Introduction
The most common condition associated with exposure to noise is occupational hearing loss. Occupational noise is an integral part of the job especially in iron and steel industries, noise may cause severe hearing loss. The hearing loss occurred due to occupational noise exposure is called as occupational noise induced hearing loss or occupational deafness. Worldwide, about 16 per cent of the hearing disability is caused by occupational noise exposure, according to Nelson et al. (2005). The workers in industries such as mining, construction, printing, crushers, drop forging, iron and steel companies are at high risk of NIHL (Nadi et al., 2008). When an individual is over-exposed to excessive sound levels, sensitive structures of the inner ear can be damaged. This can result in permanent noise-induced hearing loss (NIHL). These structures can be injured by exposure to a brief but intense sound, such as an explosion, or from regular exposure to excessive sound levels over time. Noise may cause hearing loss in three ways.
(a) Temporary threshold shift (TTS) refers to the short-term effect, i.e., a temporary reduction in the hearing ability of an individual that may follow to noise exposure. For example, if a person attend a concert or party function for 1–2 hours, in which DJ system was run at high decibel level, or an individual visit a factory in which various machines such as forge hammer and mechanical presses are running simultaneously and noise level is very high. This noise exposure will consequent in short-term hearing disability which is called temporary hearing threshold shift. The state of temporary threshold shift is usually recoverable, a person has hearing disability for some time, and then recovers the original hearing threshold level, and this is also known as auditory fatigue.
(b) Permanent threshold shift happens where the permissible limit of tolerance is exceeds in terms of; exposure time, noise level and individual vulnerability or susceptibility. The permanent hearing threshold shift (PTS) is result of chronic auditory fatigue, i.e., repeated temporary threshold shifts (TTS). It has been established by Chiu et al. (2007) that TTS driven by noise exposure is further enhanced by heat and work load. In this case recovery from threshold shift does not take place completely, rather will reduce and ultimately stop at some point of time, even after the end of the exposure.
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