The Effectiveness of Applying Different Permissible Exposure Limits

Transcripción

Balachandar S. SAYAPATHI, et al.: Review of Different PELs for Preserving HTL
J Occup Health 2014; 56: 1–11
1
Journal of
Occupational Health
Review
The Effectiveness of Applying Different Permissible Exposure
Limits in Preserving the Hearing Threshold Level:
A Systematic Review
Balachandar S. Sayapathi1, Anselm Ting Su1,2 and David Koh3,4
Centre for Occupational and Environmental Health, University of Malaya, Malaysia, 2Department of Hygiene,
School of Medicine, Wakayama Medical University, Japan, 3Occupational Health and Medicine, PAPRSB Institute
of Health Sciences, University of Brunei Darussalam and 4SSH School of Public Health, National University of
Singapore, Singapore
1
Abstract: The Effectiveness of Applying Different
Permissible Exposure Limits in Preserving the Hearing Threshold Level: A Systematic Review;
Balachandar S. SAYAPATHI, et al. Centre for Occupational and Environmental Health, University of
Malaya, Malaysia—Objectives: A systematic review
was conducted to identify the effectiveness of different
permissible exposure limits in preserving the hearing
threshold level. This review compared the limits of the
US National Institute of Occupational Safety and Health
with those of the US Occupational Safety and Health
Administration. The prevalence of occupational noiseinduced hearing loss is on an increasing trend globally.
This review was performed to reduce the prevalence of
noise-induced hearing loss. Methods: We searched
3 major databases, i.e., PubMed, Embase and Lippincott Williams & Wilkins Journals@Ovid, for studies
published up until 1May 2013 without language restrictions. All study designs were included in this review. The
studies were identiﬁed and retrieved by two independent
authors. Results: Of 118 titles scanned, 14 duplicates
were removed, and a total of 13 abstracts from all three
databases were identiﬁed for full-text retrieval. From the
full text, eight articles met the inclusion criteria for this
systematic review. These articles showed acceptable
quality based on our scoring system. Most of the studies indicated that temporary threshold shifts were much
lower when subjects were exposed to a noise level of
85 dBA or lower. Conclusions: There were more
threshold shifts in subjects adopting 90 dBA compared
with 85 dBA. These temporary threshold shifts may
progress to permanent shifts over time. Action curtailing
noise exposure among employees would be taken
Received Jun 18, 2013; Accepted Sept 27, 2013
Published online in J-STAGE Nov 22, 2013
Correspondence to: B.S. Sayapathi, Centre for Occupational and
Environmental Health, University of Malaya, 50603 Kuala Lumpur,
Malaysia (e-mail: [email protected])
earlier on adoption of 85 dBA as the permissible exposure limit, and hence prevalence of noise-induced
hearing loss may be reduced.
(J Occup Health 2014; 56: 1–11)
Key words: Effects 85 or 90dBA, Noise, Threshold
shift
The progression to noise-induced hearing loss is
reliant on a few factors such as frequency, intensity
and duration of noise exposure1, 2). Cumulative and
repetitive exposure to high-intensity noise levels and
long duration of contact with noise levels beyond the
permissible exposure limit may aggravate the development of this occupational disease 3). Noise-induced
hearing loss is a slow and progressive disease. The
duration needed for an employee to develop this irremediable disease depends on several factors. Besides
being influenced by high noise levels, the degree of
hearing loss of an employee is dependent on the institution of a hearing conservation program in an industry. This kind of program utilizes various approaches
to curb noise exposure.
The gold standard for measuring hearing loss is
use of an audiometer. The pure-tone audiometer is
capable of detecting hearing loss at various frequencies ranging from 250 to 8,000 Hertz (Hz)4). Noiseinduced hearing loss shows few characteristic features
in an audiogram. Permanent shifts will be noted relative to baseline findings of the employees5). These
shifts usually occur in both ears and involve frequencies ranging from 3,000 to 6,000 Hz1, 3). There will
be a dip at these frequencies in early phases of the
disease with recovery at 8,000 Hz. In long-standing
disease, the lower frequencies would be involved too.
These features distinguish it from a condition known
as presbycusis, in which no recovery at 8,000 Hz is
2
noted.
According to the US Occupational Safety and
Health Administration (OSHA), which is a regulatory body, the permissible exposure limit for noise
is 90 dBA6). An employee should not be exposed
beyond this level for more than eight hours. As for
the National Institute of Occupational Safety and
Health (NIOSH), USA, which is a research body, the
exposure limit recommended is 85 dBA for the same
duration in hours6). There has been an increase in the
prevalence of occupational noise-induced hearing loss
globally. The occurrence of this occupational disease
has doubled from 120 million in 1995 to 250 million
in 20047). Millions of workers in the US are exposed
to noise levels above the permissible exposure limit,
90 dBA. Besides the US, Malaysia is also adopting
90 dBA8) as the permissible exposure limit. There
were a total of 663 cases of occupational diseases
investigated in Malaysia for the year 2010. From
this total, around 70% of the cases were diagnosed as
having noise-induced hearing loss, making it the most
common occupational disease9).
