits, and chronic,
was that" ... Averaging the results for all 1OO incomplete recovery will hasten the advent of
subjects, in order to make comparisons to other PTS. The CHABA criteria also assume regularly
available data, yielded results in close agreement to spaced noise bursts, interspersed with periods
predictions based upon the work of Burns and that are sufficiently quiet to permit the necessary
Robinson, Baughn, NIOSH and Passchier-Vermeer, amount of recovery.
indicating that 10 years of exposure to a daily Leq of In addition to Suter•s<35l conclusions, there are
89 dBA causes measurable hearing loss at 4 several other reasons to change to the equal-

ACGIH® © 2006 Noise - 5

energy rule. These reasons should benefit industry poorer ear, and then dividing the total by 6.
as well as increase assessment accuracy. One of
the foremost reasons is the elimination of the IMPULSE NOISE
awkward all-or-nothing limit of 115 dBA. A short
burst of noise such as an aircraft flyover or a siren The previous approach for assessing impulse/
might exceed this limit. Yet, a burst of broadband impact noise was to allow 100 impulses or impacts
noise as long as 10 msec at 130 dB has been per day at 140 dB, 1000 per day at 130 dB, or
shown to cause almost no TTs.<51 -s3 J On the other 10,000 per day at 120 dB. Impacts or impulses refer
hand, research has shown that broad-band noise of to discrete noise of short duration, less than 500
115 dB for 15 minutes is likely to cause excessive milliseconds (ms), in which the sound pressure level
TTs.<52> Use of equal energy eliminates this arbitrary rises and decays very rapidly. One of the problems
115 dB limit. with this approach is that it is difficult, if not impos-
Second, use of equal energy better predicts the sible, to properly measure. A manufactured instru-
hazard of noises for exposure durations greater than ment is available that can sum the number of
8 hours. For an 8-hour criterion level of 85 dB, the 5- measured impulses at each 1 dB increment and
dB rule would dictate a 16-hour exposure at 80 dBA divide by the number of allowable impulses. From
and a 24-hour exposure as 77 dBA. The equal- this sum, a dose can be calculated. However, the
energy rule will allow 82 dBA for 16 hours and 80 calculation of this dose requires several assumptions
dBA for 24 hours. The threshold of any TTS to that were not explicit in the previous TLV. While
broadband noise for periods as long as 24 hours has these assumptions could be clarified here, there
been shown to be between 78 and 80 dBA.<54 -55> On exists a more accurate approach for addressing
the other hand, 85 dBA for 8 hours will cause some impulse/impact noise.
TTS. It is certainly more reasonable to anchor the Besides the complexity of using the previous
24-hour point to 80 dBA. The only time that a limit TLV, there were several fundamental problems with
lower than 80 dBA would be appropriate is the very this TLV. The first problem is that the duration of the
unusual circumstance when the exposure consists of impulse or impact was not considered. A short 1-ms
a steady pure tone. pulse was considered as harmful as a long 200-ms
A third reason is the inclusion of hearing levels pulse. This is not consistent with the CHABA
at 3000 Hz. What is often forgotten is that the benefit guidelines on impulse noise<55l or any known
of intermittency, as shown in the CHABA WG46 research. Second, impulse or impact noise was
curves,<1 8' did vary with the audiometric frequency treated separately from nonimpact noise. This
considered. Higher audiometric frequencies required separate treatment is inconsistent with research
smaller trading relations than the lower audiometric which has shown that at exposures causing
frequencies to produce equal TTS. Therefore, even moderate or high levels of TTS, combined impact
if the equal TTS 2 model was correct, inclusion of and continuous noise can cause a synergistic effect;
3000 Hz would dictate reducing the 5-dB trading i.e., the resulting effect is greater than just the
relation to a lower number. In some cases, this addition of the results from the impact exposures
number might even be slightly lower than 3 dB. and the results from the continuous noise
In summary, the equal-energy rule (3-dB rule) exposures.<57> Fortunately, these same researchers
appears to be a better predictor of noise hazard for have shown that at exposure levels which will cause
most practical conditions and is strongly recom- only a small amount of TIS, as much as the
mended by ACGIH proposed 8-hour 85 dBA threshold, this synergistic
Reference is made to the following formula for effect disappears. <55>
determining hearing impairment. The main point at The proposed method of assessing impulse or
issue is the inclusion of the hearing threshold level at impact noise resolves both these problems. By
3000 Hz in such a formula. In 1979 the American combining all sound energy between 80 dB and 140
Academy of Otolaryngology developed a new dBA, impacUimpulse noise is combined with
formula for determining hearing impairment.<13> The continuous and intermittent noise. Longer impulses
formula includes the 3000 Hz frequency and is as are considered more dangerous than short impulses.
follows: Finally, the measurement of noise becomes greatly
1. The average of the hearing threshold levels at simplified. An integrating sound level meter that
500, 1000, 2000, and 3000 Hz should be meets the requirement of IEC-804<4 > or a dosimeter
calculated for each ear. that meets the requirements of S1 .25<3> is all that is
2. The percentage of impairment for each ear required with one exception. The IEC-804<4 > for Type
should be calculated by multiplying by 1.5% the II instruments and the ANSI S1 .25<3> specify only a
amount by which the average hearing threshold minimum pulse range of 53 dB. The instruments
