Welders, fire fighters, garage mechanics, truckers, forklift operators and miners are among the most exposed groups, according to a Canadian study among 8,600 workers.
Based on data gathered by the National Institute of Occupational Safety and Health, the researchers compared the hearing of workers exposed to noise levels lower than 90 decibels for 8 hours to another group of workers exposed to noise levels above 90 decibels. In both groups, a sample of workers was also exposed to carbon monoxide.
The Université de Montréal researchers demonstrated that workers who were exposed to carbon monoxide and noise levels above 90 decibels displayed significantly poorer hearing thresholds at high frequencies (from 3 to 6 kiloHertz).
Lower oxygen levels in the blood
The damaging effects from carbon monoxide, however, manifest themselves only after many years of working in noisy surroundings.
â€In our study we saw that it took at least 15 years to start showing this increase in hearing loss. The effect increases with the number of years of exposure,â€ Adriana Lacerda of the research group told the canada.com News web site.
One of several hypotheses to explain this phenomenon is that the reduction of oxygen in the blood stream caused by carbon monoxide accelerates the deterioration of the sensory cells of the inner ear, Lacerda explained.
â€œBased on these results, we recommend that such risks as chronic exposure to carbon monoxide be considered when assessing the risk of developing a noise-induced hearing loss,â€ stated Lacerda in a press release issued by the university.
Source: UniversitÃ© de MontrÃ©al, press release, May 16, 2005