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The risk of developing lung cancer due to smoking and/or exposure to asbestos is a useful example of synergy, which occurs when the combined effect of two chemicals is much greater than their additive effects. Smokers have a ten-times greater risk of dying from lung cancer than non-smokers, and if people are exposed to asbestos, they are five times more likely to die of lung cancer compared with those not exposed to asbestos. However, the combined risk of dying of lung cancer in a smoker also exposed to asbestos is not 15 times greater than a non-smoker not exposed to asbestos, but 80 times greater.

Testing chemicals for their carcinogenicity is expensive, but two organisations which conduct a lot of carcinogenicity testing are the National Toxicology Program (NTP), part of the US government National Institutes of Health, and the International Agency for Research on Cancer (IARC), part of the World Health Organization.

Since 1972, IARC has published over 70 monographs reviewing the scientific evidence for carcinogenicity of individual chemicals, as well as mixtures and specific occupations. By 1998, about 834 chemicals, groups of chemicals, complex mixtures, and occupational exposures had been evaluated. Of these, IARC says 75 are carcinogenic to humans (group 1), 59 are probably carcinogenic to humans (group 2A), 225 are possibly carcinogenic to humans (group 2B), and the rest are unclassifiable (group 3). IARC classifications are based on the strength of the scientific evidence, not the potency of the carcinogen.

"Keep in mind that a chemical can be demonstrated to be a human carcinogen only if an opportunity exists to study it in exposed humans in a systematic way, and such opportunities are not frequently found" (J. V. Rodricks).

"Occupational cancer differs from other occupational diseases in several ways: no safe level of exposure to carcinogens is recognised; many different forms of cancer exist; cancer develops many years after exposure; occupational cancer generally resembles cancer of non-occupational origin; and competing carcinogenic exposures are present in many cases. On the other hand, occupational cancer shares at least important features with other occupational diseases: there are large data gaps in relating exposure to disease; and most cases are preventable" (H. Frumkin in Levy and Wegman).

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