level exceeds 25 dB. The impairment should be used for this TLV must have a pulse range of at
calculated up to 100% reached at 92 dB. least 63 dB if all sounds from 80 dB to 140 dB are to
3. The impairment then should be calculated by be combined. The dosimeters used for this TLV also
multiplying the percentage of the better ear by 5, must have a pulse range of 63 dB set on the 3-dB
adding this figure to the percentage from the exchange range. Obtaining instrumentation that
6-Noise ACGIH® © 2006
meets this re~uirement should not be a problem. more properly assessed. lnfrasound exposures
The IEC-804< l already requires a 63-dB pulse range (exposures below 20 Hz) will also be better
for Type I instruments. In addition, the major assessed. Such exposures are rare and, even if they
manufacturers of dosimeters in the United States all could occur at levels found in industry directly, they
have instruments with more than a 63-dB pulse are not likely to be dangerous to a person's hearing
range. or health.<62-=-a5J
It should be noted that the previous impulse The TLV does not address the case in which the
noise TLV was already based on equal energy; impulse exposure exceeds a C-weighted peak of
therefore, the major change is the combining of all 140 dB. In such cases, MIL-STD-1474c<5 should be
noise into one measure. used. The MIL-STD recommends that hearing
Support of this procedure comes from numerous protection be worn whenever exposures exceed 140
documents and standards. The revised ISO dB. In addition, guidance is provided for those
standard<5l uses this approach. The published draft situations in which double hearing protection {both
standard, ANSI S3.28 1986,<59l also has adopted this muffs and plugs) should be worn.
measurement approach.
Several European field studies also support the FETAL NOISE EXPOSURE
combining of impact noise with continuous noise. At
the Southampton meeting on this subject,<35l The significance of this note to the Noise TLV is
Passchier-Vermeer<29l presented data that indicated to alert workers and heath officials to the possibility
the possibility that equal energy may slightly of noise-induced hearing loss to the fetus. Especially
underestimate the combined effect, especially for 8- of concern are those situations in which the hearing
hour criteria levels above 90 dB. The majority of the of the pregnant female is shielded by hearing
researchers did not see the need for adjusting for protection that is better than the natural protection
the combined effect; however, the revised ISO-R- afforded the fetus. Thus, the fetus is assumed to be
1999.2<5l states in note three: "The prediction more at risk of hearing loss than the expectant
method presented is based primarily on data mother. This natural protection or sound isolation is
collected with essentially broad-band steady non- similar to a good hearing protector and consists of
tonal noise. The application of the data base to tonal the sound attenuation of the abdomen and the lack
or impulsive/impact noise represents the best of middle ear function. However, this may not
available extrapolation. Some users may, however, provide as much protection for impulse noise.
wish to consider tonal noise and/or impulsive/impact Therefore, the Documentation is separated into two
noise about as harmful as a steady non-tonal noise general discussions, one for the time-weighted
that is approximately 5 dB higher in level." average (TWA) recommenda-tion of 115 dBC and
Because the TLV is an 8-hour criterion level of one for the peak recommen-dation of 155 dBC.
85 dB, such a correction was not used nor is such a
correction recommended. The 8-Hour TWA C-Weighted Level
The selection of 140 dB as a ceiling limit for of 115 Decibels
unprotected ears remains a reasonable level. This
Background
level was reviewed by the working group that
prepared ANSI S3-28. After this review, the working There has been concern for some time that the
group recommended the continuation of this limit. fetus may be overexposed to damaging noise. In
The key research for that limit was that of Wardcsoi 1982, CHABA published a report<55l that reviewed
and Price.<51 l A report<52l by the Committee on the data known at that time. One of the conclusions
Hearing and Bio-Acoustics (CHABA) of the National was that" ... There is no conclusive evidence of
Research Council also suggests this level as the detrimental effects of high-intensity external sound in
breakpoint above which the CHABA criteria of 1969 higher mammals." The report also showed that the
should be used. natural protection is probably adequate for most
The use of a C-weighted peak resolves a long- industrial noise. However, the report went on to
standing problem with measurement of the peak. recommend
The term "unweighted peak" is undefined. Without "... Until better information is available,
specifying the low end cutoff frequency of the however, it would appear prudent for pregnant
measurement devices, different measurement women to avoid exposures of long duration (several
devices could vary greatly. For example, an hours per day) to sounds of 90 dB and above ... "
innocuous car door slam might cause a 140-dB This latter statement is not well supported and
unweighted peak on some instruments but not on undoubtedly errs on the side of excessive caution.
others. Use of C-weighting defines the frequency The problem with such caution is that pregnant
response of the instrument and eliminates very low women might be unnecessarily deprived of their
frequency impulses and sounds. The C-weighting livelihood. Since 1982, there have been additional
discounts such sounds. Thus, the low frequency investigations on this subject, none of which has
pulse that comes from closing a car door or other proven entirely conclusive. Pierson<57l provides a
