Misconceptions About Environmental Pollution, Pesticides and the Causes of Cancer

Policy Reports | Energy and Natural Resources

No. 214
Sunday, March 01, 1998
by Bruce N. Ames and Lois Swirsky Gold

Misconception #7: Synthetic Chemicals Pose Greater Carcinogenic Hazards than Natural Chemicals

Gaining a broad perspective about the vast number of chemicals to which humans are exposed can be helpful when setting research and regulatory priorities.61 Rodent cancer tests provide little information about how a chemical causes cancer or about low-dose risk. The assumption that synthetic chemicals are hazardous has led to a bias in testing, such that synthetic chemicals account for 77 percent (432 of 559) of the chemicals tested chronically in both rats and mice. [See Appendix Table IV.] The natural world of chemicals has never been tested systematically. One reasonable strategy is to use a rough index to compare and rank possible carcinogenic hazards from a wide variety of chemical exposures at levels that humans typically receive, and then to focus on those that rank highest.62 Ranking is a critical first step that can help to set priorities for selecting chemicals for long term cancer tests, studies on mechanism, epidemiological research and regulatory policy. Although one cannot say whether the ranked chemical exposures are likely to be of major or minor importance in human cancer, it is not prudent to focus attention on the possible hazards at the bottom of a ranking if, using the same methodology to identify hazard, there are numerous common human exposures with much greater possible hazards. Our analyses are based on the HERP index (Human Exposure/Rodent Potency), which indicates what percentage of the rodent carcinogenic potency (TD50 in mg/kg/day, i.e. dose to give 50 percent of test animals tumors) a human receives from a given daily lifetime exposure (mg/kg/day).63 [See Appendix Table Va and Vb.] A ranking based on standard regulatory risk assessment would be similar.

"Our results call for a reevaluation of the utility of animal cancer tests in protecting the public against minor hypothetical risks."

Overall, our analyses have shown that HERP values for some historically high exposures in the workplace (e.g., butadiene and tetrachloroethylene) and some pharmaceuticals (e.g., clofibrate) rank high, and that there is an enormous background of naturally occurring rodent carcinogens in typical portions of common foods that cast doubt on the relative importance of low-dose exposures to residues of synthetic chemicals such as pesticides.64 A committee of the National Research Council/National Academy of Sciences recently reached similar conclusions about natural vs. synthetic chemicals in the diet, and called for further research on natural chemicals.65

The possible carcinogenic hazards from synthetic pesticides (at average exposures) are minimal compared to the background of nature's pesticides, though neither may be a hazard at the low doses consumed. [See Appendix Table Va and Vb.] Appendix Table V also indicates that many ordinary foods would not pass the regulatory criteria used for synthetic chemicals. For many natural chemicals the HERP values are in the top half of the table, even though natural chemicals are markedly underrepresented because so few have been tested in rodent bioassays. Caution is necessary in drawing conclusions from the occurrence in the diet of natural chemicals that are rodent carcinogens. It is not argued here that these dietary exposures are necessarily of much relevance to human cancer. Our results call for a reevaluation of the utility of animal cancer tests in protecting the public against minor hypothetical risks.

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