It is well established that exposure to certain environmental factors will lead to hearing loss (e.g. Fransen et al., 2003). Excessive noise exposure is the most obvious cause of hearing loss. However, infections such as prenatal rubella or meningitis can lead to deafness. Certain drugs also have ototoxic effects, including aminoglyco-side antibiotics, cisplatin and diuretic agents. In some cases it is clear that one of these factors is the immediate cause of deafness, such as hearing impairment that develops over the few days after exposure to an ototoxic drug or after meningitis, but often it is not so obvious. For example, although many studies of noise exposure in animals have shown the deleterious effects of noise on auditory function, such experiments are impossible in humans so we are left trying to piece together a history of noise exposure long after the event. The National Study of Hearing in the UK found a small but significant effect of relatively high reported noise exposure on hearing levels, but not for moderate noise exposures, and the effects were very small compared with the effects of age (Davis, 1989).
Animal studies have revealed some factors that moderate or potentiate noise-induced hearing loss. For example, exposure to certain drugs and chemicals like toluene and styrene or concomitant hyperthermia potentiate noise-induced hearing loss (Davis etal., 2002; Fechter, 2004). Surprisingly, exposure to a stressful stimulus, such as heat stress, restraint stress or moderate noise levels, during the few days prior to the damaging noise exposure can reduce the amount of noise-induced damage (e.g. Yoshida et al., 1999; Yoshida and Liberman, 2000). This phenomenon is known as conditioning or toughening.
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