There is clear evidence associating noise intensity
and hearing loss. A hearing conservation program
is instituted once the noise level is above the action
level8), which is a noise dose of half the permissible exposure limit. We systematically reviewed the
evidence in order to compare the effects of 85 and
90 dBA on temporary threshold shifts to reduce the
prevalence of noise-induced hearing loss.
Methods
We considered observational studies (trials and any
other comparative design). We used medical subject
headings with the keywords “Effects 85 or 90 dBA
AND Noise AND Threshold shift” for all the databases searched. The outcome measures were temporary
threshold shifts on an audiogram.
Search strategy
We searched 3 major databases, i.e., PubMed,
E m b a s e a n d L i p p i n c o t t Wi l l i a m s & Wi l k i n s
Journals@Ovid, for studies published up until 1 May
2013. We applied no language restrictions. There
were a few inclusion criteria that needed to be
achieved for the papers to be included in this systematic review. The studies were required to include
exposure information regarding the noise source, such
as industrial or experimental noise, noise levels and
outcome of threshold levels. Initially the titles of the
papers were reviewed for applicability in this review.
Later, the abstracts were studied to find out further
whether the studies fulfilled the requirements for the
review. Finally, the full texts of the papers were
appraised to confirm that the contents met the require-
J Occup Health, Vol. 56, 2014
ments of this review. On evaluation of the full text,
once a paper was accepted for this review, information
was gathered from the paper such as the names of
authors, year of publication, country where the study
was conducted, the setting (experimental or industrial), number of participants or subjects, exposure
information including the sources of noise, duration
of exposure and usage of hearing protection devices,
the noise levels in dB and finally the outcome of the
hearing threshold level in the subjects. However,
unpublished works such as conference proceedings
were not included in this assessment.
Quality assessment
The scoring system used to assess the quality of a
paper was as depicted in Table 1. This scoring system
was a modified form of that of Su et al.10). The standards included in this scoring system were divided
into three major subheadings, i.e., population, exposure and outcome. Under the population subheading,
probability for bias among participants, nonrespondent bias and confounding variables were taken into
account, whereas under the exposure subheading,
control of confounding variables and use of effective
measurement tools to quantify exposure were recorded. Finally, the outcome subheading, use of effective
measurement tools to quantify outcome and appositeness of blinding were justified for the score. A paper
in this review was categorized as either acceptable or
poor quality based on the score gained. Acceptable
quality of a paper would mean that a score of 2 was
attained for each measurement of confounding variables, effective measurement to quantify exposure and
outcome. If any score was below 2 for these standards, a score of at least 2 was necessary for probability for bias on selection of participants or subjects
and for avoidance of nonrespondent bias standards.
Data extraction
We (Sayapathi BS and Su AT) approved and
retrieved studies independently based on the study’s
inclusion criteria. We then extracted data from the
included studies. These data later were gathered
depending on the country in which the study was
done, setting, number of subjects, exposure information, noise level and outcomes.
Results
After a thorough search by titles, we identified
a total of 47 pertinent articles through the PubMed
database and 47 articles from Lippincott Williams
& Wilkins Journals@Ovid. The search of the
Embase database identified 24 articles. Fourteen
duplicates found in Lippincott Williams & Wilkins
Journals@Ovid were then eliminated. There were a
3
Table 1. Scoring system for ranking the quality of papers
Scope Description
Population
I) Probability for bias in selected subjects
3
2
1
Survivor bias avoided
Volunteer bias avoided
Selection method not reported
II) Nonrespondent bias
2
1
Response rate ≥75%
Response rate <75% or not reported
III) Potential confounding variables between groups
2
1
NA
Potential confounders measured or both groups exposed to similar confounders
Potential confounders not measured
No comparison group
Exposure
IV) Confounding exposure controlled
2
1
Exposure confounders measured or both groups exposed to similar confounders
Exposure confounders not measured nor reported
V) Effective measurement tool used to quantify exposure
2
1
Exposure measured and reported
Exposure not measured nor reported
Outcome
VI) Effective measurement tool used to quantify outcome
2
1
Outcome measured and reported
Outcome not measured nor reported
VII) Blinding
2
1
Relevant blinding
No blinding done or reported
Judgment criteria
There were threats to the validity of a study if criteria III), V) and VI) were scored as 1. The quality of a study
was acceptable if these criteria were scores as at least 2. The scores for criteria I) and II) should be at least 2 if
criterion III) is scored as NA (not available) in order for the quality of a study to be considered as acceptable.
total of 36 relevant titles retrieved from the three databases. Of them, 20 relevant abstracts were obtained.
