Understanding and preventing noise-induced hearing loss.

Noise is a major occupational and environmental hazard, causing hearing loss, annoyance, sleep disturbance, fatigue, and hypertension. Although the extra-auditory effects of high-level noise exposure have been reported, noise-induced hearing loss (NIHL) has long been recognized as the primary and most direct health effect of excessive noise exposure. The World Health Organization reported that 16% of the disabling hearing loss in adults is attributable to occupational noise exposure. NIHL has been recognized as an occupational disease and injury since the 18th century among copper workers who suffered hearing loss as a result of hammering on metal. In the 1800s, Fosbroke also mentioned how blacksmiths suffered hearing impairment from continued exposure to noise. The occupational risk of NIHL in industries that expose workers to continuous high levels of noise is well established with more than 30 million U.S. civilian workers estimated to be exposed to potentially damaging noise levels and another 9 million at risk from other ototraumatic agents, including chemicals. In 1996, the NIOSH had established the National Occupational Research Agenda (NORA) that identified NIHL as one of the 21 priority areas to stimulate innovative research and improved workplace practices to reduce NIHL problem. In order to manage this major health problem among workers exposed to occupational noise, it is important to understand the nature of NIHL. Two characteristics of NIHL have been thoroughly established through numerous studies. First, the amount of hearing loss increases with noise intensity and duration of exposure, such that more intense and longer-duration noise exposures cause more severe hearing loss. Second, individual susceptibility to NIHL varies greatly. Not all individuals exposed to a given noise level develop the same degree of hearing loss. Although the reason that some individuals are more susceptible to NIHL than others is not well understood, several factors have been implicated, including age, previous sensorineural hearing loss, cigarette smoking, use of ototoxic medication, type 2 diabetes, and hypertension. The relationship between these factors and NIHL works in reverse as well. For example, a recent animal study by Kujawa and Liberman demonstrated that damage caused by noise exposure early in life made mice more susceptible to age-related hearing loss.