The most common cause of workplace hearing loss is exposure to damaging levels of noise, so hearing conservation efforts in the workplace are almost exclusively focused on noise control. However, some chemical exposures can either directly damage hearing or act synergistically with noise exposure to increase hearing damage.

If you’ve acted to control noise exposures in your workplace, but you’re still seeing evidence of hearing loss, you should consider the possibility of ototoxic chemical exposures.

Ototoxic chemicals

Chemicals that can lead to hearing loss, either alone or in combination with noise exposures, include those in tobacco smoke, workplace chemicals, and prescription medications.

Tobacco smoke. Tobacco smoke can deprive tissues of oxygen, and it contains hydrogen cyanide. Either or both of these factors may account for hearing loss observed in connection with tobacco smoke exposure.

Workplace chemicals. Common occupational exposures that can cause or exacerbate hearing loss include:

• Solvents. Toluene, xylene, hexane, and styrenes have long been linked to hearing loss. In addition, evidence indicates that carbon disulfide, trichloroethylene, ethylbenzene, and n-propylbenzene can damage hearing.


• Asphyxiants. Both carbon monoxide and cyanides can cause hearing impairment, probably attributable to oxygen deprivation. Low-dose exposures tend to cause reversible damage; higher dose exposures can cause permanent damage.


• Nitriles. Acrylonitrile has been observed to cause temporary hearing loss, but other nitriles can cause permanent damage, including 3,3’-iminodipropionitrile, 3-butenenitrile, cis-2-pentenenitrile, and cis-crotononitrile.


• Metals and metal compounds. Lead, mercury, and organic tin compounds have long been known to cause hearing damage and damage to the vestibular system (which controls balance and is also located in the ear). Germanium dioxide is a less commonly known metal compound that can cause hearing loss. Some evidence also suggests that cadmium and arsenic can damage hearing.


• Bromates. Exposure to high doses of sodium and potassium bromate may cause permanent hearing damage, but studies have come to conflicting conclusions.


• Prescription medications. Workers who are at risk of noise-induced hearing loss should be aware of their additional risk if they begin taking:
  
  
  • Some antibiotics, in particular the aminoglycoside antibiotics (streptomycin, gentamicin, and amikacin) and certain other antibiotics (tetracyclines, erythromycin, and vancomycin).
  
  
  • Some antitumor drugs, including cisplatin, carboplatin, and bleomycin.
  
  
  • Some painkillers and fever reducers, including salicylates (aspirin), quinone, and chloroquine.
  
  
  • Loop diuretics like furosemide, piretanide, and bumetanide, which can be especially damaging when used together with antibiotics that can cause hearing loss.

As with most chemical exposures, the hearing damage caused by chemicals can be affected by worker factors that include:

• Genetic susceptibility. Some workers are genetically more likely to suffer hearing loss caused by antibiotics.


• Existing liver or kidney damage. Such damage can increase workers’ susceptibility to chemical-related hearing loss, possibly by failing to effectively remove toxins from the system.

Identifying ototoxic effects

One of the problems with hearing loss caused by ototoxic chemicals is that the type of hearing loss caused by chemicals isn’t always identifiable using standard audiometry methods.

Conventional audiometric testing, sometimes called “pure tone audiometry,” measures hearing in the range most common to human speech, from 250 to 8,000 hertz (Hz). Federal OSHA requires that audiometric tests include at a minimum 500, 1,000, 2,000, 3,000, 4,000, and 6,000 Hz frequencies. But some ototoxic chemical damage doesn’t show up on this type of audiometric exam until the damage is extensive.

If you have ototoxic chemical exposures in your workplace, you may want to consider adding hearing tests that measure other types of hearing damage that show up before hearing loss in the speech frequencies, such as:

• Speech audiometry. Speech audiometry measures an individual’s ability to understand speech units (syllables, multisyllable words, sentences, and running discourse) at a volume level that’s comfortable for the listener. These scores generally remain normal for people with noise-induced hearing loss, but those with chemically induced damage to the acoustic nerve or the auditory cortex in the brain will have below-normal scores.


• Brainstem auditory evoked potentials. This type of testing, also called brainstem auditory evoked response testing, is often used in newborns, young children, and other individuals who can’t be tested using pure tone audiometry. It can identify nerve damage caused by ototoxic chemicals, distinguishing it from noise-induced hearing loss.