Millions of workers are exposed to noise in the workplace every day and, when uncontrolled, noise exposure may cause permanent hearing loss. Research demonstrates 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, 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. 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. Because chemicals can 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), 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
Speech discrimination dysfunction can also make working in noisy environments difficult and increase the risk of workplace injuries because of an inability to hear coworkers, environmental sounds, and warning signals.
There is growing concern among occupational health and safety professionals that ototoxicant-induced hearing loss may go unrecognized because the measure for hearing loss does not indicate the cause. For example, audiometric tests are powerful tools that show hearing impairments (i.e., threshold shifts); however, they do not differentiate between noise and ototoxic causes.
Combined Exposure: Health Effects Below the Noise Permissible Exposure Limit
OSHA standards require employers to maintain exposure to the specific substance at or below the Permissible exposure limit (PEL). However, synergistic effects from the combined ototoxicant and noise exposure could result in hearing loss when exposures are below the PEL.
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 PEL.
What Are Ototoxic Chemicals and Substances That Contain Ototoxicants?
Ototoxic chemicals are classified as neurotoxicants, cochleotoxicants, or vestibulotoxicants based on the part of the ear they damage, and they can reach the inner ear through the blood stream and cause injury to inner parts of the ear and connected neural pathways. 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. 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.