Noise-induced hearing loss (NIHL) is either temporary or permanent. Temporary NIHL occurs after exposure to high noise levels for a short time or lower noise levels for a much longer time, a person's threshold of audibility is temporarily shifted to higher levels. A typical instance of Temporary NIHL could occur after attending a concert at a music club. However, as its name indicates, it is only temporary, and the ear recovers fully after several hours. If such exposures are repeated daily, or if the ear is not allowed to recover from this "auditory fatigue" over a quiet night before it is exposed to noise again, temporary NIHL can lead to permanent NIHL.
There has been significant research conducted over the last 40 years on industrial and military populations, which has resulted in a reasonable understanding of the development of NIHL, including the amount of hearing loss caused by combinations of noise level, frequency spectrum, and duration of exposure. Detailed criteria have been developed that identify maximum noise exposures to prevent NIHL. The U.S. Occupational Safety and Health Administration (OSHA) regulation identifies the maximum permissible A-weighted exposure of 90 dB for eight hours.
However, a World Health Organization report states that “even a lifetime exposure to environmental and leisure-time noise with an LAeq24h<70 dBA [approximately equivalent to DNL 70 dBA] would not cause hearing impairment in the large majority of people.”i
i World Health Organization, Guidelines for Community Noise, 1999.