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Evaluating Occupational Noise Exposures: The Effects of VOC Exposures
Noise induced hearing loss (NIHL) is a fairly common illness. According to statistics reported by OSHA in 2010, NIHL accounted for approximately 12 percent of all recordable illnesses in the United States.1 This equated to approximately 18,748 NIHL cases reported for that year (Figure 1), and may be lower than the number of actual cases.
To identify areas of concern, many safety professionals and industrial hygienists use a sound level meter (SLM) to identify areas in a facility where noise levels exceed 85 dBA. The 85 dBA level is used because it is the action level set by OSHA for an eight-hour shift and the threshold limit value (TLV) set by the American Conference of Governmental Industrial Hygienists (ACGIH). OSH professionals who have worked with noise exposures know the 85 dBA level is just the action level for an eight-hour shift and must be adjusted for extended work shifts to determine the 50 percent dose or the dose where the exposure is 50 percent of the OSHA permissible exposure level (PEL) (90 dBA).
Since sound levels are measured on a logarithmic scale, determining the 50 percent dose is not as simple as dividing an exposure in half. It is complicated further by the use of a 5 dB doubling scale by OSHA and a 3 dB doubling scale by the ACGIH and National Institute for Occupational Safety and Health (NIOSH). Using the OSHA formula, we can determine that the action level for a 10-hour shift is 83.4 dBA and the action level for a 12-hour shift is 82.1 dBA.
Consider Additional Exposures
Once areas of the facility with potential high-noise exposures are identified, most noise evaluations will then include personal sampling using noise dosimeters. Based on the results of the noise dosimetry, individuals are typically included in a formal hearing conservation program if their exposure exceeds the action level. Some facilities will attempt to reduce noise levels if the results exceed the OSHA PEL. Some facilities will simply provide hearing protection and not attempt to reduce noise levels.
In fact, few facilities consider any exposures other than the recorded noise levels when addressing NIHL. There are other potential exposures that should be considered when assessing the risk of NIHLs in a facility, including chemicals. Many chemicals have been identified with the potential to increase the risk of hearing loss even without high-noise exposures. These ototoxic chemicals have the potential to damage parts of the auditory system.2
Acute and chronic exposures to these chemicals can cause temporary or permanent hearing loss. Since this is caused by chemical exposure instead of noise exposure, this would not technically be NIHL. We might call it chemical induced hearing loss (CIHL). Many workplaces contain chemicals that are considered ototoxic. Table 1 is a list of some common ototoxic chemicals published by the National Institute for Occupational Safety and Health (NIOSH, 2018). Per the table, there are several volatile organic compounds (solvents), asphyxiants, and metals that are commonly present in industrial operations in the U.S.
Types of Hearing Loss
There are two types of hearing loss: conductive and sensorineural. Conduction loss Is the inability of sounds to be conducted from the outer ear to the inner ear where the nerve endings would be stimulated. Conductive hearing loss can be caused by ear infections and physical blockages, including a buildup of ear wax.
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 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 advisable. These actions should be evaluated even if industrial hygiene studies show that exposures are less than the published permissible exposure levels and ACGIH TLVs.
This article originally appeared in the May 1, 2021 issue of Occupational Health & Safety.
About the Author
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.