such innocuous very low frequency pulses can be summary of these investigations. Three retro-
ACGIH® © 2006 Noise - 7
spective studies have looked at the hearing level of for 500 Hz, 89 dB for 1000 Hz, 80 dB for 2000 Hz,
children whose mothers were noise-exposed during 77 dB for 3000 Hz, 75 dB for 4000 Hz, and 77 dB for
pregnancy.<5a-7oJ None of these had a sufficient 6000 Hz.
sample size or made use of proper research Using the sound isolation values of Gerhardt et
methodology, but they did show hearing levels a1.,<75l the abdomen provides at least 25 dB of
indicative of some effect.<57•71 l attenuation at 500 Hz, 35 dB at 1000 Hz, and well
Several key animal studies have experimented over 40 dB at 2000 Hz and above. The sound
with pregnant sheep. Dunn et a1.<72l used 18 isolation data from Gerhardt et al. <75l show the
pregnant ewes. Nine of these ewes were exposed to reduction of sound from air to the fetal inner ear.
pink noise 4 hours/day, 5 days/week at an overall Subtracting these values from 115 dB results in
level of 130 dB for approximately the last half of their levels below the threshold of injury levels, even if the
pregnancy. Nine ewes were used as controls and occupational noise was a tone rather than
followed the same regimen, including sitting in the broadband noise. The 115 dB limit is also lower than
reverberant noise exposure room without the noise the TWAs of the animal exposures described earlier.
source for 4 hours/day. Hearing of the lambs born of Specifically, 120 dB for 16 hours<73,74l is like an 8-
both the noise-exposed and control animals was hour TWA at 123 dB, and the 4-hour exposure at
evaluated by recording auditory brainstem 130 dB<72l is like an 8-hour TWA at 27 dB.
responses (ABR) from scalp electrodes when the C-weighting the noise measurements is recom-
lambs were between 30 and 40 days of age. The mended rather than A-weighting because, just like
ABRs for both groups were found to be in the range the attenuation of most hearing protectors, the atten-
of normal thresholds. The lambs were then uation of the abdomen declines as the frequency
sacrificed, and examination of the cochleae showed decreases. In fact, as with hearing protection, this
that approximately half of the 18 exposed ears had de-cline is approximated by the inverse of the A-
morphological anomalies, while approximately one weighted curve for frequencies below 1000 Hz.
quarter of the control ears had the same anomalies. Thus, to estimate a safe A-weighted level at the ear
Dunn et al. concluded that " ... Even though the of the fetus, the C-weighted TWA should be used.
exposed ears exhibited almost twice the number or This same concept is used in calculating the NRR
morphological anomalies found in the control group, for hearing protectors_<77l
we are reluctant to interpret these findings as being
directly related to noise exposure since they The Peak Exposure of 155 dBC
occurred in both groups. We are inclined to interpret
them as being postmortem fixation artifacts. On the Background
other hand, the higher incidence of cellular The 155 dBC recommendation is supported by
anomalies in the cochleae of the noise-exposed the study of Gerhardt et al. <75l Eleven pregnant
lambs than of the controls could not be sheep at gestational day 127 were exposed to 20
explained."<72l impulses using a shock tube 4 feet from the sheep.
The other animal studies include a series on the With the sheep removed, average peak noise levels
exposure of pregnant ewes by Griffiths et a1.<73l and of 169.7 dBC were measured at the position of the
Pierson et a1.< 74l In contrast to Dunn et a1.,< 72l these fetus. Using a hydrophone within the uterus, the
studies evaluated the effect of a single 120 dB differences in attenuation between the air and the
broadband noise for 16 hours on the ABR thresholds uterus varied between 2 dB and 20 dB. Slight
of in utero fetal sheep. Griffiths et a1.< 73l showed elevations of evoked potential threshold were noted
temporary changes in the ABR thresholds when the for low-frequency stimuli. Scanning electron
gestation age was from 126 to 134 days at the time microscopy revealed damage to hair cells in the
of exposure (average gestation period is 145 days). middle and apical turns of the cochlea.
In a follow-up experiment, Pierson et a1.,<57,74l The 155 dBC TLV-Ceiling was derived using
showed some small permanent changes in the ABR two approaches. The first apwoach assumed, during
thresholds when the exposure to the same 120 dB the study by Gerhardt et al.,< 5l that the average
for 16 hours noise occurred on day 113 of gestation. attenuation between the air measurements and the
The high exposure levels of these studies gave fetal head was 11 dB ([2 + 20]/2). This would imply
support to the belief that one does not need to be that 158. 7 dBC (169. 7 - 11) at the head of the fetus
overly concerned with fetal noise exposure. could result in damage. However, the worst-case
However, these studies also suggested that some situation of the fetal head next to the surface of the
upper limit of noise exposure should be set. abdomen could result in such hair-cell injury
The TWA recommendation of 115 dBC as an occurring 9 dB lower or at a peak of 161 dBC
upper exposure limit is based on the assumption that because of reduced attenuation. Thus, injury to the
the ears of the fetus should be protected at least as hearing of the fetus could occur at a peak of 160.7
well as an adult. Thus, the sound levels at the dBC (158.7 + 2) as measured outside the abdomen.
cochlea should not exceed the threshold of injury Because there is only one experimental point, the
levels used in ISO-R-1999.2<5l or ANSI S3.44- threshold of injury is difficult to predict. However,
1995<75l recommendations. These levels are 93 dB reducing the peak pressure by at least a factor of 2