Finally, a total of 13 relevant full texts were retrieved
from these databases. After thorough assessment, it
was found that two similar papers from Lippincott
Williams & Wilkins@Ovid and three similar papers
from Embase had also been retrieved from PubMed.
Hence, after removing the five similar papers from
these databases, there were a total of eight papers that
fulfilled the requirements for this systematic review.
The process of identifying these eight articles is
shown in Fig. 1.
All eight articles in the current review had effective
measurement tools used to quantify noise exposure
and an instrument to measure outcome due to noise.
Only three studies had not controlled for confounding
bias, namely, those of McBride et al.11), Kvaerner et
al.12) and Chen and Tsai13). However, the three studies
had avoided selection bias, and the response rate of
respondents in these studies was at least 75%. Hence,
all eight articles showed acceptable quality based on
the scoring system as outlined in Table 2. Most of
the studies were carried out in the US. There was
also a study conducted in the east, in Taiwan by Chen
4
Fig. 1. Flow diagram of search strategy
a
Resources included Journals@Ovid Full Text, UM Library Full Text Journals &
LWW, EBM Reviews-ACP Journal Club (Cochrane Central Register of Controlled
Trials, Cochrane Database of Systematic Reviews 2005, Cochrane Methodology
Register, Database of Abstracts of Reviews of Effects, Health Technology
Assessment, NHS Economic Evaluation Database and ACP Journal Club), Biological
Abstracts, CAB Abstracts, EconLit, GEOBASE, ICONDA and Ovid MEDLINE
(In-Process & Other Non-Indexed Citations, without Revisions, Daily Update, Ovid
OLDMEDLINE).
b
Total number of papers searched for systematic review; 2 similar papers from
Lippincott Williams & Wilkins@Ovid were retrieved from PubMed; 3 similar papers
from Embase were retrieved from PubMed.
and Tsai13). In Europe, there were two studies, which
were conducted in Denmark and Norway, while one
study was conducted in the southwest Pacific, i.e.,
New Zealand. Therefore, in this review most of the
regions of the world were covered with respect to the
effects of different noise levels on hearing threshold
level, thereby eliminating effects of ethnic variability.
A summary of studies is shown in Table 3.
Stephenson et al. 14) conducted an experimental
study to identify the minimum noise intensity level
that would result in an asymptotic temporary thresh-
old shift. Below this level, one may be exposed to
noise for an unlimited period of time, since continuous exposure would not result in permanent damage
to hearing. This study was conducted among 12
volunteers who happened to be all males. These
volunteers had normal hearing threshold levels, 15 dB
or below at each frequency tested, before the study
was conducted. They were then exposed to noise
levels of 65, 70, 75, 80 and 85 dBA in a room using
a random noise generator. They were exposed to
different noise levels for 24 hours with a one-week
5
Table 2. Quality assessment of included papers in this review
Paper
Stephenson et al.,
1980
Mc Bride et al.,
2003
Mills et al., 1983
Yates et al., 1976
Kvaerner et al.,
1995
Chen and Tsai,
2003
Rubak et al.,
2006
Melnick, 1977
Avoid
Avoid nonselection respondent
bias
bias
Comparable
study and
control groups
Control for
confounding
bias
Valid
exposure
measures
Valid
outcome
measures
Blinding
Overall
quality and
validity
1
2
2
2
2
2
2
A
2
2
1
1
2
2
1
A
2
2
3
2
2
2
2
2
1
2
2
1
2
2
2
2
2
2
1
1
1
A
A
A
3
2
1
1
2
2
1
A
2
1
2
2
2
2
1
A
1
2
2
2
2
2
1
A
A, acceptable
interval. Hearing threshold levels were measured over
the left ear at various intervals upon exposure and
after exposure to noise. It was noted that the mean
asymptotic of threshold shifts more than two minutes
after exposure to noise were higher among subjects
exposed to 85 dBA, 10−13 dB at 4 kHz, compared
with those exposed to other noise intensities. These
significant changes were also noted for other test
frequencies among subjects exposed to 85 dBA, but
they were not evident when subjects were exposed
to less than 80 dBA. It was also found that upon
exposure to 85 dBA, the subjects tended to have more
threshold shifts, with the average for six frequencies
being 4.4 dB and the recovery period of these shifts
being prolonged. This was contrary to the results
for subjects exposed to 80 dBA, in which the mean
threshold shift was only 1.9 dB and the recovery
period was much faster. The investigators came to
the general conclusion that one may be exposed to
noise levels below 80 dBA for an indefinite period of
time without causing permanent changes in hearing.
