Published in Cancer Detection and Prevention 2000; 24(Supplement 1).

Environmental and occupational exposures to carcinogens

JM Rice PhD

IARC Monographs Programme, International Agency for Research on Cancer, Lyon, France,

Most environmental exposures to known carcinogens (other than infectious agents) are at low levels and the results in terms of increased cancer incidence are difficult to measure. However, true epidemics of cancers caused by intensive environmental or occupational exposures to carcinogens have occurred both in the past and in recent times. These include bladder carcinomas from occupational exposures to aromatic amines; mesothelioma and lung cancer from asbestos; liver cancer from hepatitis B infections; thyroid carcinoma in children exposed to radioactive iodine from the Chernobyl accident; and the entire spectrum of tobacco-related cancers including lung cancer. Such epidemics could in principle be prevented. The IARC Monographs on the Evaluation of Carcinogenic Risks to Humans was established in 1972 to provide information on causes of human cancer for use in cancer prevention. The Monographs review the published scientific evidence for carcinogenicity of chemical, biological, and physical agents, complex chemical mixtures, occupational exposure circumstances and lifestyle factors such as tobacco use. Evaluations of the strength of the total evidence classify each agent into one of five categories, the most conclusive of which is Group 1 - carcinogenic to humans. IARC evaluations are qualitative in nature and do not necessarily reflect the relative potencies of different agents or their relative importance in terms of the sizes of populations at risk. IARC has evaluated 869 agents and exposure circumstances as of June 2000, some of them as many as four times as new evidence has appeared in the scientific literature. Group 1 now includes 87 agents and exposures, the majority of which are environmental or occupational carcinogens. These include 10 chronic infections or infectious agents; 25 chemicals (other than drugs), respirable dusts and mineral fibers; 13 industrial processes; solar radiation; various forms of ionizing radiation; and tobacco smoke. Group 1 infectious agents and processes are estimated to play a role in causing approximately one-sixth of all cancer cases globally. These include the viruses EBV, HTLV-I, and HPV types 16 and 18, and infections with HIV-1, hepatitis B and C viruses, Schistosoma haematobium, Opisthorchis viverrini, and Helicobacter pylori. This list is almost certainly incomplete, and additional biological agents will continue to be evaluated by the Monographs. The synergistic effects of combined exposure to multiple carcinogenic agents can be dramatic (aflatoxin and hepatitis virus B; tobacco smoking and radon inhalation). Combined effects of exposures to multiple chemical agents, or of exposure to chemical and either biological or physical agents, can be multiplicative rather than additive, and this aspect of environmental carcinogen exposure deserves greater attention. The most satisfactory hazard identifications define the carcinogenic agent or mixture as precisely as possible, but for a significant number of occupational circumstances the exposures are too complex for precise characterization. Industrial processes evolve rapidly over time and those included in Group 1 may be re-evaluated when additional studies more precisely define the carcinogenic agent or the aspect of the process that conveys a carcinogenic risk.

KEY WORDS: environment, occupational exposures, risk.

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Paper presented at the International Symposium on Impact of Biotechnology on Cancer Diagnostic & Prognostic Indicators; Geneva, Switzerland; October 28 - 31, 2000; in the section on environment & lifestyle.