Chemicals, cancer and cancer biology.

Chemicals can cause cancer in humans and animals. Two significant questions are how and how frequently do these neoplasms arise? The first documentation of chemically induced cancer in humans was of the occupationally related "soot wart" in 1775. Since that time various carcinogens have been identified. Some compounds act directly on cell populations, whereas others must be metabolized by a host to produce a "proximate" or "ultimate" carcinogen. Because of the variety in carcinogen structure and the multiplicity of modifications to the cellular macromolecules, a simple explanation for chemical effect is unlikely. Furthermore, true neoplastic growth involves at least two and possibly more steps, some of which are reversible. Evidence suggests that cancer represents an altering of differentiation and that chemical agents may act at the level of DNA or on epigenetic regulatory phenomena. The method for selecting the neoplastic cell from those that are normal is not known. Because we cannot explain the mechanisms for cancer formation or the role of chemicals in the process, prudence is needed in determining the significance of human exposure and in relating this exposure to the risk of neoplastic disease.