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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: 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.
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