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Several carcinogens (cancer-causing agents) have already been mentioned. However, it is more usual to discuss cancer-causing chemicals together as a group, irrespective of which organs or systems they affect.

Cancer is not one, but dozens of diseases affecting different organs and tissues. Cancer kills over one in five of the population of industrialised countries, due to both inherited and environmental (including diet and lifestyle) factors as well as workplace exposures.

How much cancer results from workplace exposures has been argued about for years. In 1978, scientists from the US National Cancer Institute, National Institute of Occupational Safety and Health, and National Institute of Environmental Health Sciences estimated 20-40% of cancers were work-related (M Firth et al.). Two UK scientists, Richard Doll and Julian Peto, said in 1981 that only 2-8% of all cancers were due to work. These figures have been debated ever since, with arguments from a variety of standpoints (Hazards 54).

A 1996 review of occupational lung cancers said that 9,000-10,000 men and 900-1,900 women develop lung cancer each year in the USA due to past occupational exposure to carcinogens. This represents 9% of male and 2% of female US lung cancer cases. Others say that 17% of US male lung cancers stem from exposure to carcinogens at work (K Steenland et al., Review of occupational lung carcinogens, American Journal of Industrial Medicine 1996, 29, 474-490).

Cancers are complex diseases, because they usually result from several factors, and often take many years to develop. There are also several different steps involved in the process which turns normal cells into cancerous tumours. As a result, carcinogenesis (the development of cancer) is described as a multi-step process. The two most important stages are initiation and promotion. Some chemicals or their metabolites act as initiators, causing permanent changes in a cell's genetic makeup (mutation). Others will then promote the development of these abnormal cells into a tumour. Some chemicals act as both initiator and promoter. However, not all chemicals that cause mutations (mutagens) are carcinogens, and some chemicals cause cancer without damaging DNA.

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