8-Noise ACGIH® © 2006

(or 6 dB) appears reasonable. This results in an Thus, 70 dBA is the safer level when nothing is
estimated peak exposure limit of about 155 dBC. known about the spectrum of the noise.
The second approach is to adjust the current When there is a steady background noise, there
peak (TLV-C) of 140 dBC for impulse noise by a can be other noise exposure, such as the sound
reasonable estimate of the amount of protection from communication headsets, that can also cause
afforded by the abdomen and the lack of middle ear TTS. The background level can affect the recovery
function. As shown in the previous approach, the of this TTS. Research has shown that TTS recovery
womb may provide as little as 2 dB of attenuation; can be hampered by background octave band levels
thus, the peak limit of 140 dBC can be raised to 142 from 65 to 80 dB.caor Ward called this level required
dBC to account for the attenuation of the womb. The to prevent the delay TTS recovery as "Effective
lack of middle ear function in the fetus results in Quiet". If an A- weighted level is used, the
attenuation values that range from 10 to 40 dB at suggested level to allow recovery is an A-weighted
frequencies of 125 Hz to 2000 Hz.<75> This would level of 75 dB. However, for noise with strong tonal
indicate that a limiting level of noise impulses in air components, "Effective Quiet" may need to be at
could be anywhere from 152 (142 + 10) dBC to 182 least as low as 70 dB. "Effective Quiet for an octave
(142 + 40) dBC. The 152 dBC is a worse-case band of noise at 4000 Hz starts at 65 dB, however,
estimate for both frequency and fetal position. Thus, the situation in which all the sound energy is only in
a slightly higher value seems reasonable. The 155 this octave band is expected to be rare. The
dBC TLV-C, measured at the abdomen, is close to committee believes that the 70 dBA is the proper
the lower end of this range and is considered to be a level.
reasonable estimate of what the limited available For the above reasons the committee has
data suggests is necessary to protect the hearing of recommended an A-weighted level of 70 dB or
the developing fetus. below as the background noise exposure for
sleeping and relaxation for situations in which the
EXTENDED EXPOSURES work time extends beyond 24 hours.

Allowing the daily dose to exceed unity if the OTOTOXICITY

weekly exposure is less than 5 is allowed under the
current international standard that relates noise Exposure to certain chemicals, either alone or in
exposure to noise induced hearing loss (ISO R- concert with noise, may result in hearing loss.
1999). In fact, the international standard allows a Certain chemical substances have shown some
daily fraction to be 10 times (10 dB) the yearly ototoxic effects but may not be ototoxic in typical
average. The proposed note allows much less daily occupational settings. Since the exposure threshold
variability and is more protective. Based on an 8- where such ototoxic effects is not known, the only
hour level of 85 dBA, the factor of 3 could equate to reliable way to know if the substance is affecting the
a level of 90 dBA for 1 day, 82 dBA for 4 days, and hearing of exposed workers is to take audiograms.
less than 80 dBA for 2 days. While audiometric data is useful for any worker
The noise exposure guidelines depicted in Table exposed to any measurable level of a chemical
1 assume that after a work shift, the worker would be substance, yearly audiograms are highly
able to relax and sleep at home for a time such that recommended for workers whose exposures are at
if any temporary shifts in hearing did occur during 20% or more of the TLV for the substance in
the time at work, these changes would recover question. This 20%, while somewhat arbitrary, will
during the time the worker was away from work. This ensure data from sub-TLV exposures. If a worker is
assumption is most likely violated when the worker is currently participating in a hearing conservation
confined to a set of spaces such as a ship, space program due to excessive noise, the reviewers of the
station, jail cell, or similar situation. Studies of long audiometric data should be alert to possible
duration broadband noise exposures have shown synergistic effects between exposure to noise and
that temporary changes in hearing or Temporary the chemical substance and, if necessary, suggest
Threshold Shifts (TTS) can occur at levels of 80 reducing the exposure to one or both.
dB.<79> At 75 dB this does not occur. Long duration A summary of the evidence for ototoxicity of the
studies have also shown that after 8 to 16 hours of substances listed in Note 2 follows.
exposure, TTS stops increasing. This behavior is
interpreted to mean that if TTS has not occurred by
Trichloroethylene
the time a persons been exposed for 24 hours to
some level of steady noise, then TTS will not occur A study<51 > conducted in 1976 demonstrated
regardless of the duration of the noise exposure. For bilateral, high frequency sensori-neural hearing loss
this reason, the level of 75 dBA is considered the in 26 out of 40 workers exposed to excessive
safe level for causing hearing loss for long duration concentrations (above international recommended
broadband noise. If the noise is tonal in nature, this values at that time) of trichloroethylene (TCE).
level needs to be reduced by as much as 5 decibels. Cases with previous or current noise exposure were

