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.
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.1,6 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.1 The relationship between these factors and NIHL works in reverse as well.
Exposure to loud noise can result in temporary threshold shifts (TTS) and/or permanent threshold shifts (PTS). Moderate exposure in a short time period may initially cause TTS. Hearing loss after a TTS may fully recover within 24–48 h.12 But this common scientific understanding (possible full recovery from TTS without lasting consequences) has been challenged by the study results on mice that showed completely reversible TTS may increase nerve degeneration and accelerate age-related hearing loss in later life.13 A possible late-life consequence of neural losses following even a TTS is a particularly important message to deliver to young children and young adults who expose themselves to various recreational noises such as loud computer games or portable music players
