Solvent-induced hearing loss: mechanisms and prevention strategy

Area of Science:

• Occupational health
• Auditory toxicology
• Environmental health

Background:

• Noise exposure is a primary cause of occupational hearing loss.
• Emerging evidence implicates organic solvents as significant contributors to hearing disorders.
• Limited measures exist in the European community to mitigate combined exposure to solvents and noise.

Purpose of the Study:

• To propose guidelines for enhanced hearing protection for individuals exposed to solvents or combined solvent-noise exposure.
• To re-evaluate and suggest revised exposure limits for occupational solvents based on toxicological data.

Main Methods:

• Analysis of toxicological data on styrene-induced hearing loss in rats.
• Investigation of toluene's effects on the auditory efferent system using cochlear microphonic and compound action potentials.
• Evaluation of the anticholinergic-like effects of toluene on middle ear reflex.

Main Results:

• A proposed reduction in the French threshold limit value for styrene from 50 to 30 ppm, based on a lowest-observed-adverse-effect level (LOAEL) and a safety factor.
• Demonstration that toluene can inhibit the middle ear reflex by affecting cholinergic receptors, potentially at the brainstem level.
• Evidence suggesting toluene's anticholinergic effect may increase cochlear susceptibility to acoustic energy in combined exposures.

Conclusions:

• Revised, lower exposure limits for solvents like styrene are necessary for better hearing protection.
• Hearing protection should be employed at lower exposure action values (e.g., Lex,8h=80 dB(A)) in environments with both noise and solvent pollution.
• Understanding the mechanisms of solvent ototoxiciy, including effects on the auditory efferent system, is crucial for developing effective prevention strategies.