This study also showed that the action level should
be 80 dBA for institution of a hearing conservative program and that the permissible exposure limit
should preferably be 85 dBA.
In New Zealand, McBride et al. 11) performed a
cross-sectional study in an engineering industry.
Ninety-two employees of a railway workshop took
part in this study. These employees were exposed to
levels either below 85 dBA, 85 to 90 dBA or more
than 90 dBA in the workplace. The noise exposures
of these subjects were recorded using a noise dosimeter. Baseline audiometry was performed for these
employees initially, followed by post-shift audiometry
using manual pure-tone audiometry. The standard for
the threshold shifts was based on the NIOSH’s criteria for a recommended standard on noise exposure.
Usage of hearing protection devices during the study
was not reported by the authors. A total of 50%
of the subjects had temporary threshold shifts upon
exposure to noise levels below 85 dBA compared
with 29% when exposed to noise levels between 85
and 90 dBA and 39% when exposed to noise levels
beyond 90 dBA. However, there were no statistically
significant differences in threshold shifts between
these three groups. In this study, no changes were
observed in occurrence of temporary threshold shifts
regardless of adoption of different permissible exposure limits. However, the lengths of time these workers had worked in this industry were not mentioned
by the authors. Moreover, the post-shift audiometry
was performed a day after the baseline audiometry;
noise in any industry usually fluctuates and hence,
a longer duration of exposure is usually required to
observe any significant changes in threshold shifts
upon adopting different permissible exposure limits.
Mills et al.15) conducted an experiment to identify
temporary threshold shifts among students after exposure to low-frequency noises. There were 52 students;
all participants in the study were male. Prior to the
study, their hearing threshold levels were within the
accepted range. There were primarily divided into
two groups, whereby the first group of students was
exposed to three octave bands centered at 63, 125
and 250 Hz at 84 dBA for a period of 24 hours, while
the second was exposed at 90 dBA for a period of 8
hours. Another group of students were exposed to
90 dBA at only 63 Hz for 8 hours. Noise was gener-
Experimental
Industry
Experimental
Experimental
Industry
Industry
Industry
Experimental
New Zealand,
McBride et al.,
2003
USA, Mills et al.,
1983
USA, Yates et al.,
1976
Norway, Kvarner
et al., 1995
Taiwan, Chen and
Tsai, 2003
Denmark, Rubak
et al., 2006
USA, Melnick,
1977
Setting
USA, Stephenson
et al., 1980
Country, authors,
date
n=9
n=649
n=384
n=13
(excluded one
ear)
i. n1=6 at 85
dBA
ii. n2=6 at 90
dBA
i. n1=7−8 each
at 84 dBA
ii. n2=7−8 each
at 90 dBA
i. n1=30
ii. n2=34
iii. n3=28
n=12
Study subjects
Exposure information
Noise generated through loudspeakers (24 hours)
All noisy trades (20 years and more)
Oil refinery (8 hours)
Global iron works (7 hours each day
for 3 days)
Noise generating equipment
(7 hours)
Noise generating equipment with
loudspeakers (24 hours for 84 dBA;
8 hours for 90 dBA)
Railway workshop (7 hours
13 minutes)
General Radio Model 1,382
Random Noise Generator (24 hours)
Noise source (duration of exposure)
Table 3. Summary of studies on different noise intensities
≤85 dBA
≤85 dBA
and>85
dBA
≤85 dBA
>85 dBA
≤85 dBA
and>85
dBA
≤85 dBA
and>85
dBA
≤85 dBA
and>85
dBA
≤85 dBA
Noise level
Exposure to 80 dBA:
i. 9.3 dB at 4 kHz
7.2 dB at 6 kHz
Exposure to 85 dBA:
i. 17.8 dB at 4 kHz
ii. 14.6 dB at 6 kHz
i. Exposure between 80 and 84 dBA, no statistically significant change,
OR 1.92 (95 % CI, 0.77 to 4.80)
ii. Exposure between 85 and 89 dBA, OR 3.05 (95% CI, 1.33 to 6.99)
Hearing loss (average hearing threshold level more than 25 dBA),
mean noise level was 81 dBA:
i. 9.6% - low frequencies (0.5 to 2 kHz)
ii. 38.3% - high frequencies (3 to 6 kHz)
a) Temporary threshold shifts, exposure between 85−90 dBA:
i. Median=3.8 dB (range=−10.0 to 16.7 dB), p=0.001 at 4 kHz
ii. Median=5.5 dB (range=−5.0 to 11.7 dB), p<0.001 at 6 kHz
b) Transient evoked otoacoustic emissions showing median=0.65
(range=−0.8 to 3.3 dB), p=0.001
a) Mean temporary threshold shifts (combined frequencies):
i. For 85 dBA - mean (SD)=6.26 (9.15)
ii. For 90 dBA - mean (SD)=10.13 (7.45)
b) No statistically significant difference in temporary threshold shifts
for individual frequencies.
