Efficacy of hearing protection devices

The efficacy of hearing protection devices (HPDs) is evaluated by calculating the exposure level when hearing protectors are worn (EN ISO 4869-2 methods). In this standard, the amount of protection received is estimated by both the sound pressure levels to which the worker is exposed and the attenuation offered by the protector itself. This latter attenuation is determined following the REAT (Real-Ear Attenuation at Threshold) technique that evaluates hearing threshold levels on a subject with and without the HPD. According to the EN 24869-1 standard, 16 selected subjects fully informed of the proper HPDs wearing procedures (trained-fit) have to be tested. This method provides unrealistically high attenuation values which are not representative of those found in the majority of workplaces, where workers are not highly trained and motivated. Although these values are reported on the HPD label, as manufacturer's attenuation data, they should not be used in a reliable hearing conservation program. A better estimate of realistic attenuation may be provided by a test protocol applicable to subjects naive with respect to hearing protector use and training (subject-fit). Although scientifically validated and already published as standard (ANSI S12.6, ISO/TS 4869-5), this procedure is still not compulsory and not adopted by manufacturers. Meanwhile, derating schemes proposed by NIOSH and by OSHA may be applied for reducing the labeled attenuation data, in spite of their roughness. However another problem arises, especially when exposure levels are high or hearing loss is worsening: attenuation values statistically assessed do not permit definition of how well HPDs work on individuals. This problem may be worked out with a direct measure of the attenuation provided by a hearing protector. The MIRE (Microphone In Real Ear) technique, in fact, makes objective measurements of HPD attenuation through the use of either a miniaturized microphone, or a probe inserted in the ear canal. In any case, this technique is not easily applicable, as it requires appropriate instrumentation and particular experience by the experimenter. The protection individually obtained by HPD may be evaluated by using REAT technique on workers. Using loudspeakers, the background noise in the test site must be very low. An alternative system having a large volume headset as sound source was developed. It allows for individual fit testing in most environments and could be integrated into the annual audiometry program.