The Definition of Ototoxic Chemicals

Definition of Ototoxic Chemicals

Ototoxic chemicals, or ototoxicants, are substances that can damage the inner ear or the nerves involved in hearing and balance. Exposure can cause hearing loss, ringing in the ears (tinnitus) or balance problems, either on their own or by making the ear more vulnerable to noise.

Most workplace hearing programs focus on noise, but chemicals can harm hearing too. Some solvents, metals and asphyxiants can damage hearing even at exposures below their occupational limits, and the combined effect of noise plus an ototoxicant can be greater than either one alone. This means workers can lose hearing even when noise levels and air concentrations both look acceptable on paper.

Ototoxicants matter wherever chemicals and noise occur together, such as manufacturing, painting, printing, boat building, fueling, construction, firefighting and mining. Because many of these chemicals can be absorbed through the skin as well as inhaled, exposure can be missed if only air monitoring is considered.

How Ototoxic Chemicals Affect Hearing

Routes of Exposure

Ototoxicants can enter the body by breathing in vapors, dust or fumes, by swallowing contaminated material, or through skin contact. Once in the bloodstream, they can reach the inner ear and the auditory nerve pathways. Some damage the sensory hair cells of the cochlea, while others affect the nerves that carry signals to the brain, which can make speech harder to understand even when sounds are audible.

Chemicals Named by OSHA and NIOSH

The 2018 OSHA and NIOSH Safety and Health Information Bulletin, Preventing Hearing Loss Caused by Chemical (Ototoxicity) and Noise Exposure (SHIB 03-08-2018, DHHS (NIOSH) Publication No. 2018-124), lists several groups of ototoxicants:

• Solvents: Carbon disulfide, n-hexane, toluene, xylenes, ethylbenzene, n-propylbenzene, styrene and methylstyrene, and trichloroethylene. See also organic solvent exposure and xylene exposure.
• Asphyxiants: Carbon monoxide, hydrogen cyanide and its salts, and tobacco smoke.
• Nitriles: Several nitrile compounds, including acrylonitrile.
• Metals and compounds: Mercury compounds, germanium dioxide, organic tin compounds and lead.
• Pharmaceuticals: Certain antibiotics, loop diuretics, some pain relievers and antimalarials, and some cancer drugs.

The bulletin also notes more limited evidence of ototoxicity for substances such as cadmium, arsenic, manganese and some insecticides.

Interaction With Noise

The bulletin explains that combined exposure to noise and some ototoxicants can produce more hearing loss than either exposure alone, even when each is below its permissible exposure limit. This is why a worker in a moderately noisy area who also handles solvents may need closer attention than noise readings alone suggest.

Regulations and Good Practices

Regulatory Context

OSHA's occupational noise standard for general industry, 29 CFR 1910.95, sets an 8-hour time-weighted average action level of 85 dBA that triggers a hearing conservation program, including audiometric testing. The standard does not account for ototoxic chemicals. Individual chemicals are covered by their own exposure limits, the Hazard Communication standard (1910.1200) requires hazard information on labels and safety data sheets, and respiratory protection and PPE rules (1910.132 to 1910.138) apply where needed. The 2018 bulletin is guidance, not a new requirement, but it describes steps OSHA and NIOSH recommend.

Identifying Exposures

• Review safety data sheets: Check SDSs for ototoxic ingredients, and look again whenever a worker reports hearing changes.
• Map noise and chemicals together: Identify jobs where workers face both, even at moderate levels.
• Consider skin exposure: Note tasks with hand or body contact, such as cleaning parts with solvents or handling wet paint.

Controlling Exposures

• Substitute where possible: Replace ototoxic solvents with less hazardous alternatives.
• Use engineering controls: Enclose processes, isolate noisy equipment and use local exhaust ventilation.
• Adjust work practices: Eliminate unnecessary tasks and schedule noisy work when fewer people are present.
• Provide suitable PPE: Chemical-resistant gloves, sleeves and aprons reduce skin absorption, and respirators and hearing protectors address inhalation and noise.
• Test hearing even below the action level: The bulletin recommends hearing protection and audiometric testing for workers exposed to ototoxicants, even when noise and chemical exposures are below the regulatory limits.
• Train workers: Explain that some chemicals can damage hearing, in language workers understand, as part of hazard communication training.

Example of Ototoxic Chemicals

A fiberglass boat manufacturer runs a hearing conservation program for workers in the grinding area, where noise exceeds 85 dBA. Workers in the lamination area, where styrene resins are applied, are not enrolled because measured noise there averages about 80 dBA. After two laminators report trouble following conversations at home, the safety manager reviews the safety data sheets and the 2018 OSHA and NIOSH bulletin and recognizes styrene as a listed ototoxicant.

The company enrolls laminators in baseline and annual audiometric testing, improves ventilation in the layup booths, switches to a lower-styrene resin for some products and provides better-fitting chemical gloves and sleeves. Hearing protection is offered in the lamination area, and supervisors cover ototoxicity in the next safety meeting.

Frequently Asked Questions: Ototoxic Chemicals

Can chemicals cause hearing loss without loud noise?

Yes. Some ototoxicants can damage hearing on their own. The greater concern at work is often the combination of chemicals and noise, which can be more harmful than either alone.

Does OSHA have an exposure limit for ototoxicity?

No. OSHA's limits for individual chemicals were not set with hearing effects in mind, and the noise standard does not adjust for chemical exposure. The 2018 bulletin recommends going beyond these limits where ototoxicants are present.

Which industries have the most exposure?

The bulletin points to manufacturing, mining, utilities, construction and agriculture, along with activities such as printing, painting, fueling, firefighting and pesticide spraying.

How can workers tell if they are affected?

Early signs include tinnitus, muffled hearing or difficulty understanding speech in noisy settings. Regular audiometric testing is the most reliable way to detect changes before they become severe.