Temporary threshold shifts:
i. 84 dBA−13, 9 and 9 dB at 63, 125 and 250-Hz
ii. 90 dBA−17, 14 and 18 dB at 63, 125 and 250-Hz
Recovery of threshold shifts:
i. 48 hours for 84 dBA
ii. 12 hours for 90 dBA
Temporary threshold shifts were not statistically significant among the
three groups (70.4−84.9; 85.1−89.8; 90.2−104.2), x2=2.39, p=0.24
Mean asymptotic threshold shifts of various noise intensities from six
frequencies (0.5−6 KHz):
85 dBA−4.4 dB
80 dBA−1.9 dB
75 dBA−0.73 dB
70 dBA−0.7 dB
65 dBA−0.06 dB
Outcomes
6
ated in a room using noise generating equipment with
loudspeakers. The threshold shifts were recorded an
average of four minutes after exposure to noise among
the subjects by using a Bekesy type of audiometer.
The temporary threshold shifts were found to be
increased among students exposed to 90 dBA by more
than 10 dB for all three conditions. Among students
exposed to 84 dBA, there were threshold shifts too,
amounting to more than 10 dB for all three conditions;
the threshold shifts increased under the 250 Hz condition but decreased in the 63 and 125 Hz settings after
12 hours of exposure. Crude estimation by comparing
the threshold shifts among subjects exposed to two
different noise levels, showed a more damaging effect
among those exposed to noise levels of 90 dBA;
that is, the subjects showed threshold shifts of 17,
14 and 18 dB for the 63, 125 and 250 Hz conditions
compared with 13, 9 and 9 dB for 84 dBA. Recovery
from threshold shifts took up to 48 hours for subjects
exposed to 84 dBA, since the duration of exposure
was longer, while those exposed to noise levels of
90 dBA took around 12 hours. This study had showed
that there were temporary threshold shifts at 84 dBA
but that they were comparatively worse when subjects
were exposed to noise levels of 90 dBA. The author
also reported that a median shift of 5 dB would be
produced if noise levels were between 78−80 dBA at
250 Hz. So it is advisable to have the action level at
80 dBA with the permissible exposure limit at 85 dBA
rather than 85 and 90 dBA for the action level and
permissible exposure limit respectively.
Yates et al.16) evaluated the damage risk of noise
exposure at 85 and 90 dBA. The damage risk was
evaluated by comparing temporary threshold shifts
when exposed to these two noise intensities. There
were six subjects in two groups, one group exposed
to 85 and the other exposed to 90 dBA. There were
equal numbers among genders in both groups. This
experiment was conducted in a room where noise was
produced by using either one of two noise generators
with four loudspeakers. All the subjects had normal
hearing threshold levels, which was prerequisite to be
selected for the study. Each half of the subjects in a
group was exposed to noise either for half a day or
a full day. The Bekesy type of audiometer was used
to measure threshold shifts ranging from 1 to 8 kHz
for both ears, with the measurements being performed
more than two minutes after exposure among the
subjects. The outcome of this study showed that the
mean temporary threshold shifts for the frequencies
combined more than two minutes after exposure were
higher on exposure to 90 dBA (mean [SD]=10.13
[7.45]) compared with 85 dBA (mean [SD]=6.26 [9.15])
among subjects exposed for a full day. This result
was found to be significantly different upon conduct-
7
ing Duncan’s multiple range tests. This test however
showed no statistically significant difference when
comparing the individual frequencies under the fullday exposure condition. It also revealed that there
were no significant differences between exposure to
85 dBA for a full day and 90 dBA for a half day.
This indicated that the damages occurred on exposure
to 85 dBA full-day was similar to that of 90 dBA halfday. Yates et al.16) also noted that the mean threshold
shift measured at various time intervals by individual
frequencies or frequencies together were similar to
those at two minutes after noise exposure. This study
emphasized that the effect on hearing upon exposure
to 90 dBA was far more detrimental than that of 85
dBA. Hence, based on this study, it was advisable to
adopt NIOSH’s recommended exposure limit.
