Long-term noise exposures: A brief review

Occupational regulations are developed to conserve the hearing of a worker noise-exposed for 8 h per day. Workers often work longer shifts. In order to account for longer exposures, regulators have adopted variations of the equal energy hypothesis in which the ear is modeled as an integrator of acoustic energy over time. Thus time and energy can be traded to produce equal hearing loss. This results in the exchange rate. (A 24 h period is assumed to be the integration period. Thus each worker begins their exposure anew at the start of the work day.) U.S. occupational regulations limit the 8 h exposure of workers to noise of 90 dBA or less. With every doubling of exposure time, the exposure level must decrease by an exchange rate of 5 dB (CFR, 29 1910.95). The National Institute for Occupational Safety and Health (NIOSH) recommends that workers not be exposed to more than 85 dBA for more than 8 h per day with an exchange rate is 3 dB (NIOSH, 1998). Three dB corresponds to a doubling of acoustic energy. In an effort to provideworkers with safe noise exposure levels in excess of 8 h industrial hygienists utilize an equation in conjunction with a 5 or 3 dB exchange rate (see Appendix A). As an example, the NIOSH Criteria Document (1998) in Table 1e1 lists a Permissible Exposure Level of 80 dBA for 25 h 24 min. There are occupations and situations where workers may be in an environment in which exposures may last 24 h or even longer. Examples include air craft carrier crew, oil platform workers and over-the-road truck drivers