Basic And Applied Aspects Of Noise Induced Hearing Loss

In September 1985, NATO sponsored an Advanced Study WOl'kshop entitled, "Noise-Induced Hearing Loss: Basic and Applied Aspects." Tne meeting was held in a mountain retreat near Lucca, Italy and was attended by scientists, clinicians, and public officials from 12 countries. This was the third in a series of such conferences organized by the authors. The first two were supported by the United States National Institute of Occupational Safety and Health; their proceedings were published as "The Effects of Noise on Hearing" in 1976 and "New Perspectives on Noise-Induced Hearing Loss" in 1982. The Organizing Committee approached NATO because it was felt that the problem of noise was common to all industrialized countries and was an especially serious problem for the military. Thus, the NATO sponsorship and the Italian site of the meeting were part of the Organizing Committee's plan to obtain an international and thorough repr'esentation on the problem of noise-induced hearing loss. The NATO meeting and proceedings followed the format of the previous two symposia with an initial focus on the anatomical and physiological disturbances resulting from noise-induced hearing loss. This was followed by sections devoted to studies of a more applied nature involving general auditory performance in noise, issues associated with the establishment of noise-exposure criteria, nonauditory effects of noise, and the interaction of noise with other agents.

In September 1985, NATO sponsored an Advanced Study WOl'kshop entitled, "Noise-Induced Hearing Loss: Basic and Applied Aspects." Tne meeting was held in a mountain retreat near Lucca, Italy and was attended by scientists, clinicians, and public officials from 12 countries. This was the third in a series of such conferences organized by the authors. The first two were supported by the United States National Institute of Occupational Safety and Health; their proceedings were published as "The Effects of Noise on Hearing" in 1976 and "New Perspectives on Noise-Induced Hearing Loss" in 1982. The Organizing Committee approached NATO because it was felt that the problem of noise was common to all industrialized countries and was an especially serious problem for the military. Thus, the NATO sponsorship and the Italian site of the meeting were part of the Organizing Committee's plan to obtain an international and thorough repr'esentation on the problem of noise-induced hearing loss. The NATO meeting and proceedings followed the format of the previous two symposia with an initial focus on the anatomical and physiological disturbances resulting from noise-induced hearing loss. This was followed by sections devoted to studies of a more applied nature involving general auditory performance in noise, issues associated with the establishment of noise-exposure criteria, nonauditory effects of noise, and the interaction of noise with other agents.

Exposure to loud noise continues to be the largest cause of hearing loss in the adult population. The problem of NIHL impacts a number of disciplines. US standards for permissible noise exposure were originally published in 1968 and remain largely unchanged today. Indeed, permissible noise exposure for US personnel is significantly greater than that allowed in numerous other countries, including for example, Canada, China, Brazil, Mexico, and the European Union. However, there have been a number of discoveries and advances that have increased our understanding of the mechanisms of NIHL. These advances have the potential to impact how NIHL can be prevented and how our noise standards can be made more appropriate.

In the Occupational Safety and Health Act of 1970, Congress declared that its purpose was to assure, so far as possible, safe and healthful working conditions for every working man and woman and to preserve our human resources. In this Act, the National Institute for Occupational Safety and Health (NIOSH) is charged with recommending occupational safety and health standards and describing exposure concentrations that are safe for various periods of employment-including but not limited to concentrations at which no worker will suffer diminished health, functional capacity, or life expectancy as a result of his or her work experience. By means of criteria documents, NIOSH communicates these recommended standards to regulatory agencies (including the Occupational Safety and Health Administration [OSHA]) and to others in the occupational safety and health community. Criteria documents provide the scientific basis for new occupational safety and health standards. These documents generally contain a critical review of the scientific and technical information available on the prevalence of hazards, the existence of safety and health risks, and the adequacy of control methods. In addition to transmitting these documents to the Department of Labor, NIOSH also distributes them to health professionals in academic institutions, industry, organized labor, public interest groups, and other government agencies. In 1972, NIOSH published Criteria for a Recommended Standard: Occupational Exposure to Noise, which provided the basis for a recommended standard to reduce the risk of developing permanent hearing loss as a result of occupational noise exposure [NIOSH 1972]. NIOSH has now evaluated the latest scientific information and has revised some of its previous recommendations. The 1998 recommendations go beyond attempting to conserve hearing by focusing on preventing occupational noise-induced hearing loss (NIHL). This criteria document reevaluates and reaffirms the recommended exposure limit (REL) for occupational noise exposure established by the National Institute for Occupational Safety and Health (NIOSH) in 1972. The REL is 85 decibels, A-weighted, as an 8-hr time-weighted average (85 dBA as an 8-hr TWA). Exposures at or above this level are hazardous. By incorporating the 4000-Hz audiometric frequency into the definition of hearing impairment in the risk assessment, NIOSH has found an 8% excess risk of developing occupational noise-induced hearing loss (NIHL) during a 40-year lifetime exposure at the 85-dBA REL. NIOSH has also found that scientific evidence supports the use of a 3-dB exchange rate for the calculation of TWA exposures to noise. The recommendations in this document go beyond attempts to conserve hearing by focusing on prevention of occupational NIHL. For workers whose noise exposures equal or exceed 85 dBA, NIOSH recommends a hearing loss prevention program (HLPP) that includes exposure assessment, engineering and administrative controls, proper use of hearing protectors, audiometric evaluation, education and motivation, recordkeeping, and program audits and evaluations. Audiometric evaluation is an important component of an HLPP. To provide early identification of workers with increasing hearing loss, NIOSH has revised the criterion for significant threshold shift to an increase of 15 dB in the hearing threshold level (HTL) at 500, 1000, 2000, 3000, 4000, or 6000 Hz in either ear, as determined by two consecutive tests. To permit timely intervention and prevent further hearing losses in workers whose HTLs have increased because of occupational noise exposure, NIOSH no longer recommends age correction on individual audiograms.