In a paper published by Kvaerner et al. 12), the
authors evaluated temporary threshold shifts upon
exposure to noise in an industry. Thirteen employees
took part in this intervention study. These employees were tested and found to have normal hearing
threshold levels and tympanograms prior to selection
for the study. The subjects were exposed to noise
levels between 85 and 90 dBA. Their hearing levels
were examined for three days, and hearing protection
devices were used for the first two days. The hearing
levels were measured at 1 to 6 kHz using air conduction pure-tone audiometry, tympanometry and transient evoked otoacoustic emissions. Both ears were
examined, since the correlations between them were
low. The outcome showed that there were significant
threshold shifts at 4 kHz (p=0.001) with a median
value of 3.8 dB (range=−10.0 to 16.7 dB) and at 6
kHz (p < 0.001) with a median value of 5.5 dB (range=
−5.0 to 11.7 dB) when comparing before and after
work exposure. Consequently, there was a reduction
in amplitude in the otoacoustic emissions (median=0.65
dB, range=−0.8 to 3.3 dB, p=0.001). The results
showed that there were significant temporary threshold shifts when workers were exposed to noise levels
beyond 85 dBA. This study again supports the notion
that the permissible exposure limit preferably should
be at 85 dBA.
In Taiwan, Chen and Tsai 13) performed a crosssectional study on hearing loss among workers. The
authors included all the workers in an oil refinery, but
only 384 of them were selected for the study, as they
were exposed to noise levels above 80 dBA and had
no past history of chronic otitis media. All of them
happened to be males. The average noise exposure
among the subjects was 81 dBA. The left ears were
analyzed using audiometry in the range of 0.5 to
8 kHz. The workers were categorized as having hearing loss if their average hearing threshold levels were
more than 25 dBA between frequencies 0.5 to 2 kHz
8
and 3 to 6 kHz. The use of hearing protection devices during the study was not reported by the authors.
The results showed that 9.6% of the workers had
hearing loss at low frequencies compared with 38.3%
who had hearing loss at high frequencies. Most of
the threshold shifts were noted at 6 kHz among the
subjects. This study highlighted that the action level
should be 80 dBA, since slight hearing losses were
observed among subjects at a noise exposure below 85
dBA, and that the permissible exposure limit should
preferably be 85 dBA.
In Denmark, Rubak et al.17) performed a prevalence study on risk of noise-induced hearing loss.
The authors included 91 companies in the study,
which comprised of 649 workers from noisy environments and 104 residents as a reference population. Dosimeters were used to measure noise areas.
Questionnaires were distributed to extract information
such as duration of work, ear diseases and exposure to noise during relaxation hours. The hearing
threshold levels of the subjects were measured using
pure-tone audiometry by a qualified technician. If
the average hearing threshold levels were more than
20 dB from frequencies 2 to 4 kHz, then the subject
was considered to have hearing impairment. The
workers were wearing hearing protection devices.
The results showed that risk of hearing impairment
was statistically significant among subjects exposed to
noise levels between 85 and 89 dBA [OR 3.05 (95%
CI 1.33−6.99)]. These subjects worked for more than
20 years. The risk was mainly seen among subjects
working in construction and basic metals trades. The
subjects exposed to noise levels between 80 and 84
dBA and subjects who had worked less than 20 years
showed no significant changes in hearing impairment.
This finding indicated that if employees were exposed
to noise levels beyond 85 dBA and continued to be
exposed for many years, then the risk of hearing loss
among them was much higher. Hence, the NIOSH’s
recommended limit is far more appropriate based on
the outcome from this study.
Melnick18) performed an experiment on development
of temporary threshold shifts on exposure to noise
for 24 hours. This study was conducted among nine
subjects having normal hearing threshold levels that
were not more than 15 dB. Hearing threshold levels
were measured among subjects during three time periods: before, during and after exposure to noise. The
hearing threshold levels were as the average of the
levels at 8, 12, 16, 20 and 24 hours of noise exposure. All nine subjects were exposed to 80 and then
85 dB through loudspeakers, with the octave band of
the noise being centered at 4 kHz. The workers were
given hearing protection devices if they were exposed
high noise levels in the workplace. When the subjects
were exposed to 80 dBA, they developed asymptotic
threshold shifts of 9.3 dB at 4 kHz and 7.2 dB at
6 kHz, while at 85 dBA, the threshold shifts were
17.8 dB at 4 kHz and 14.6 dB at 6 kHz. Recovery of
the threshold when subjects were exposed to 85 dBA
took more than a day compared with those exposed
to noise levels of 80 dBA. This indicated that more
damaging effects were seen on exposure to noise
levels of 85 dBA. Hence, the action level should be
lower preferably at 80 dBA, as threshold shifts were
also noted at this noise intensity.
Discussion
We conducted a systematic review of sources of
evidence to compare adoption of 85 or 90 dBA for
preserving the hearing threshold. A total of eight
papers were found in our systematic search of the
three main databases. These studies were considered
to have acceptable quality.
