Considerations for Controlling Ototoxicant Exposures
By Ed Rutkowski
Ototoxicants, or chemicals that can cause hearing loss, were first identified in the early 1900s. But as Alex Cardone, MPH, CIH, explained in an educational session held May 20 at AIHA Connect 2025 in Kansas City, much about these substances remains mysterious. It isn’t clear, for example, by what mechanism they affect hearing and to what degree they are responsible for hearing loss in exposed individuals. And there is still uncertainty about which chemicals have ototoxic effects. ACGIH, for example, has developed Threshold Limit Values for just four ototoxicants, all of them solvents.
A partial list of recognized ototoxicants includes solvents such as xylene, toluene, methyl ethyl ketone, and benzene; the metals cadmium, lead, mercury, and manganese; pharmaceuticals such as aspirin and non-steroidal anti-inflammatory drugs (NSAIDs); and the asphyxiant carbon monoxide. Though Cardone hasn’t found methylene chloride (MC) on any list of ototoxicants, he considers it to be an ototoxicant since exposure to MC can create carbon monoxide in the liver and potentially lead to carbon monoxide poisoning.
While more research on ototoxicants is needed, studies show that they primarily target the middle or inner ear, Cardone said. Exposure to solvents or carbon monoxide can affect the cilia, the hair-like structures that help convert sound vibrations into electrical signals for the brain to process. Benzene compromises the stapedius, a muscle in the middle ear that activates to diminish large sound pressure waves.
Ototoxicant exposure also has possible additive or synergistic effects on hearing loss when combined with noise. Cardone discussed a 2017 study of personnel at a shipyard in Portsmouth, Virginia, which found that workers exposed to ototoxicants were 2.4 times more likely to experience a permanent threshold shift in their hearing. Other studies suggest that the combination of noise exposures with some ototoxicants corresponds with a higher rate of threshold shift in workers. The good news, Cardone said, is that evidence increasingly shows that some hearing loss can be reversed if exposure to ototoxicants is controlled.
The first step in controlling ototoxicants is to identify which ones are present in the workplace. Resources that can help with this task include safety data sheets, AIHA’s Noise Manual, the Journal of Occupational and Environmental Hygiene, and the ACGIH TLV manual. Reviewing chemical logs can provide an idea of the prevalence of ototoxicants at a site.
To evaluate ototoxicant exposures alone—that is, without considering noise—Cardone recommended prioritizing inhalation exposures that are greater than 50 percent of the relevant occupational exposure limit. Workers with significant dermal exposures or whose inhalation exposures exceed the action level are placed in the hearing conservation program.
For workers who are exposed to both ototoxicants and noise, OEHS professionals must consider possible additive or synergistic effects. In both cases, Cardone’s organization—he is the supervisory industrial hygienist at a United States Air Force base—has developed benchmarks for assigning workers to the hearing conservation program. For example, workers exposed to both noise and an additive ototoxicant are placed in the program if their chemical exposure exceeds 25 percent of the OEL. For synergistic ototoxicants, the benchmark is an exceedance of 10 percent of the OEL.
Cardone acknowledged that many workplaces are hesitant about addressing ototoxicants due to uncertainties around the science. But given that hearing loss is one of the most common workers’ compensation claims, both workers and employers stand to benefit when OEHS professionals are proactive about ototoxicants.
Ed Rutkowski is editor in chief of The Synergist.
Read more coverage of AIHA Connect 2025.
