Solvent Exposure Can Lead to Hearing Loss

By Ralph Birch

While exposure to noise has been identified as the most significant contributor to occupational hearing loss in the lab, recent European studies have shown that exposure to organic solvents can also lead to hearing disorders.

Animal and human studies over more than four decades have revealed disturbances in both the central auditory and vestibular systems, and other ototoxic effects from industrial solvents.

Exposure to both solvents and noise results in a synergistic effect between the two, enhancing the pattern of the trauma. Despite these findings, few measures have been taken to limit exposure to noise or solvents in laboratory settings.

Balancing Act

Research has definitively shown industrial solvent ototoxicity in rats. Most solvents caused a loss of auditory sensitivity in the mid-frequency range due to changes in outer hair cells. (Inner hair cells were generally unaffected.) Researchers have also found that spiral ganglion cells are vulnerable to trichloroethylene.

Along with producing neurotoxic effects, solvent exposure can influence the vestibulo-oculomotor system in both animals and humans. Humans may also experience postural sway, defined as horizontal movement of the body’s center of gravity when one is standing still.

Studies of humans have shown ototoxicity even at low exposure levels, and disturbances in peripheral and central auditory pathways. Hearing loss in humans can occur in a wide range of frequencies or just in the high-frequency region.

As a result of these findings, employers are looking for effective ways to encourage safety and hearing conservation in workplaces.

Currently, few workers exposed to solvents are required to have a hearing test, because the noise level they’re exposed to is not considered hazardous.

The Need for More Research

Hearing loss among workers is generally attributed to a combination of age and noise exposure. But evidence continues to emerge that suggests exposure to industrial solvents and the combination of chemicals and noise has an adverse and permanent effect on auditory sensitivity and the vestibular system in rats.

Animals show varying levels of susceptibility to solvent exposure between species, and human study results vary by individual. These differences are not well understood but could help determine which individuals are more prone to adverse effects from solvent exposure.

Balance disturbances from solvent exposure have been largely neglected, but evidence suggests that the incidence may be significant. Research in this field has been limited, which makes the need for further study even more important.

Future Prevention

In many countries, protection against ototrauma is limited to reduced exposure to high-intensity noise. But solvent exposure is significant and is estimated to affect as many as 10 million European workers every day.

Few workers who are exposed to solvents are currently required to have hearing tests if their noise level exposure is not considered to be hazardous. And, while it is now recommended that workers exposed to solvents have their hearing tested periodically, there are no regulations requiring that testing. 

Regular testing to examine both the peripheral and central elements of hearing and balance would be an effective means of monitoring the impact of solvent exposure. These tests would need to be quick, easy to administer, and acceptable to the workforce.

If sufficient work practice and exposure protocol changes are the ultimate goal, solid evidence regarding the effects of exposure to solvents is critical for industry decision makers. Gathering that evidence will require a standardized approach to the assessment of hazards in both animal and human studies.