Half of the studies in this review were experimental in design, increasing the implications with regard
to causality and outcome. According to DeVries and
Berlet19), the evidence from these experimental studies was of high quality. The other studies in this
review though were cross-sectional in design and were
appropriate for reporting on the occurrence of temporary threshold shifts, as they fulfill the aim of this
systematic review, which was to compare the effects
of 85 dBA and 90 dBA on temporary threshold shifts.
There was only one study 14) that used a blinding
process. However, all the studies in this review had
valid exposure and outcome measures. Probability
of bias on selection may be noted in two studies14,18);
however, the exposure and control groups were
comparable, and confounding bias was controlled for
in these studies. The search was not restricted to a
specific study design, since only limited numbers of
published studies were available. Any discrepancies
were resolved by consensus through discussion before
the studies were included.
All four experimental studies, those of Stephenson
et al.14), Mills et al.15), Yates et al.16) and Melnick18),
were conducted on human subjects. The subjects
were exposed to noise generated by equipment with
loudspeakers. The noises introduced were of a
continuous type. The other studies, those of McBride
et al.11), Kvaerner et al.12), Chen and Tsai13) and Rubak
et al.17), were conducted in an industrial setting in
which employees were exposed to a fluctuating type
of noise. The precise machines involved in generating
noise in these industries were not reported. Hearing
protection devices were used by the workers during
the studies of Kvaerner et al.12) and Rubak et al.17) but
use of hearing protection devices was not reported by
McBride et al.11) and Chen and Tsai13). There were
studies comparing 80 dBA and 85 dBA, such as those
of Stephenson et al.14) and Melnick18), and there was
also a study on subjects exposed to 81 dBA, that of
Chen and Tsai13). We included these studies to show
the detrimental effects of exposure to 85 dBA and that
the negative effects would be greater if employees
were exposed to noise levels reaching 90 dBA.
Our study recommends adoption of 85 dBA for
conservation of the hearing threshold. Most of the
studies indicated that the temporary threshold shifts
were much lower when subjects were exposed to
noise levels of 85 dBA or lower. However, the study
of McBride et al.11) showed that there were no statistically significant changes in temporary threshold shifts
among subjects exposed to less than 85, between 85
and 90 dBA and more than 90 dBA. These findings of McBride et al.11) may not be applicable, since
post-shift audiometry was done within a day and the
employment durations of the workers exposed to these
noise levels were not reported. The temporary threshold shifts among the employees may result in permanent shifts after continuous exposure to high noise
levels20). These permanent shifts may result due to
damage to hair cells in the organ of Corti2) which is
irreversible. The cochlea contains the organ of Corti,
which is embedded with sensory and non-sensory
cells. The sensory cells appear to be hair-like cells
referred to as outer or inner cells. The outer sensory
cells are not only more abundant but are also arranged
in three rows compared with the inner cells, which
are arranged in only one row5). The outer hair cells
are more prone to damage from noise and ototoxic
drugs. Prolonged exposure to sounds ranging 3,000
to 6,000 Hz may damage these cells1,3).
In the current review, there are four comparative
studies among subjects exposed to noise levels of 85
and 90 dBA. Of them, three studies showed that there
were more temporary threshold shifts with 90 dBA
than with 85 dBA. One of the studies showed that
when subjects were exposed to noise levels between
85 and 89 dBA, the OR was 3.05 (95% CI, 1.33 to 6.99),
whereas no change in threshold shifts was observed
on exposure to noise levels between 80 and 84 dBA17).
Another study, that of McBride et al.11), showed that
there was no statistically significant change in threshold shifts, as data collection was performed within a
day after baseline audiometry in an industry setting.
