Environmental Pollutants and Breast Cancer: The Evidence from Animal and Human Studies

Abstract

Clinicians are often called on by patients and the news media to comment on the environmental and preventable causes of breast cancer. Thoughtful, evidence-based comments must take into account animal and cell studies of biological mechanisms that may link chemicals from consumer products, workplace exposures, and pollution to the disease. They should also report the small but increasingly sophisticated body of epidemiologic results in humans that provide evidence of risks associated with some chemicals. Likewise, clinicians must state what we do not know. The majority of chemicals in use today have never been assessed for their roles in breast cancer, so the common statement that there is “no evidence” of a link between this disease and unstudied exposures can lead people to mistake a lack of evidence of harm for evidence of safety. A review volume, “Environmental Factors in Breast Cancer,” published by Cancer this year and 2 online databases (the Mammary Carcinogens Review Database and the Epidemiology Reviews Database, accessible at www.silentspring.org/ sciencereview) provide new resources for clarifying the status of scientific evidence in several key topic areas: animal studies that identify chemicals as mammary gland carcinogens1 and epidemiologic studies of environmental pollutants,2 diet (assessed prospectively),3 body size, and physical activity.4 This volume and the databases, both developed with support from Susan G. Komen for the Cure, include critical reviews of approximately 450 primary epidemiology research articles and information on 216 chemicals that increased the number of mammary gland tumors in animal studies. Substantial evidence shows that increasing physical activity, avoiding weight gain and obesity after menopause, and reducing alcohol consumption contribute to lowering risk of breast cancer, and these factors are now used to support clear public health messages to reduce incidence. Less familiar are laboratory findings suggesting that environmental chemicals may influence breast cancer risk by acting as classic carcinogens that damage DNA,5 by hormonal mechanisms that signal tumor cells to proliferate,6,7 or by altering mammary gland development in early life to increase susceptibility.8 Epidemiologic studies have also shown evidence of increased vulnerability to the effects of these chemicals when exposure occurs early in life and of interactions between exposure and susceptibility.2,9 Chemicals of concern include certain banned organochlorine compounds, air and water pollutants, organic solvents, and other ubiquitous agents, including consumer products and pharmaceuticals. These chemicals are discussed here and reviewed at length in the Cancer articles.1-4