It is well documented that occupational noise exposure is a significant health hazard that leads to permanent noise-induced hearing loss (NIHL) and we have the National Exposure Standard for Occupational Noise to deal with this.  It is less well known that a substantial number of medications and common industrial chemicals can also cause hearing loss themselves or exacerbate the effects of noise.  These chemicals are said to be ototoxic (oto = ear, toxic = poisonous).  

They may damage the cochlea in the inner ear and/or the auditory neurological pathways leading to hearing loss, tinnitus and vertigo.  Hearing damage is more likely if exposure is to a combination of chemicals or a combination of the chemicals and noise.

Ototoxic chemicals can be divided into two general classes: workplace chemicals and medication. Currently it is thought that more than 750 different groups of chemicals are potentially ototoxic, but only a few of these have been studied in any depth.  

Activities where noise and ototoxic chemicals often combine include: painting, printing, boat building, construction, furniture making, manufacture of metal, fibreglass, leather and petroleum products, aircraft maintenance, assay labs, radiator repair, fuelling vehicles and aircraft, fire fighting, pesticide spraying and weapons firing.
Research is still being carried out to establish human exposure-response relationships for workplace ototoxic chemicals, either alone or in combination with noise.

Exposure standards for chemicals and noise have not yet been altered to take account of increased risk to hearing.  Material Safety Data Sheets in many cases do not contain warnings about potential hearing loss. 

Until revised standards are established, it is recommended that the 8-hour equivalent continuous noise level of workers exposed to any of the chemicals listed in Table 1 be reduced to 80 dB(A) or below. They should also undergo audiometric testing and be given information on ototoxic chemicals.  

Annual audiograms are highly recommended for workers whose airborne exposures (without regard to respiratory protection worn) are at 50% or more of the exposure standards stated in the Safe Work Australia Hazardous Substances Information System for the chemical in question, regardless of the noise level.  If no air monitoring has been carried out, workers should have an annual audiogram if they have frequent, long duration exposure to an ototoxic chemical in circumstances where: 

• The efficiency of ventilation is not known or there is no mechanical ventilation; and/or  
• Workers have reported health concerns that may be due to the chemical; and/or  
• It is difficult to estimate exposure.

Some potentially ototoxic chemicals may be absorbed through the skin (See Table 1).  If skin exposures cannot be controlled and are ongoing, annual audiograms are also recommended. 

For workers currently participating in an audiometric testing program due to excessive noise, suitably trained reviewers of the audiometric data should be alert to the relationship between the exposure to noise and ototoxic chemicals.  

If workers exposed to ototoxic chemicals complain of hearing difficulties, but have normal audiometric test results, they should be referred for more comprehensive audiological tests to evaluate the more central parts of the auditory system.   

Control measures such as substitution, isolation and local ventilation should be implemented to eliminate or reduce chemical exposures.  Personal protective equipment should be used to prevent skin and respiratory absorption when other controls are insufficient. 

Table 1  Possible workplace ototoxic chemicals 

Substances have been included in this table based on the list given in: 
Morata T.C. (2007) Promoting hearing health and the combined risk of noise-induced hearing loss and ototoxicity, Audiological Medicine, Vol.5, Issue 1, pp33-40. 

Some medications have been identified as ototoxic such as some anti-cancer, anti-inflammatory, anti-thrombotic, anti-malarial and anti rheumatic drugs, loop diuretics and antibiotics.

Information about the effects of these drugs on hearing should be included in training programs and workers should be encouraged to discuss any concerns they may have about medication with their doctor or pharmacist.

• Protect your hearing - chemicals can damage hearing