ACGIH® © 2006 Noise-9

excluded from the study. Another study,!52l on the hearing losses resulting from causes other than
health status of populations exposed to trichloro- exposure to noise_!57l Nevertheless, 7 of 18 workers
ethylene {TCE) through contaminated water {n = displayed abnormal results in central auditory
4281) indicated a significant increase in reported system testing. Styrene and noise exposures were
hearing losses for the 0 through 9 years of age meticulously assessed for 299 workers in the fiber-
group. No human study has investigated occupa- reinforced industry. !55l Noise levels were found to be
tional TCE exposures for ototoxicity at the current in the range between 85 and 90 dBA, while styrene
TLV, either alone or in concert with noise. levels were generally below the recommended level
of 50 ppm. The association between noise
Carbon Disulfide exposure, based on the developed lifetime noise-
dose estimate, and hearing loss was significant.
Three studies were conducted with workers from That was not the case for styrene exposure. Styrene
viscose rayon plants, where exposure to carbon exposure approached significance for hearing loss
disulfide constitutes its main hazard_(53-85 l The only at some specific frequencies.
Sulkowski study!53l of workers exposed to both noise Styrene may not be ototoxic at the current TLV
and carbon disulfide showed that a larger percent- of 50 ppm, but since no human study investigated
age of the workers had been poisoned with carbon this level alone or in combination with noise, styrene
disulfide, than merely exposed to carbon disulfide, should be considered potentially ototoxic.
and incurred sensori-neural hearing losses. A large
percentage of these had retrocochlear losses.
Toluene
Morata!MJ studied workers exposed simultaneously
to carbon disulfide at 29 ppm and noise between 86 A review paper!59l on the ototoxicity of toluene
and 89 dBA. Levels for both agents were excessive. discusses the abuse of toluene {from glue sniffing)
Carbon disulfide-exposed workers had worse and cites four references showing dramatic hearing
hearing than those ex~osed to noise alone. The loss originating from the central auditory pathways.
study by Hirata et al? l was conducted with workers In another study,!90l an accidental acute exposure to
currently exposed {exposure duration,> 240 months toluene produced hearing loss in half of the exposed
[n=34]), current exposed {exposure duration, 24-84 {N=6).
months), and former workers {exposure duration, > A study of 190 workers!91 l was carried out with
120 months). Exposure to carbon disulfide ranged rotogravure printing workers. The hearing and
from 3.3 to 8.2 ppm, averaging 4.76 ppm. The balance functions of a group of printers exposed
Industry exhaust ventilation apparatus had under- simultaneously to noise (88-98 dBA) and toluene
gone major improvement 14 years before the (100-365 ppm) were compared with a group of
investigation was conducted. Thus, workers with printers exposed to noise alone (88-97 dBA), a
exposure duration longer than 14 years were ex- group exposed to a solvent mixture in which toluene
posed to undetermined higher levels than the ones was the major component, and a group neither
reported. Auditory brainstem responses were signifi- exposed to noise nor toluene. The adjusted relative
cantly altered only for the group with the longest risk estimates for hearing loss were 4 times greater
exposure {> 20 years) and a history of excessive for the noise group, 11 times greater for the noise
exposure. The results suggested that chronic and toluene group, and 5 times greater for the
exposure to carbon disulfide in humans affects the solvents group. Acoustic reflex measurements
ascending auditory tract in the brainstem_!55l suggested that the hearing losses found in the group
Carbon disulfide may not be ototoxic at the exposed to both agents might be due to lesions in
current TLVof 10 ppm; however, since Hirata did not the central auditory system.
report the values workers were exposed at before The effects of toluene on the auditory system
the ventilation system was improved, and no other was studied in a group of rotogravure printers
human study investigated this level alone or in through the use of auditory brainstem responses
concert with noise, these data indicate that carbon (ABR).!92l Forty workers with normal hearing ability
disulfide is potentially ototoxic. {assessed by pure tone audiometry), who had been
exposed at an average of 97 ppm styrene for 12 to
Styrene 14 years, were selected to participate. Their results
were compared with those from a group of workers
Workers exposed to low levels of styrene did not of the same age but not occupationally exposed to
appear to have increased age-dependent hearing solvents. The study indicated that exposure to tol-
loss at high frequencies_!55l However, a comparison uene was able to induce a statistically significant
within the group of exposed workers between the alteration in the evoked responses, visible for all
least exposed and the most exposed {limit- waves and all the intervals studied. The auditory
normalized exposures ranging from 14 to 106 ppm), brainstem responses demonstrated auditory nervous
in hearing thresholds at high frequencies. Routine system modifications before the occurrence of neu-
hearing tests of workers exposed to styrene {levels rological clinical signs due to chronic exposure to
not reported) in a plastic boat plant did not indicate toluene.

