Noise levels above a certain threshold and duration lead to first reversible and subsequently permanent injury to the cochlear hair cells. Duration of the noise exposure is important as well as absolute volume. Continued exposure to sounds above 85 dB in volume is likely to lead to damage. Initially there is a reversible hearing loss on short exposure to noise, known as temporary threshold shift, on prolonged, repeated exposure, this will become irreversible. The outer hair cells of the cochlea are characteristically damaged. There may be a genetic predisposition to NIHL and older people are more likely to be affected.
The hair cells most affected are in the area of the cochlea which deals with sound in the frequencies 3-6KHz. This gives the characteristic 4Khz notch in the audiogram that is pathognomonic of NIHL.
Industrial noise related deafness has been much better recognised and ear defenders are compulsory in noisy areas of factories but twenty to thirty years ago, workers were often exposed to quite high volumes without protection. Groups most affected include: * miners * factory workers, especially printing and sheet metal * armed forces, especially artillery and tanks * construction site workers - road drills etc.
The growing popularity of portable music players and other items that attach directly to the ears - including cell phones - is contributing to hearing loss in younger people. Hearing specialists are seeing more people in their 30s and 40s - many of them among the first Walkman users - who suffer from pronounced tinnitus, an internal ringing or even the sound of whooshing or buzzing in the ears. Usually, however, they have lost the ability to hear higher frequencies, evidenced at times by mild ear-ringing or trouble following conversations in noisy situations.
A survey published in 2005 by Australia's National Acoustic Laboratories found, for instance, that about 25 percent of people using portable stereos had daily noise exposures high enough to cause hearing damage. Research by Britain's Royal National Institute for Deaf People determined that young people, ages 18 to 24, were more likely than other adults to exceed safe listening limits.