There are two misperceptions about how many cancers are really caused by chemical substances in the environment.

First, the major risk comes from heavy exposure over several years, for instance work-related exposure, that is related to cancer risk, rather than the much lower risks resulting from exposure to the same chemical at environmental levels.

Second, “risk” may mean something different when relating to the individual compared with the entire population. A relatively low risk for the individual, such as that of lung cancer caused by air pollution or second-hand smoke, may result in a significant number of cancer cases in the population if many people are exposed, whereas heavy exposure in a relatively small group of workers will result in a high cancer risk among the exposed individuals but will cause only a small number of cancers in the entire population.