Page 33 - OHS, May 2021

Sensorineural loss is caused by damage to one or more components of the middle or inner ear. Noise-induced hearing loss is always sensorineural and mostly occurs because of damage to the tiny hairs in the cochlea of the inner ear which work to stimulate the nerve endings.3
Exposure to ototoxic compounds can cause sensorineural hearing loss by damaging various parts of the middle and inner ear. As such, OSHA classifies ototoxic chemicals by the area of the ear that is damaged. Neurotoxicants can damage the nerve fibers in the inner ear which conduct the sound to the brain. Cochleotoxicants can damage the hairs in the cochlea that are commonly damaged by high-noise exposures.
Understanding Chemical
Exposure Hearing Loss
How does exposure to chemicals like the solvents listed in Table 1 cause damage to the ear when there is no direct contact between the airborne compounds and the inner ear? Ototoxic chemicals reach the ear after they are absorbed into and transported by the blood. The toxicity can be from the original compound or from one or more metabolites of the original compounds. The exposure might be through inhalation, ingestion, or dermal absorption. In many cases, it can be extremely difficult to distinguish noise- induced hearing loss from chemical induced hearing loss.
Another variable that is sometimes overlooked is the combined effects of noise exposure and exposure to ototoxic compounds. Ototoxic compounds can result in hearing loss even with the absence of exposure to high noise levels. A meta-analysis of epidemiology studies about ototoxic chemical exposures and noise related to hearing loss reported an increased odds ratio of 2.05 (95% Ci: 1.44-2.9) for exposure to a mixture of solvents alone.4
With exposure to a mixture of ototoxic chemicals, study subjects had an increased risk of developing chemical induced hearing loss that was 1.4 to 2.9 times greater than workers with no exposure. Exposure to both ototoxic compounds and noise can cause a synergistic increase in the risk of hearing loss. When noise exposure was also included the odds ration increased to 2.95 (95% CI: 2.1-4.17).
Another study looked at the increase risk associated with exposure to just styrene or to n-hexane and toluene simultaneously. The odds ratio for hearing loss with exposure to just styrene was reported as 3.9 (95% CI: 2.4-6.2). The odds ratio for hearing loss with exposure to n-hexane and toluene simultaneously was 5.3 (95% CI: 2.6-10.9). The reported odds ration increased more than 20 times when noise exposure was also present.
These studies reinforce the warning provided by OSHA that the synergistic effects of noise exposure and exposure to ototoxic compounds can result in hearing loss even when noise and chemical exposures are below the published permissible exposure levels. This adds a new consideration for OSH professionals when dealing with the risk of hearing loss in occupational settings. One approach may be to implement hearing conservation programs when noise exposures exceed the action level and evaluating controls, based on the Hierarchy of controls, and when noise exposures exceed the permissible exposure level.
In most cases, workers are required to wear hearing protection when exposures exceed the action level or the permissible exposure level. For chemical exposures, most facilities do not take
Many chemicals have been identified with the potential to increase the risk of hearing loss even without high-noise exposures.
actions unless personal exposures have been shown to exceed an OSHA PEL or ACGIH TLV. The most common controls used based on the hierarchy of controls are general or local exhaust ventilation systems and respiratory protection.
Reducing the Risk
Whenever potential exposure to ototoxic chemicals exists along with exposure to sound levels approaching 80 dBA, OSH professionals may consider implementing controls to reduce the risk of occupationally induced hearing loss in workers. I know of facilities where standard threshold shifts have been identified and noise exposures did not exceed the OSHA action level. The potential for the synergistic effects of exposure to ototoxic compounds and noise both at levels below the published OSHA permissible exposure levels or ACGIH TLVs might account for these anomalies.
Using the hierarchy of controls, the first approach should be to determine if the ototoxic compound can be eliminated from the workplace, or if a chemical that has not been identified as being ototoxic can be substituted for the ototoxic compound.
If that option is not viable, engineering controls should be evaluated to reduce both the exposure to the ototoxic compound (i.e., ventilation) and the noise (i.e., shielding). If a significant risk continues, respiratory protection and hearing protection might also be advisableto mitigate the hazards. These actions should be evaluated even if industrial hygiene studies show that exposures are less than the published permissible exposure levels and ACGIH TLVs.
Greg Boothe, Ph.D., CIH, CSP® is the faculty lead for occupational safety and health at Columbia Southern University. He is a founding member and past president of the Middle Tennessee section of AIHA and a professional member of the Middle Tennessee section of ASSP. Dr. Boothe has more than 35 years of experience in OSH.
REFERENCES
1. https://doi.org/10.13075/ijomeh.1896.01024
2. https://www.cdc.gov/niosh/docs/2018-124/pdfs/2018-124.
pdf?id=10.26616/NIOSHPUB2018124
3. http://stagesd.acoem.org/acoem/media/News-Library/Occupational_ Noise_Induced_Hearing_Loss.pdf
4. https://doi.org/10.1016/j.etap.2004.12.018
5. https://www.osha.gov/dts/osta/otm/new_noise/
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