Abstract :
Every year, approximately 30 million people in the United States are occupationally exposed to hazardous noise. Noise-related hearing loss has been listed as one of the most prevalent occupational health concerns in the United States for more than 25 years. Thousands of workers every year suffer from preventable hearing loss due to high workplace noise levels.
Noise-induced hearing loss (NIHL) has been listed as one of the most prevalent occupational health concerns in all regions of the world. According to the WHO, only in the United States and Germany, respectively 30 and 4?5 million (12?15% of the workforce) workers are annually exposed to hazardous noise levels. It is estimated that thousands of workers every year suffer from preventable significant, permanent hearing loss mainly due to high workplace noise levels. However, noise exposure does not seem to be the only cause of occupational hearing loss. Today in many workplaces around the world, workers are commonly exposed to multiple harmful agents.
Thereʹs growing evidence that physiological interactions of some mixed exposures can contribute to an increase in the severity of harmful effects. Such interactions of course not only are applied to the combination of interfering chemical substances, but also in certain cases to the co?action of chemical and physical factors. Recently, for instance, the possibility of noise/solvents interaction was raised as a concern in occupational settings, when Bergstr?m and Nystr?m (1986) published the results of a 20?year epidemiological follow?up study in Sweden, started in 1958 and involving regular hearing tests in workers. Interestingly, a large proportion of workers employed in the chemicals divisions of companies suffered from hearing impairment, although noise levels were significantly lower than those in sawmills and paper pulp production. Indeed, the results from animal and epidemiological studies suggest that apart from occupational noise exposure, co-exposure to several ototoxicants (substances that may affect the structures and/or the function of the inner ear) and noise may lead to occupational hearing loss. The categories of chemicals with “good” (confirmed ototoxic substance) evidence of the qualitative ability to induce ototoxic effects are:
Pharmaceuticals with potential ototoxic side-effects such as Antibiotics (e.g. chemotherapeutic agents, streptomycin, dihydrostreptomycin, gentamycin, amikacin tetracycline antibiotics, erythromycin, vancomycin) and certain antineoplastics (antitumour drugs, e.g. cisplatin, carboplatin, bleomycin).
Solvents (toluene, ethylbenzene, n-propylbenzene, styrene and methylstyrenes, trichloroethylene, p-Xylene, n-Hexane and carbon disulfide).
Asphyxiants (carbon monoxide and hydrogen cyanide and its salts (cyanides).
Nitriles (acrylonitrile, 3,3’-iminodipropionitrile, 3-butenenitrile, cis-2-pentenenitrile, cis-crotononitrile).
Metals and metal compounds (lead and lead compounds, mercury (methyl mercury chloride, mercuric sulfide), tin, organic compounds, germanium (germanium dioxide).
Although these catagories are known to be ototoxic by themselves, investigations which deal with combined exposures to different ototoxic substances or simultaneous exposure to ototoxic substances and noise have shown adverse interactive (e.g. additive or synergistic) effects on hearing.
Regarding the standards, there is no common regulation that requires the monitoring of hearing for workers exposed to ototoxic chemicals without significant noise exposure and explicit requirements relating to co-exposure to noise and ototoxic substances.
More frequent medical surveillance should be considered for workers co-exposed to noise and ototoxic substances, irrespective of the noise exposure level, and workers’ health results should be recorded in order to detect early changes at individual and collective levels. The aim of health surveillance is to have a system which identifies early symptoms of hearing impairment. Otoacoustic emission measurements (in particular TEOAE,) could be a valuable complement to pure tone audiometry (PTA) recordings.
Ideally, an interview by an occupational doctor should take place with subsequent listing of potential ototoxic drugs consumed during a hospitalisation period before returning to work. Based on the precautionary principle, the use of individual hearing protectors from an exposure limit of 80 dB(A) in a complex occupational environment (noise plus chemical ototoxic substances) should be recommended. A special label for ototoxic substances may be considered.
Moreover, it is important not to neglect the importance of the education and motivation of the relevant stakeholders in hearing conservation programmes including exposure to chemicals.
In many cases the exposure to ototoxic chemicals may occur through dermal uptake, for which air-concentration-based occupational exposure limits provide no protection. In order to control the total body burden, biomonitoring is needed. Biological tolerance values, however, exist for only a small number of ototoxic chemicals. Moreover, these limit values are based on endpoints other than ototoxicity.
Occupational exposure limits are based on “critical effects”. A critical effect is the adverse health effect that is detected at the lowest exposure level – regardless of its nature. Ototoxicity is not tested as a matter of routine workers’ medical examinations. This endpoint, which in addition could occasionally be used as an early indicator of neurotoxicity, should be given higher priority when evaluating the toxicity of industrial chemicals and establishing occupational exposure limits.
Tests for ototoxicity therefore have to be standardised and incorporated into national and international guidelines. Relevant regulatory research should include bioassays applying minimum- and sub-effect concentrations of the individual stressors. Nevertheless, a deeper insight in the mode of action of ototoxic substances and their interaction with noise is an essential prerequisite for adequate risk management measures. It is hoped that in this context the ototoxic effects of workplace substances can be addressed more systematically.
