Epidemiologic Notes and Reports
Hearing Protectors: Field Measurements
In 1977 and 1981, the National Institute for Occupational
Safety
and Health (NIOSH) conducted field investigations to determine the
amount of noise reduction (attenuation) afforded to industrial
workers
who use earplugs. Tests of 420 workers at 15 industrial plants
indicated that 50% of the workers received less than half the
potential protection demonstrated in laboratory testing.
Earplug distributors label their products with noise-reduction
indexes based on data from standard audiometric laboratory tests.
Although earplugs can provide adequate protection from noise
hazards,
workers generally wear earplugs incorrectly; thus, distributors'
estimates may greatly exceed the actual protection of earplugs.
The field investigations included evaluations of five general
types of earplug design: twin-flanged (pre-formed in "small" and
"regular" sizes); single-flanged (pre-formed in five sizes);
acoustic
wool (two types made of user-formed cotton-like material, one with
a
pre-formed plastic shroud); custom-molded (two types, one vented
with
a "noise filter"); and expandable acoustic foam (two types
differing
only in color).
Twenty-eight workers who used the same type of earplug were
randomly selected at each plant. The attenuation provided by the
earplug was audiometrically measured for each worker and was then
plotted against the tested sound frequency (1,2). These results
were
compared with the results of previous laboratory tests of
attenuation
at the same frequencies, and, in most cases, revealed substantial
differences between the attenuation values recorded in the field
and
those recorded in the laboratory.
The noise reduction afforded each worker was calculated using
the
attenuation value at each test frequency and a typical industrial
noise spectrum adjusted according to a frequency contour (known as
"A-weighting") approximating the human ear response (Table 1).
Overall, the median reduction value was 13 decibels (dB) under
actual
working conditions versus 28 dB estimated from data provided by the
distributors.
Reported by Div of Biomedical and Behavioral Science, National
Institute for Occupation Safety and Health, CDC.
Editorial Note
Editorial Note: There are many reasons for differences between the
results of field and laboratory testing. Sizing, fit, and method
of
insertion are usually less than optimal in pre-formed and
user-formed
earplugs. Effectiveness of the custom-molded types depends on
preparation of the impression materials and fit of the final mold.
The expandable foam earplugs may not be inserted fully and are
often
not held in place to prevent slippage as they expand.
Noise-induced hearing loss is one of the most serious and
common
occupational diseases. More than three million workers wear
hearing
protectors in industrial environments where adequate engineering
controls are unavailable to reduce noise to acceptable limits. The
large differences between the laboratory-derived attenuation values
provided by distributors and actual attenuation in industrial
settings
should be considered by employers when choosing earplugs for their
employees. Workers can be endangered from excessive noise exposure
if
employers assume that workers will be protected to the extent
indicated by laboratory tests.
References
NIOSH. A field investigation of noise reduction afforded by
insert-type hearing protectors. (Pub #79-115): National
Institute
for Occupational Safety and Health, CDC, 1978.
NIOSH. A second field investigation of noise reduction afforded
by
insert-type hearing protectors, final report (#210-81-3001,
unpublished). National Institute for Occupational Safety and
Health, CDC, 1982.
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