Millions of workers who are exposed daily to uncontrolled workplace noise may face the prospect of partial or permanent hearing loss.

Research has shown that exposure to certain chemicals, called ototoxicants, may cause hearing loss or balance problems, regardless of noise exposure.

Substances, including certain pesticides, solvents, and pharmaceuticals that contain ototoxicants, can negatively affect how the ear functions, causing hearing loss, and/or affecting balance.

The risk of hearing loss is increased when workers are exposed to these chemicals while working around elevated noise levels, according to the federal Occupational Safety and Health Administration.

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.

This hearing impairment affects many occupations and industries, from machinists to firefighters.

Effects on Hearing

Harmful exposure to ototoxicants may occur through inhalation, ingestion, or skin absorption.

Health effects caused by ototoxic chemicals vary based on exposure frequency, intensity, duration, workplace exposure to other hazards, and individual factors such as age.

Effects may be temporary or permanent, can affect hearing sensitivity, and result in a standard threshold shift.

Health effects caused by ototoxic chemicals vary based on exposure frequency, intensity, and duration.

Since chemicals can affect central portions of the auditory system—such as nerves or nuclei in the central nervous system, the pathways to the brain or in the brain itself—not only do sounds need to be louder to be detected, but also they lose 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.
  • Spatial resolution: the inability to localize sound.

Undetected Hearing Loss

Speech discrimination dysfunction can also make working in noisy environments difficult and increase the risk of workplace injuries due to an inability to hear co-workers, environmental sounds, and warning signals.

There is growing concern among occupational health and safety professionals that ototoxicant-induced hearing loss may go unrecognized since the measure for hearing loss does not indicate the cause.

For example, audiometric tests are powerful tools that show hearing impairments, such as threshold shifts; however, they do not differentiate between noise and ototoxic causes.

Hearing loss can be even greater with exposure to both ototoxic chemicals and noise than exposure to either noise or the ototoxic chemical alone.

Many ototoxic substances have a greater-than-additive (e.g., synergistic) effect on hearing loss with noise exposure and in particular with impulse noise.

Several studies have suggested that some ototoxic chemicals, such as certain solvents, might exacerbate noise-induced hearing loss even though the noise level is below OSHA's permissible exposure limit.

Ototoxic Chemicals

Ototoxic chemicals are classified as neurotoxicants, cochleotoxicants, or vestibulotoxicants, based on the part of the ear they damage.

They can reach the inner ear through the blood stream and cause injury to inner parts of the ear and connected neural pathways.

Several studies have suggested that some ototoxic chemicals might exacerbate noise-induced hearing loss.

Neurotoxicants are ototoxic when they damage the nerve fibers that interfere with hearing and balance.

Cochleotoxicants mainly affect the cochlear hair cells, which are the sensory receptors, and can impair the ability to hear.

Vestibulotoxicants affect the hair cells on the spatial orientation and balance organs.

Limited Research

The research on ototoxicants and their interactions with noise is limited.

The dose-response, lowest observed effect level, and no observed effect level have been identified in animal experiments for only a few substances.

Here is more information on the specific industries and chemicals most likely to be impacted by ototoxicants.

To further assist companies in combating hearing loss, CONN-OSHA has organized the Connecticut Hearing Conservation Team to provide support and educational information.

The team is comprised of CONN-OSHA, OSHA, the Connecticut Council for Occupational Safety and Health, the state Department of Public Health, occupational health clinics, and the U.S. Department of Labor.

For information on how this group can assist your company, contact DPH's Deborah Pease (860.509.7771).


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