We are all aware of the problem of noise in the workplace and how hearing can be irreparably damaged. But did you know that hearing can be damaged by certain chemicals in the workplace? Yesterday we looked at findings concerning solvents in the workplace and hearing loss. Today we will review findings that suggest that exposure to metals and asphyxiants in the workplace can also have an adverse effect on hearing.
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According to the National Institute for Occupational Safety and Health (NIOSH), in addition to noise, exposure to ototoxic chemicals in the workplace can also damage hearing. Ototoxic refers to drugs or other substances that have a harmful effect on hearing and balance.
Examples of ototoxic chemicals in the workplace are:
- Organic solvents, such as styrene, trichloroethylene, toluene, and solvent mixtures;
- Heavy metals, such as mercury, lead, and trimethyltin; and
- Asphyxiants, such as carbon monoxide, hyrdrogen cyanide, and acrylonitrile.
NIOSH claims that about 10 million workers are exposed to solvents and an unknown number are exposed to other ototoxicants.
In a collaboration with the Swedish Nordic Expert Group for Criteria Documentation of Health Risks from Chemicals (NEG), NIOSH conducted research to evaluate occupational exposure to chemicals and hearing impairment.
Here’s a snapshot of some of the research findings for metals and asphyxiants commonly found in the workplace.
Findings for Metals
Lead. Studies conducted with lead-exposed workers consistently report an association between lead exposure and central auditory effects. Chronic lead exposure impaired the auditory nerve and the auditory pathway in the lower brainstem. Noise levels were not always reported, but the researchers concluded that because of the nature of the work performed, it is likely that the studied workers were also exposed to noise. One study reported significant correlation between blood lead levels and hearing thresholds. The exposed workers, with a mean blood lead value of 37 micrograms per deciliter (µg/dl) had significantly elevated hearing thresholds compared to the controls.
Note. According to NIOSH, an elevated blood lead level for adults is anything greater than 5 µg/dl.
Mercury. Studies conducted with mercury-exposed workers were consistent in reporting an association between mercury exposure and central auditory effects. Chronic mercury exposure impaired auditory nerves and the auditory pathway in the lower brainstem. Noise levels were not always reported, but the researchers again concluded that because of the nature of the work performed, it is likely that the studied workers were also exposed to noise.
In one study, 40 workers were exposed to mercury, 37 workers were exposed to chlorinated hydrocarbons, and a control group of 36 subjects was exposed to no neurotoxic substances. The mean duration of exposure to mercury and chlorinated hydrocarbons was 15.5 and 15.8 years, respectively. The air concentration of current mercury exposure was 8 micrograms per cubic meter (µg/m3). The mean blood mercury level was inconsistently reported but was presumably 5 µg/l. Results showed that a higher percentage of workers exposed to mercury and chlorinated hydrocarbons had a higher degree of hearing loss compared to control subjects (42% and 34% versus 18%, respectively).
Findings for Asphyxiants
Carbon monoxide (CO). Several reports have documented that hearing loss is one of the outcomes associated with acute CO poisoning. (Noise exposure was not a necessary factor for the auditory problems to occur in humans; however, animal studies have shown the combination of noise and CO to significantly effects hearing.) One occupational study examined 700 patients suspected of having CO poisoning. Most of the participants were from occupations in which workers were exposed to CO from gas generators used in automobiles (56% were drivers). The participants’ mean exposure duration was 6 years. Two hundred sixty-three participants (38%) were diagnosed with chronic CO poisoning. Among those, 78% had hearing loss. The researchers concluded that CO poisoning was associated with hearing loss despite the lack of excessive noise exposure.
Other asphyxiants. Although there is limited research on the auditory effects of other asphyxiants on humans, animals studies have shown that exposure to hydrogen cyanide, acrylonitrile, and 3,3’-Iminodipropionitrile can cause hearing loss either alone or combined with noise.
Takeaway
The major route for exposure in the workplace for these ototoxic chemicals is the respiratory system. This suggests that the implementation of an effective respiratory program would serve to protect hearing as well.
Chemical hazards are present in most workplaces. Check out BLR’s TRAC360 Chemical Lists to see if the chemicals in your workplace are on critically important regulatory lists.