Our findings are consistent with a survey done
earlier in the US. According to the US NIOSH21),
the survey was conducted to estimate material hearing
impairment in industries over a 40-year working lifetime. Employees from 13 industries were surveyed
from 1968 to 1971. Material hearing impairment is
said to occur if the hearing threshold level is beyond
25 dBA at 1,000, 2,000, 3,000 and 4,000 Hz. Only
9
1,172 audiograms were taken for data analysis from
more than 4,000 audiograms collected, since the
others did not fulfill the inclusion criteria. The abnormal hearing levels of the employees and levels of
noise exposure were obtained through questionnaires
and audiometry assessment. Excess risk estimates
were then calculated; the difference between employees exposed to noise resulting in levels beyond the
maximum acceptable hearing loss and the unexposed
population exceeded these levels in percentage. The
excess risk estimates increased as duration of exposure
to noise increased among the population exposed to
noise levels of 85 and 90 dBA. Employees exposed
to noise levels of 85 dBA had excess risk estimates of
1.4 (95% CI, 0.3 to 3.2), 2.6 (95% CI, 0.6 to 6.0), 4.0
(95% CI, 0.9 to 9.3) and 4.9 (95% CI, 1.0 to 11.5) at
30, 40, 50 and 60 years old, respectively, for a duration of exposure to noise of between 5 to 10 years,
compared with those exposed to 90 dBA, who had
excess risks of 5.4 (95% CI, 2.1 to 9.5), 9.7 (95% CI,
3.7 to 16.5), 14.3 (95% CI, 5.5 to 24.4), and 15.9 (95%
CI, 6.2 to 26.2). For employees who were exposed
to noise for more than 10 years, the excess risk estimates for 85 dBA were 2.3 (95% CI, 0.7 to 5.3), 4.3
(95% CI, 1.3 to 9.4), 6.7 (95% CI, 2.0 to 13.9) and 7.9
(95% CI, 2.3 to 16.6) at 30, 40, 50 and 60 years old,
respectively, compared with those exposed to 90 dBA,
who had excess risks of 10.3 (95% CI, 5.8 to 16.2),
17.5 (95% CI, 10.7 to 25.3), 24.1 (95% CI, 14.6 to
33.5), and 24.7 (95% CI, 14.9 to 34.3). In summary,
at 60 years old, the excess risk estimates were 8% for
employees exposed to 85 dBA and 25% for employees exposed to 90 dBA.
It is claimed that subjects or employees exposed to
noise with 90 dBA considered the permissible exposure limit may experience a more damaging effect
on hearing compared with those exposed 85 dBA.
Temporary threshold shifts may result in permanent
shift of hearing thresholds over time if continuous
exposure to noise ensues20). According to Kryter22),
temporary threshold shifts that developed in his study
reflected one day’s exposure to noise. These temporary threshold shifts occurred two minutes after exposure to noise (TTS2). The author also stressed that
temporary threshold shifts at two minutes may result
in permanent threshold shifts if one is exposed to
noise continuously for a longer duration. It is hypothesized that permanent threshold shifts would occur
at about 10 years due to this continuous noise insult.
Kryter22) also revealed that if the temporary threshold
shifts at two minutes was beyond 40 dB, the recovery
period of threshold shifts were prolonged and development of permanent threshold shifts may ensue.
According to Lawton20), noise levels beyond 80 dBA
produced temporary threshold shifts that recovered
10
quickly upon cessation of the noise insult. Noise
levels at or below 80 dBA may not produce hearing
loss at 4,000 Hz, the frequency that most susceptible
to noise. Lawton also mentioned that employees
exposed to noise levels beyond 85 dBA over a period
of eight hours would acquire some degree of hearing
loss. Hence, it would be more appropriate to institute a hearing conservation program1) at 80 dBA in
which action can be taken to reduce noise exposure
among employees, also known as the action level, and
to adopt 85 dBA as the permissible exposure limit.
The findings from this systematic review also suggest
adoption of the recommendations of the US NIOSH
rather than the US OSHA’s occupational exposure
limit. The Japan Society for Occupational Health23)
has also recommended the permissible exposure limit
of the US NIOSH, that is, 85 dBA for a period of
eight hours.
Another experimental study24) has been conducted
on noise-induced temporary threshold shifts in Japan.
In the exposure group, ten subjects were exposed to
noise levels in between 65 and 86 dB for 24 hours.
The control group was not exposed to noise. The
hearing threshold levels were measured over the
right ear for both groups. There were no statistically
significant changes in threshold levels among the
control group. However, among the exposure group,
temporary threshold shifts increased as noise levels
increased. The threshold shifts at 86 dBA were below
10, 15, 25, 25 and 20 dB as compared with subjects
exposed at 83 dBA, the threshold shifts of which were
5, 12.5, 15, 20 and 15 dB at 2, 3, 4, 6 and 8 kHz,
respectively, after 24 hours of noise exposure. The
study of by Yates et al.16) showed that the damaging
effects on threshold shifts of subjects exposed to a
full day of noise at 85 dBA were equivalent to those
of subjects exposed to a half day of noise at 90 dBA.
Moreover, most of the detrimental effects were seen
at 4 kHz16,18).
Conclusions
There were more threshold shifts in subjects
exposed to 90 dBA compared with 85 dBA. These
temporary threshold shifts over time may lead to
noise-induced hearing loss, which is irreversible. By
adopting 85 dBA as the permissible exposure limit,
action to minimize noise levels from sources and
exposure among employees would be taken earlier.
Early action taken may reduce occurrence of temporary threshold shifts and hence reduce the prevalence
of noise-induced hearing loss.
Acknowledgments: This study was supported by a
Postgraduate Research Grant (Grant number PV106/
2011A) from the University of Malaya.
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The Effectiveness of Applying Different Permissible Exposure Limits

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