Noise attenuation of earplugs as measured by hREAT and F-MIRE methods in a Japanese metal manufacturing plant.

OBJECTIVE The aim of this study was to compare the noise attenuation (NA) properties of earplugs by using the headphone-based real-ear-attenuation-at-threshold (hREAT) and field microphone-in-real-ear (F-MIRE) techniques. METHODS The subjects were 89 male workers (mean age: 44.8 ± 12.1 yr) exposed to noise above 85 dBA (mean noise exposure period: 14.3 ± 11.3 yr) in a Japanese nonferrous metal manufacturing plant. They were confirmed to have pure-tone air-conduction hearing threshold levels (HTLs). The hREAT and F-MIRE NA values were measured by a Rion AG-20A and a 3M E-A-Rfit, respectively. RESULTS The NA values could not be measured by hREAT for four workers (hREAT-group). The mean NA of the earplugs for subjects for whom hREAT measurements were possible (hREAT+ group) was 26.0 ± 10.0 dB. The NA of the earplugs could be measured for all subjects using F-MIRE, and the NAs of the hREAT- and hREAT+ groups were 9.5 ± 8.7 and 21.0 ± 7.3 dB. The mean HTL value at 500 Hz to 2 kHz was 45.8 ± 3.1 dB for the hREAT- group, which was significantly lower than the value for the hREAT+ group, 18.0 ± 8.6 dB. CONCLUSION Because there is a difference between the NA values obtained by hREAT and F-MIRE, it may be necessary to compensate for this difference. In addition, workers with hearing loss and the length of the measurement time need to be taken into consideration. Finally the F-MIRE method may be useful for educating workers about using earplugs in noisy workplaces.