Abstract
Workers are commonly exposed to multiple factors and substances that are hazardous to health. For instance, in 2005. approximately 30% of the European workers reported being exposed to noise during at least a quarter of the time spent in their work environment, 11 % reported inhaling vapors such as solvents and thinners: 19% reported exposure to smoke, fumes, powder or dust; and 14% reported handling chemical substances. Although occupational noise exposure has long been recognized in the United States and in Europe as the most deleterious factor to hearing the impact of chemical-induced hearing loss on workers should not be underestimated. A cursory survey of the literature for the last three decades reveals that concern about the effects of chemicals on hearing has grown steadily. The proliferation of work-related substances and medicinal drugs (mostly antitumor drugs and aminoglycosides) has been accompanied by an equivalent increase in the number of scientific publications on the hearing risks encountered by chemical-exposed persons. Cause for concern is even greater when the synergistic risks of co-exposures are considered. For example, physiological factors may increase the severity of a chemical's effect on hearing. Today, robust evidence also confirms that the effects of ototoxic substances on ear function can be aggravated by noise, which remains a well-recognized cause of hearing impairment. In an expert forecast supervised by the European Agency for Safety and Health at Work' (EU-OSHA, 2009), a "combined exposure to noise and ototoxic substances" was rated as an emerging risk. Industrial ototoxic chemicals have predominantly been assessed through animal studies. which are supported by data from epidemiologic studies on human workers from various industries. Bergstrom and Nystrom (1986) published the seminal results of a 20-year longitudinal study performed with 319 Swedish employees from different industrial sectors. This study began in 1958 and involved testing workers' hearing regularly. Its findings showed that a large proportion (23%) of the employees working in a chemical division suffered from hearing impairment, despite their exposure to lower noise levels than other divisions. Based on this type of evidence, researchers have long hypothesized that industrial solvents pose an additive risk to hearing. Scientific findings in animals are generally considered qualitatively relevant to human health. provided no substantial difference in biological response (e.g. metabolism) exists between test animals and humans. Despite these findings. (urrent research is limited by the following: (1) a lack of detailed exposure histories; (2) the presence of confounding factors (ototoxic drugs, tobacco, alcohol consumption. aging. and exposures outside the workplace); (3) and the fact that chemical exposure scenarios used in experimental investigations are qualitatively different from real-world occupational settings. In this extensive, but not exhaustive, review of the literature, the authors hope to share the evidence indicating that noise is not the only source of work-related hearing damage and to draw greater attention to the matter of chemically induced hearing loss. The present publication intends to provide occupational physicians and other health professionals with a clear picture of what is known about how chemical substances may affect hearing ability in the general population, and more specifically. in workers. The article describes the following: (1) chemicals that can be noxious to the inner ear; (2) work areas where exposure to ototoxic substances are likely; (3) and the basic features of the physiological mechanisms leading to hearing impairment. This article also addresses the limitations of pure-tone air-conduc tion audiometry when assessing chemical-induced hearing loss and proposes a more complete approach to evaluate the auditory neurosensory hearing receptor and the neural pathways involved in the stapedial reflex. With the proposed battery of tests, phYSicians will be able to evaluate both ototoxicity and neurotOXicity. Throughout this manuscript. emphasis is placed on the need for stronger links between primary care and occupational physicians. This could help preserve patients' hearing status as a part of their overall health.
