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Millions of US workers are exposed to substances that are known to cause cancer in humans. The National Institute for Occupational Safety and Health (NIOSH) has estimated that 3–6% of cancers worldwide are caused by occupational exposures and that 46,000–92,000 US workers are afflicted with cancer due to past workplace exposures every year.

The identification of occupational carcinogens is important in part because most occupational cancers are completely preventable with appropriate exposure controls, personnel practices, and strict protective legislation. NIOSH has concluded that for carcinogens that are directly genotoxic, there is no threshold exposure level below which cancer risk disappears and, for others with indirect genotoxic or nongenotoxic mechanisms, while there may be a safe exposure threshold, such a threshold is typically very difficult to prove empirically.


Several governmental agencies and other organizations determine the carcinogenicity of chemical agents. The primary agencies are the International Agency for Research on Cancer (IARC), the National Toxicology Program (NTP) of the U.S. Public Health Service, the NIOSH, the American Conference of Governmental Industrial Hygienists (ACGIH), and, in California, the Office of Environmental Health Hazard Assessment (OEHHA) of the California Environmental Protection Agency (EPA), which determines those chemicals to be included on the list of chemicals known to the state to cause cancer pursuant to the Safe Drinking Water and Toxic Enforcement Act of 1986 (“Proposition 65”).

The IARC Monographs on the Evaluation of Carcinogenic Risks to Humans are published by the World Health Organization’s (WHO) IARC. Each monograph represents the consensus of an international working group of expert scientists. The monographs include a critical review of the pertinent peer-reviewed scientific literature as the basis for an evaluation of the weight of the evidence that an agent may be carcinogenic to humans. Published continuously since 1972, the scope of the monographs has expanded beyond chemicals to include complex mixtures, occupational exposures, lifestyle factors, physical and biological agents, and other potentially carcinogenic exposures. Through 2017, IARC had classified 47 occupational chemicals and other substances as “carcinogenic to humans” (group 1 classification) and at least an additional 19 as “probably carcinogenic to humans” (group 2A classification). Unfortunately, IARC has to-date evaluated less than 2% of chemicals manufactured or processed in the United States for carcinogenicity, meaning that virtually all chemicals to which workers are exposed on a daily basis are of unknown carcinogenic potential.

IARC classifies agents in five categories:

  • Group 1: Agents that are carcinogenic to humans

  • Group 2A: Agents that are probably carcinogenic to humans (agents for which there is limited evidence of carcinogenicity in humans and sufficient evidence of carcinogenicity in experimental animals)

  • Group 2B: Agents that are possibly carcinogenic to humans (agents for which there is limited evidence of carcinogenicity in humans and an absence of sufficient evidence of carcinogenicity in animals, or when there is inadequate evidence of carcinogenicity ...

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