In 1981 Doll and Peto reviewed current scientific knowledge regarding the environmental causes of human cancer, and they concluded that environmental pollution, in its various forms arising from human activity, could account for only a small fraction of total cancer mortality.1 Their best estimate for such mortality arising from all forms of pollution, principally affecting air, water, and food, was 2%, within a range of uncertainty extending from less than1% to 5%. In contrast, the use of tobacco was judged to account for 30% and dietary factors (excluding food additives) for 35% of cancer deaths.
These relative contributions to the human cancer burden are unlikely to have changed over the past 15 years. Nonetheless, despite this striking contrast in levels of risk, public concern regarding the role of environmental pollutants in causing human cancer remains intense, and for no specific class of potential pollutant is concern more intense than for chemical pesticides.
The increasing level of such concern in Canada (similar to concerns voiced in the U.S.) led the Canadian Cancer Society and the National Cancer Institute of Canada in 1994 to form a Panel to review current scientific knowledge regarding pesticides and cancer. I participated in that Panel's deliberations as one of two members from the U.S.; the Panel's full report is published in this issue of Cancer.2
Issues regarding pesticides and cancer are complex. Aside from scientific evidence regarding pesticide exposures and the potential importance of certain pesticides for carcinogenesis, one must consider regulatory aspects, socioeconomic implications, and public perceptions. In covering these several topics, the Panel's report concluded that no increase in overall cancer risk has appeared since the 1981 review, that safety regulations and procedures provide a wide margin of safety, and that agricultural uses of pesticides play a substantial role in providing high quality food products, especially fruits and vegetables, that contribute strongly to population health and to the primary prevention of cancer. This is not to say that scientific knowledge is complete, that regulatory systems have no flaws, that current uses of pesticides in agriculture and elsewhere do not need continuous scrutiny, and that alternate pest control approaches may not be required. Instead, the report calls for continued research to fill gaps in existing knowledge and to assure adequate risk assessment and risk management of existing and new pest control methods. It also calls for similar ongoing attention to the regulatory process, noting the need for continued advances in methods of toxicity testing (risk assessment), for sustained support for consumer education, and for the enforcement of adequate food inspection procedures.
Two classes of pesticides are of particular research interest, partly because of their history of extensive use, past and present: phenoxy herbicides, and organochlorine herbicides and insecticides. Phenoxy compounds have been extensively studied, especially 2,4-dichlorophenoxyacetic acid or 2,4-D. Unanswered research questions principally concern farm workers, where sustained, relatively high dose exposures may increase the risk of non-Hodgkin's lymphoma and perhaps soft tissue sarcomas. However, at low dose levels and under nonoccupational exposure conditions, cancer risks appear negligible.
For organochlorine compounds, much of the current research interest lies with tissue residues of dichlorodiphenyltrichloroethane (DDT) and its metabolic byproducts. Although DDT has long been banned from use in North America, its propensity for environmental persistence and for storage in tissue fat raises questions of long term exposure and possible chronic disease health effects. Most prominent has been interest in the estrogen-like activity of such chlorinated compounds and their potential implications for causation of human breast carcinoma. Although the most recent epidemiologic studies cast doubt on any large effects in this area, considerably more research is needed.3
Among the Panel's 14 recommendations, the final one emphasizes the need for research regarding approaches for explaining concepts of risk to the general public. This by itself is a difficult and complex matter. Not only are abstract concepts of risk hard to communicate effectively, but public perceptions of risk in different settings are affected greatly by considerations other than just the current state of scientific knowledge. Regarding pollution, health risks often are seen against a background of concern over a deteriorating global environment. That larger concern involves escalating population growth and uncontrolled exploitation of natural resources in addition to chemical pollution, yet it is often that pollution potential for human health effects, cancer especially, that receives the most attention. This may be because of the immediate and personal implications of possible effects on health. Those implications often can command far more lasting attention than other hazards that are more remote and impersonal, even though the actual risks may be very small.
For whatever reason, often it is observed that people feel more concern for risks over which they have no personal control than for risks associated with their familiar everyday life habits. Risks appear more dangerous if they are imposed from outside, regardless of the size of such risks. Hence concern regarding cancer risks arising from tobacco use and unbalanced diets often are greatly overshadowed by concern for possible pesticide exposures, even though the risks are vastly different. By the same token, risks that arise from man-made sources often are perceived as far more dangerous than those of natural origin. The existence of natural pesticides of various sorts in virtually all plant species, and hence regular exposure to such chemicals in one's daily diet receives far less attention than the low levels of pesticide residues that occasionally can be detected in foods and that nearly always are at levels well below regulatory safety limits. This is not to say that vigilance should be relaxed in food safety programs; the potential for gross misuse always exists, and adequate safeguards must be maintained. In this context, the central problem may lie with the tendency to perceive risk while ignoring dose. It obviously is important that all communications regarding risk should focus strongly on information regarding levels of dose and pathways of exposure.
Difficult as these issues are regarding risk perception and risk communication, public concern regarding pesticides as possible causes of cancer must not divert attention from eliminating the use of tobacco and improving dietary habits. Although continued research is needed to clarify relations between pesticide exposures and cancer risk, the low level of such risk in the general population does not warrant any major readjustment in current priorities for cancer control.
FOOTNOTES AND ACKNOWLEDGMENTS
See referenced original article on pages 2019-33, this issue.
Address for reprints: Clark W. Heath, Jr., M.D., American Cancer Society Inc., Epidemiology and Statistics, 1599 Clifton Road, NE, Atlanta, GA 30329.