The combined effect of noise and carbon monoxide on hearing thresholds of exposed workers

Animal models have been used to demonstrate the potentiation of noise-induced hearing loss (NIHL) by carbon monoxide. It has been shown that the addition of carbon monoxide to otherwise safe noise exposure levels produces significant NIHL in rats. However, the effects of chronic exposure to low level of carbon monoxide in a noisy work environment are still unknown. The aim of this study was to compare the hearing thresholds of a group of workers exposed to noise and carbon monoxide (Group 1) to another group of workers where carbon monoxide exposure is absent or negligible (Group 2). The analysis was based on 9396 audiograms collected by the Quebec National Public Health Institute between 1983 and 1996. The results show significantly poorer hearing thresholds at high frequencies (3, 4, and 6 kHz) for the carbon monoxide exposed group (p<0.001). The potentiation effect also varied according to years of exposure in work place; a larger effect is observed for workers with between 15 to 20 years of exposure (p<0.001). This study provides the first demonstration of a potentiation effect of NIHL by carbon monoxide in humans.