Pesticides--how research has succeeded and failed in informing policy: DDT and the link with breast cancer.

Investigation of chemical exposures as possible etiologic factors for breast cancer has not been a research priority in the United States, which is surprising given the evidence from animal studies that environmental chemicals cause cancer and reproductive dysfunction. Study of environmental chemicals has also been indicated by the failure of traditional epidemiologic methods to account for significant proportions of breast cancer incidence with other risk factors. The fact that breast cancer risk is strongly associated with reproductive hormones is a further clue that environmental chemicals should be investigated. In addition to cancer, specific outcomes that need to be explored are reproductive dysfunction, immunotoxicity and neurotoxicity. Policy guiding our research should encourage toxicologic investigations of exposures to environmental chemicals that use state-of-the-art methods to determine exposure and human health effects. Using the approach suggested by John McLachlan, functional toxicology should be used to assess the activity of chemicals with regard to these outcomes. Just as dioxin toxicity can be expressed as toxic equivalents, estrogenic activity, for example, can be characterized in terms of estrogenic equivalents. In addition to the need to undertake this kind of research, needs for methods development and creative research funding mechanisms are discussed. Prevention of breast cancer may require intervention at an early age. Better understanding of breast cancer etiology, and especially its environmental components, may lead us toward that goal.