Ototoxicants commonly found in occupational environments can be found in three general classes of chemicals: solvents, (heavy) metals, and asphyxiants. Some studies include a limited number of nitriles and pesticides. Exactly which chemical substances under each category are considered ototoxic is a matter of debate and depends on the parameter used to define an “ototoxic” chemical as well as the robustness of the particular research being considered. Various entities around the world—including the European Agency for Safety and Health at Work (EU-OSHA), Safe Work Australia, Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST), the U.S. Army, and the Nordic Expert Group for Criteria Documentation of Health Risks from Chemicals—have provided different classification protocols, designating ototoxicants with qualifiers such as “confirmed,” “suspect,” “questionable,” “possible,” “non-conclusive,” “common,” “potential,” or simply “ototoxic.” The Nordic Expert Group used a three-tiered quantitative classification scheme based on the quality of the human and animal data available. Before adopting ototoxic classifications from any of these entities for use in your organization, discuss the research and decision logic with management and other stakeholders. Substances relevant to the occupational environment that are frequently listed as ototoxic solvents include carbon disulfide, ethylbenzene, styrene, toluene, trichloroethylene, and xylene. Commonly listed under the category of ototoxic metals are lead and mercury. The most common ototoxic asphyxiant listed is carbon monoxide. Many other substances are potentially ototoxic. Some organizations that classify chemicals as ototoxicants list only the chemicals that are specific to their operations. Noise is prevalent in many industries, and it isn’t hard to find ototoxicants in work environments where noise is also an agent of concern, creating an additional risk. Animal research and human epidemiological studies suggest that the combined effects of dual exposure to noise and ototoxicants can be either additive, potentiate, or synergistic. It is important to note that in cases of dual exposure, noise levels may not need to be above exposure limits for the combined effects to manifest themselves, and that impulse noise may be more harmful than continuous noise in the presence of some solvents, according to a paper in the March 2018 . In general terms, the effect of exposure to noise and ototoxic solvents is found to be additive at best, and synergistic at worst (as with toluene); exposure to carbon monoxide may potentiate noise-induced hearing loss. Research suggests that exposure to other chemicals and metals (such as lead) may interact with physical noise exposure, but the association has not been confirmed. Another consideration is that many workers are exposed to both occupational and non-occupational noise, as well as a variety of chemicals both inside and outside the work environment. This can make it difficult for researchers to isolate the effects of a single chemical on hearing loss or distinguish the impact of occupational exposure from environmental exposure. 

It is also worth noting that occupational exposure limits are established to address a few critical effects, yet hearing loss due to ototoxicants or combined exposure is hardly ever included under these adverse health effects for the purposes of OEL development. Experts are calling for more research in these areas. In the meantime, the safety and hygiene community can help mitigate the risk presented by ototoxicants and co-exposures with noise by adding ototoxicants to risk assessment matrices, educating stakeholders about the risks associated with occupational ototoxicants, and adhering to the hierarchy of controls when addressing ototoxicants and noise in the workplace. The U.S. Army, Safe Work Australia, EU-OSHA, and ACGIH have published recommendations for addressing ototoxic exposure. These recommendations include:

Where dual exposure to noise and ototoxicants exists, published recommendations from EU-OSHA and Safe Work Australia include:

Some of these recommendations are required by the health and safety regulations of certain countries (mostly in Europe).  The prevalence of noise as a physical agent in many workplaces and its potential for illness is unquestioned. Industrial hygienists are compelled to familiarize ourselves with information regarding ototoxicants and include these agents in hearing conservation programs and guidelines. Further research is necessary to advance our understanding of the concentrations of specific chemicals that elicit adverse auditory effects, identify all potential ototoxicants, and determine which chemicals are ototoxic alone and which are ototoxic only when combined with noise. But sufficient peer-reviewed evidence is available to make informed decisions for the benefit and better health of workers.    thanks LCDR N. Cody Schaal, PhD, CIH, CSP, and Jeremy M. Slagley, PhD, CIH, CSP, for reviewing this article. Schaal is an assistant professor in the Department of Preventive Medicine and Biostatistics at Uniformed Services University in Bethesda, Md. Slagley is an assistant professor in the Department of Systems Engineering and Management at the Air Force Institute of Technology. Send feedback to The Synergist.