Breast cancer and environmental risk factors: epidemiological and experimental findings.

Breast cancer has long been associated with reproductive hormone exposures. Recently, greater attention has been focused on environmental exposures that may be responsible for some proportion of breast cancer incidence. Several etiologic aspects are discussed. A number of chemicals induce breast cancer in rodents--including solvents, pesticides, and polycyclic aromatic hydrocarbons--and these might serve as leads for studies in humans. In women, strong links have been established between breast cancer risk and ionizing radiation. Evidence for nonionizing radiation (electromagnetic field) exposures and breast cancer is suggestive, albeit limited. Occupational exposures have not been identified as breast cancer risks, but several associations need further study, including solvents and pesticides. Time of life when exposures take place is important, and this claim is strongly supported by data on cigarette smoking and radiation. Also, basic research has demonstrated that mammary tissue is more susceptible to carcinogenesis at certain periods of breast development. Likewise, prenatal, neonatal, and adolescent exposures deserve continuing attention. Research on etiology of breast cancer should measure environmental exposures and take into account the time of life at which these occur. Complex interactions between exogenous and endogenous carcinogenic agents need further focus, as modulated by varying genetically determined individual susceptibilities.