10-Noise ACGIH® © 2006

 Another study on rotogravure printers!93l noted intoxication from peeling lead paint or dust brought
an association between hearing loss and the bio- home from parents had their auditory systems
logical marker (of hippuric acid in urine, normalized examined through ABR.!95l The results of children
to the creatinine levels) for exposure to toluene. The with elevated blood lead levels showed that the
odds-ratio estimates for hearing loss were 1. 76 latencies of waves Ill and IV increased linearly with
times greater for each gram of hippuric acid per blood lead level (range, 6-59 g/dl), indicating a
gram of creatinine. Urinary hippuric acid of 2.5 gig slowing of auditory nerve conduction velocity due to
creatinine was first recommended by ACGIH in 1984 lead exposure. A 5-year follow-up of the children
as one of the Biological Exposure Indices (BEls) for with low to moderate lead exposures (range, 6-30
occupational exposure to toluene at a TLV-TWA of g/dl) showed persistence of the prolonged latencies
100 ppm_!94 ,95l ACGIH revised the BEi to 1.6 grama at repeat testing_!95l Audiometric data were obtained
of hippuric acid per gram creatinine when TLV-TWA by the Second National Health and Nutrition
was lowered to 50 ppm_!94l Field studies conducted Examination Survey on 5717 children, most of whom
during the 1970s and 1980s indicated that hippuric had blood levels measured?9l The probability of
acid levels correlated well with occupational expo- elevated hearing thresholds increased significantly
sure to toluene at air concentrations of 100 ppm_!95l with increasing blood lead levels ~range, 0-50 g/dl).
Due to its high background levels in many countries Auditory brainstem responses! 100• 01 l and auditory
(1 to 1.5 g), hippuric acid is no longer considered as event-related potentials! 102l were recorded in
a good biological marker for occupational exposure workers occupationally exposed to lead who had
to toluene below 50 ppm. It is still, however, recom- their blood lead levels monitored. Discalzi et a1.! 101 J
mended as an easy-to-analyze biological marker for divided workers into two groups by lead
exposure to toluene when nonoccupational back- concentration: above or below 50 g/dl. In the Araki et
ground levels of toluene are low?6l In the printers al. study,! 102l blood lead concentration ranged from
study, low hippuric acid levels were observed for the 12 to 59 g/dl (median 30 g/dl). Blood lead levels
majority of the group studied, which had no or little were significantly correlated with abnormalities in the
occupational exposure to toluene (52% with ≤ 0.5 g recorded evoked potentials.! 100-102i
hippuric acid/g creatinine; 75% with 1 g/g or less).
The authors stated that it provided valuable informa- Mercury
tion on occupational exposure.
At the previously recommended BEi of 2.5 g Although less pronounced than the alterations
hippuric acid/g of creatinine (corresponding to a caused by lead exposure, significant alterations in
TLV-TWA of 100 ppm), the odds ratio for hearing the ABR were also observed in the case of
occupational exposure to mercury.! 101 l Mercury
loss estimated in the present study is 4.4 (odds of
1. 76 per gram x 2.5 grams of hippuric acid /gram of intoxication has been associated with hearing
loss.!103l Gerstner and Hutr104l found in mercury-
creatinine = 4.4; 95% confidence interval [Cl] =
2.50-7.45). poisoned clinical patients that hearing loss develops
The current ACGIH TLV–TWA of 50 ppm is one early in mercury poisoning, extends over nearly the
of the lowest recommended international limits for entire frequency range, and results in difficulty in
toluene, !97l and even in this concentration, the understanding speech when other voices are
present. Autopsies of these patients showed no
estimated odds ratio is greater than 2. The U.S.
Occupational Safety and Health Administration evidence of cochlear damage, an indication of more
(OSHA) set a permissible exposure limit 4 times the central lesions.
TLV recommended by ACGIH (i.e, 200 ppm TWA). Up to 80% of patients treated for the fatal
The U.S. National Institute for Occupational Safety Minimata disease (due to ingestion of mercury
and Health (NIOSH) recommends a recommended contaminated food and water) suffered hearing loss.
exposure limit of twice that of ACGIH (i.e., 100 ppm Long-term follow-up studies have been
TWA)_!97l Each of these exposure limits may be reported.! 105•106l Twenty-eight percent of the patients
adequate for preventing a series of health outcomes, retested showed deterioration of hearing, while 7%
showed an improvement. Bekesy audiometry and
but none of them seem to be adequate for preven-
ting hearing loss. On the other hand, there is a pos- the Short Increment Sensitivity Index indicated that
sibility that peak, nontrivial exposures to solvents the early and middle stages of mercury intoxication
may be contributing considerably in causing the may have resulted from cochlear lesions, whereas
losses. Thus, a lowering of limit-normalized levels hearing impairments in late stages may have
resulted from retrocochlear lesions.!105J Brain
might not eliminate the risk. More research is
needed on solvent-induced hearing loss, both to autopsy studies of the mercury intoxicated patients
address the adequacy exposure limits and the showed demyelination in the temporal lobes and
ototoxicity of toluene at these levels. heavy deposition of heavy metals in the transverse
temporal gyri.!106l
Although mercury is ototoxic, it appears that this
Lead
happens mainly in cases of acute poisoning, and it is
Children considered to be at risk for lead accompanied by other neurological symptoms.

