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Effects of Organic Solvents and Hazardous Noise Exposure on Hearing Loss

Exposure to solvents, particularly toluene, is now being considered as a contributor to the development of hearing loss. Evidence now suggests that noise interacts synergistically with various chemicals. However, as early as 1984 it was suggested that an "ototraumatic" interaction may exists between noise and organic solvents.

Organic solvents, due to their neurotoxic effects, can cause central and peripheral injuries in the nervous system. Consequently, it has been suggested that solvents could injure the sensory cells of the cochlea. Furthermore, knowing the effects solvents have on the brain, the possibility of retrocochlear pathology can not be ruled out.

A 20-year longitudinal study of over 300 employees was conducted to study this issue. It was found that 23% of the workers exposed to organic solvents (a mixture of toluene, xylene, and ketones) showed significant, high frequency hearing loss. This in comparison to an incidence of hearing loss of only 8% in the non-chemically exposed workers. This is a particularly important finding in light of the fact that the chemically exposed workers were in a relatively low noise environment (80-90 dB) as compared to the non-chemically exposed workers who worked in relatively high levels of noise (95-100 dB). *

Following otoscopy, a routine hearing test was administered. In addition, advanced auditory tests (tympanometry and acoustic reflex and decay tests) were also administered to obtain information as to the anatomical location of the pathology. The findings of these tests strongly suggest that chronic exposure to organic solvents had a toxic effect on the auditory system and that "it is very likely that an ototraumatic interaction between noise and toluene took place.". Furthermore, it was found that the hearing losses in the chemically exposed group were the result of an intraaxial brainstem lesion. This in unlike the typical cochlear lesion found in noise-induced hearing losses. This finding would support other research which found a significant problem in discriminating or understanding speech in groups of workers with a history of long-term occupational exposure to solvent mixtures and high intensity noise.

These findings raise the question as to whether current hearing conservation regulations appropriately define who should be audiometricly monitored in the workplace. In the future, it is suggested that managers of hearing conservation programs take the employee’s chemical exposure into consideration when obtaining a case history, administering audiometry, or when making an appropriate medical referral.

* Morata, T. C. et al, Effects of Occupational Exposure to Organic Solvents and Noise on Hearing, Scand J Work Environ Health, 1993; 19:245-54.