Examples of the agents for which IARC found sufficient evidence of carcinogenicity are aluminium production, asbestos, vinyl chloride, tobacco smoke, hepatitis B virus and ionizing radiation. These include petroleum refining, formaldehyde, diesel engine exhaust and the HIV virus. A further 209 agents are classified as possibly carcinogenic, based only on animal and laboratory studies. These include DDT, a dioxin, saccharin, and petrol engine exhaust. Whether or not an agent is carcinogenic is of little relevance unless people come in contact with that agent.
Exposure is a necessary condition for cancer to develop.
Given the lack of reliability of some of the evidence and the limits of epidemiology, they proposed a best estimate and a range of acceptable estimates. Doll and Peto's estimates are widely accepted though there is some dissension. However, the evidence for tobacco smoking is particularly reliable and all researchers agree that successful measures to reduce smoking will eventually reduce many kinds of cancer, especially lung cancer. There is less certainty about what it is exactly in the diet which increases the risk of cancer and even disagreement that diet should have such a large share. Doll and Peto suggest that it is not carcinogens in food such as pesticide residues which make diet so risky.