ACGIH® © 2006 Noise-11

Manganese 4. International Electrotechnical Commission:
Integrating-Averaging Sound Level Meters. IEC 804.
Altered hearing and balance functions have IEC, New York (1985).
been reported in a study that examined workers 5. U.S. Department of Defense: Noise Limits for Military
exposed to manganese alone or in concert with Materiel (Metric). MIL-STD-1474C. U.S. DOD,
noise.< 107> Pure-tone audiograms of manganese Washington, DC (1991).
exposed workers were affected in both low and high 6. International Organization for Standardization:
frequencies. Manganese ototoxicity appeared to be Acoustic Determination of Noise Exposure and
Estimation of Noise Induced Hearing Impairment.
accelerated and exacerbated by noise exposure. ISO-R-1999.2. ISO, Geneva (1990).
7. Jones H: Industrial Noise and Hearing Conservation,
Arsenic Chap. II. Olishijski J; Harford E (Eds.). National
Safety Council, Chicago (1975).
An epidemiologic study was conducted on a 8. U.S. National Institute for Occupational Safety and
population of children living by a plant responsible Health: Criteria for a Recommended Standard:
for an emission of arsenic in the air.< 108•109 Analysis Occupational Exposure to Noise. DHEW (HSM) Pub.
of the children's hair, blood, and urine revealed No. 73-11001. In: NIOSH Criteria Documents Plus
elevated arsenic content. Arsenic concentrations CD-ROM. DHHS (NIOSH) Pub. No. 97-106; NTIS
ranged from 0.01 to 10.33 g/g in hair and from 0.001 Pub. No. PB-502-082. National Technical Information
to 0.1050 mg/g in urine. Significant hearing losses Service, Springfield, VA (1997).
were observed in the low frequencies of the audio- 9. Ad Hoc lntersociety Guidelines for Noise Exposure
Control: Am Ind Hyg Assoc J 28(5):418-424 (1967).
gram when they were compared with unexposed
10. Botsford JH: Simple method for identifying
children. Current ACGIH BEi for Arsenic is 35 g acceptable noise exposures. J Acoust Soc Am
arsenic/g urine. 46:418 (1967).
Although data indicate that arsenic is potentially 11. Passchier-Vermeer W: Hearing loss due to exposure
ototoxic, no studies have been conducted on the to steady-state broadband noise. Report No. 35.
effects of occupational exposure to arsenic and Research Institute for Public Health Engineering,
ototoxicity. Delft, Netherlands (1968).
12. Cohen A; Amticaglia JR; Carpenter P: Temporary
threshold shift in hearing from exposure to different
Carbon Monoxide noise spectra at equal dBA level. J Acoust Soc Am
Cases of accidental carbon monoxide poisoning 51 :503 (1972).
which caused severe neurologic, psychiatric 13. American Academy of Ophthalmology and
Otolaryngology: Guides for evaluation of hearing
symptoms and hearing impairments that partially
handicap. Otolaryngol Head Neck Surg 87:539-551
improved with time were reported.< 110 -111 > A 78% (July-August 1979).
prevalence of sensori-neural hearing loss among 14. American Academy of Ophthalmology and
700 cases of carbon monoxide intoxication was Otolaryngology: Guides for the evaluation of hearing
observed.< 112> Auditory brainstem responses were impairment. Trans Am Acad Ophthalm Otolaryngol
studied in 32 patients with acute carbon monoxide 63:236-238 (1959).
poisoning.< 113> The abnormalities observed were 15. American Academy of Ophthalmology and
divided into two patterns: a peripheral pattern of Otolaryngology: Guides for the evaluation of hearing
prolongation of wave I latency without the impairment: ear, noise, throat and related structures.
JAMA 177(7):489--501 (1961 ).
prolongation of interpeak latency (6 cases), and a
16. Suter AH: Speech recognition in noise by individuals
central pattern of prolongation of latencies for all with mild hearing impairment. J Acoust Soc Am
waves and interpeak latencies (2 cases). The 78:887-900 (1985).
prevalence of ABR abnormalirx increased with the 17. American Standards Association: The relations of
duration of unconsciousness.< 13> hearing loss to noise exposure. Z24-X-2 Committee
Report. ASA, New York (1978).
TLV Chronology: Noise 18. Committee on Hearing and Bio-Acoustics: Some
damage risk criteria for exposure to sound. Draft of
Table 2 provides a summary of the development Technical Report. CHABA Working Group, St. Louis,
of the Noise TLVs (see page 13). MO (1956).
19. Eldred KM; Gannon WJ; van Gierke HE: Criteria for
short time exposure of personnel to high intensity jet
References
aircraft noise. WADC Technical Note 55-355. Wright-
1. American National Standards Institute: Specification Patterson AFB, OH (1955).
for Audiometers. ANSI S3.6-1989. ANSI, New York 20. Air Force Medical Services: Hazardous noise
(1989). exposure. FR 160-3. U.S. Dept. of the Air Force,
2. American National Standards Institute: Specification Washington, DC (1956).
for Sound Level Meters. ANSI S1.4-1983. ANSI, New 21. International Organization for Standardization: Noise
York (1983). rating numbers with respect to conservation of
3. American National Standards Institute: Specification hearing, speech communication and annoyance.
for Personal Noise Dosimeters. ANSI S1 .25-1991. ISO/TC 43. ISO, Helsinki (1961 ).
ANSI, New York (1991 ). 22. Kryter KD: Exposure to steady-state noise and

