Individual susceptibility to noise-induced hearing loss

Abstract

The new EU directive (10/2003/EU) sets new limits for allowable noise levels and sets new requirements to accommodate the individual susceptibility factors, but does not define these factors. We aim to evaluate the effects of environmental and biological factors on the damage to hearing by noise. The history of work exposure, use of hearing protective devices (HPD), audiometric data and information on environmental and biological factors were collected from well over 1000 workers. The group consisted of 406 paper mill workers, 400 miners, 250 metal factory workers, 176 shipyard workers and 124 forestry workers. For comparison we evaluated 685 noise-exposed subjects visiting a hearing aid clinic for fitting of a hearing aid. They were exposed in their work to noise levels of 80–115dB(A). The environmental and biological factors encompassed smoking habits, the use of analgesics, serum cholesterol, systolic or diastolic blood pressure as well as noise exposure. There were a number of confounders which were significant factors in the hearing losses found in the noise-exposed workers. Sensitivity to sunburn, the presence of Raynaud's disease, a history of head injury and the use of analgesics correlated significantly with the extent of noise induced hearing loss (NIHL). The role of serum cholesterol levels as a risk factor for NIHL depended on the age of the subject, with the levels of middle-aged subjects having the greatest effect on NIHL. Hypertension or use of anti-hypertensive medication and use of analgesics correlated with hearing loss and their effect was additive. The correlation of tobacco smoking was also confounded by age. In analyses where the subjects were matched in pairs by age, exposure, blood pressure and serum cholesterol level, the elderly subjects were more susceptible to NIHL than younger subjects. In conclusion, among several susceptibility factors smoking and cholesterol are independently but causally related to NIHL and may cause the subject to be more susceptible for noise damage. Ageing further confounds some of these factors. The current ISO noise standard (1999–1990) has methodological deficiencies in not encompassing these susceptibility factors.

• ageing of inner ear
• analgesics
• blood pressure
• cholesterol
• risk factors for hearing loss
• smoking