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.1 The World Health Organization reported that 16% of the disabling hearing loss in adults is attributable to occupational noise exposure.2 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.3 In the 1800s, Fosbroke4 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.5 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.