12-Noise ACGIH® © 2006

TABLE 2. History of the Noise TLVs
1968: Proposed CONTINUOUS
ƒ 85 dB average for 500, 1000, and 2000 Hz octave bands; narrow band correction; 92 dBA, 8-
hour TWA
IMPULSE
ƒ140 dB ceiling
1969: Adopted CONTINUOUS
ƒ 90 dBA, 8-hour TWA
ƒ 115 dB ceiling
ƒ 5 dB doubling rate
IMPULSE
_________________________ƒ140 dB ceiling --------------------------------------------------------------------------------------
1970: Adopted CONTINUOUS
_________________________ƒ
Continuous, 90 dBA, measurement threshold ------------------------------------------------------
1973: Proposed CONTINUOUS
_________________________ƒ85 dBA, 8-hour TWA; 5 dB doubling rate; measurement threshold 80 dBN16 hours ____________ _
1975: Adopted CONTINUOUS
ƒ85 dBA, 8-hour TWA
ƒ115 dB ceiling
ƒ5 dB doubling rate
ƒ80 dBA measurement threshold
Proposed IMPULSE
ƒ140 dB, ceiling; 3 dB doubling rate; 120 dB measurement threshold; 10,000 impulses at 120
___________________________ dB, 1000 at 130 dB, and 100 at 140 dB ----------------------------------------------------------
1976: Adopted IMPULSE
ƒ140 dB, ceiling
ƒ
3 dB doubling rate
ƒ120 dB measurement threshold
______ƒ10,000 impulses at 120 dB, 1000 at 130 dB, and 100 at 140 dB------------------------------------
1993: Proposed CONTINUOUS and IMPULSE
__________________________
ƒ85 dBA, 8-hour TWA; 140 dBC ceiling; 3 dB doubling rate; 80 dBA measurement threshold ___ _
1994: Adopted CONTINUOUS and IMPULSE
ƒ85 dBA, 8-hour TWA
ƒ140 dBC ceiling
3 dB doubling rate
ƒ
ƒ80 dBA measurement threshold
-------------------------------------------------------------------------------------------------------------------------------
1999: Proposed CONTINUOUS and IMPULSE
Note on fetal hazard*, 115 dBC 8-hour TWA, 155 dBC ceiling, 3 dB doubling rate, 80 dBC
measurement threshold
-------------------------------------------------------------------------------------------------------------------------------
2000: Adopted CONTINUOUS and IMPULSE
Note on fetal hazard
-------------------------------------------------------------------------------------------------------------------------------
2000: Proposed CONTINUOUS and IMPULSE
Note on extended exposures to
ƒ Allow TWA averaging over 7 days if any daily TWA < 90 dB
_________________________ƒ
Recommended rest and sleep time at< dBA. ______________________________________________________ _
2001: Adopted CONTINUOUS and IMPULSE
______________________ Note on continuous exposure -------------------------------------------------------------------------
2002: Proposed CONTINUOUS and IMPULSE
Revision to Note 2.
-------------------------------------------------------------------------------------------------------------
2006 Adopted CONTINUOUS and IMPULSE
Revision to Note 2.
* Measured at the pregnant worker's abdomen.

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