Hearing impairment can be caused by exposure both to noise and to chemicals, and the impairment from either of those two exposures can be worse if there is exposure to both. Damage to hearing from chemicals—called ototoxicity; the chemicals themselves are called ototoxicants—is a relatively unknown cause of injury to workers, a state of affairs OSHA is endeavoring to change with a recent Safety and Health Information Bulletin.
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“Research demonstrates exposure to certain chemicals, called ototoxicants, may cause hearing loss or balance problems, regardless of noise exposure,” states OSHA. “Substances including certain pesticides, solvents, and pharmaceuticals that contain ototoxicants can negatively affect how the ear functions, causing hearing loss, and/or affect balance. The risk of hearing loss is increased when workers are exposed to these chemicals while working around elevated noise levels. This combination often results in hearing loss that can be temporary or permanent, depending on the level of noise, the dose of the chemical, and the duration of the exposure.”
The many ototoxicants include both well-known chemicals (e.g., toluene, p-xylene, carbon monoxide, and mercury compounds) and lesser-known pharmaceuticals (e.g., certain loop diuretics and antineoplastic agents). The lengthy list of industries where ototoxicants are found include machinery, textile and apparel, chemical (including paint), plastics, and furniture manufacturing.
Nervous System Affected
Ototoxicants affect central portions of the auditory system (e.g., nerves or nuclei in the central nervous system, the pathways to the brain, or in the brain itself). Given that the nervous system is impacted, the effects include both loss of hearing (i.e., sounds need to be louder to be detected) and lose of clarity. Specifically, speech discrimination dysfunction—the ability to hear voices separately from background noise—may occur and involve compressed loudness (sound distortion); frequency resolution (the inability to differentiate two sounds with similar frequency); temporal resolution (the inability to detect time gaps between sounds); and spatial resolution (the inability to localize sound).
According to OSHA, speech discrimination dysfunction can be particularly dangerous in noisy environments because workers may not be able to hear coworkers, environmental sounds, and warning signals.
Exposure Routes
Measures to protect against ototoxicants can be more challenging than measures to protect against noise since exposure to ototoxicants can occur through multiple routes—inhalation, ingestion, and skin absorption.
Also, the exposure threshold for ototoxicity varies for each chemical based on its compound family, properties, exposure route, exposure concentration and duration, synergy with noise, and noise exposure, along with an individual’s risk factors. The dose-response, lowest observed effect level (LOEL), and no observed effect level (NOEL) have been identified in animal experiments for only a few ototoxicants. Also, many chemicals are not identified as ototoxicants on their safety data sheets (SDSs).
Prevention
The first step in preventing exposure to ototoxicants is to determine if they are in the workplace. As noted, SDSs may not specifically mention ototoxicity or ototoxicants. When this is the case, information on the chemical’s general toxicity, nephrotoxicity, and neurotoxicity may provide clues about potential ototoxicity. “Most chemicals that are known to affect the auditory system are also neurotoxic and/or nephrotoxic,” says OSHA.
Once the presence of an ototoxicant has been confirmed, the employer must provide employees with safety information and training on the dangers of the chemical and measures to be followed to keep exposure at safe levels. Furthermore, employers must assess and determine the appropriate personal protective equipment (PPE) according to the general requirements in 29 CFR 1910.132, the respiratory protection requirements in 29 CFR 1910.134, and the hand protection requirements in 29 CFR 1910.138.
OSHA’s occupational noise exposure standard at 29 CFR 1910.95 only requires audiometric testing at the noise action level (i.e., an 85-decibel 8-hour time-weighted average). However, wearing hearing protection and using audiometric testing to detect early signs of hearing loss, even in workers exposed below the action level and ototoxic chemicals below the permissible exposure limit (PEL), may prevent hearing loss from their synergistic effects.
Other protective measures may include:
- Replacing ototoxicants with a less toxic chemical;
- Using engineering controls, such as isolation, enclosures, and ventilation to control exposure to ototoxicants and noise; or
- Eliminating unnecessary tasks that cause noise or ototoxicant exposure and operating noisy equipment when workers are not near.
Finally, employers that receive complaints from workers about hearing loss should investigate SDSs for ototoxicants.