Publications from the Carcinogenic Potency Project

Ames, B. N., and Gold, L. S. The causes and prevention of cancer: Gaining perspectives on management of risk. In: Improving Risk Management: From Science to Policy (ed. R. W. Hahn), Oxford University Press, pp. 4-45 (1996).

Several factors that are likely to have a major effect on reducing rates of cancer have been identified through epidemiology: reduction of smoking, increased consumption of fruits and vegetables, and control of infections. Other factors are avoidance of intense sun exposure, increased physical activity, and reduced consumption of alcohol and possibly red meat. Already, risk of many forms of cancer can be reduced, and the potential for further reductions is great. If lung cancer (90% of which is due to smoking) is excluded from the analysis, cancer death rates are decreasing in the United States for all other cancers combined. We review the latest research on the causes of cancer, and show why much cancer is preventable.

The idea that synthetic chemicals such as DDT are major contributors to human cancer is not supported by the evidence, as discussed in this chapter. Pollution appears to account for less than 1% of human cancer; yet public concern and resource allocation for chemical pollution are very high, in good part because of the use of animal cancer tests in cancer risk assessment. Animal cancer tests, which are done at the maximum tolerated dose (MTD), are being misinterpreted to mean that low doses of synthetic chemicals and industrial pollutants are relevant to human cancer. About half of the chemicals tested, whether synthetic or natural, are carcinogenic to rodents at these high doses. A plausible explanation for the high frequence of positive results in rodent bioassays is that testing at the MTD frequently can cause chronic cell killing and consequent cell replacement, a risk factor for cancer that can be limited to high doses. Ignoring this effect greatly exaggerates low-dose risks. Scientists must determine mechanisms of carcinogenesis for each substance and revise acceptable dose levels as understanding advances.

Almost all chemicals ingested by humans are natural. For example, 99.99% of the pesticides we eat are naturally present in plants to ward off insects and other predators. Half of the natural pesticides that have been tested at the MTD are rodent carcinogens. Reducing exposure to the 0.01% of pesticides that are synthetic will not reduce cancer rates. On the contrary, although fruits and vegetables contain a wide variety of naturally-occurring chemicals that are rodent carcinogens, inadequate consumption of fruits and vegetables doubles the human cancer risk for most types of cancer. Making these foods more expensive by reducing synthetic pesticide use is likely to increase cancer because of reduced consumption. Humans also ingest large numbers of natural chemicals from cooking food. Over a thousand chemicals have been reported in roasted coffee: more than half of those tested (19/26) are rodent carcinogens. By weight, there are more rodent carcinogens in a single cup of coffee than potentially carcinogenic pesticide residues in the average American diet in a year, and there are still a thousand chemicals left to test in roasted coffee. This does not mean that coffee is dangerous, but that animal cancer tests and worst-case risk assessment build in enormous safety factors and should not be considered true risks.

Humans can eat the tremendous variety of natural chemicals that are "rodent carcinogens" because humans, like other animals, are extremely well protected by many general defense enzymes, most of which are inducible (i.e., whenever a defense enzyme is in use, more of it is made). Since the defense enzymes are equally effective against natural and synthetic chemicals one does not expect, nor does one find, a general difference between synthetic and natural chemicals in the frequency of positive results in high-dose rodent tests.

There is no evidence for an increase in human cancer caused by exposure to synthetic industrial chemicals. If lung cancer (90% of which is due to smoking) were eliminated, then cancer death rates would have declined 14% in the last 40 years. In addition, there is a steady rise in life expectancy in the developed countries. Linear extrapolation from the MTD in rodents to low level exposure in humans has led to grossly exaggerated mortality forecasts. Such extrapolations cannot be verified by epidemiology. Furthermore, relying on such extrapolations for synthetic chemicals while ignoring the enormous natural background, leads to an imbalanced perception of hazard and allocation of resources. The perception of hazard is also greater now because major advances in analytical techniques enable the detection of extremely low concentrations of chemicals, often a million times lower than could have been detected 30 years ago. Moreover, low levels of rodent carcinogens of natural origin are ubiquitous in the environment. It is thus impossible for humans to be totally free of exposure to rodent carcinogens or to background radiation. It is the progress of scientific research and technology that will continue to lengthen human life expectancy.

Society must distinguish between significant and trivial cancer risks. Regulating trivial risks or exposure to substances erroneously inferred to cause cancer at low doses can harm health by diverting resources from programs that could be effective in protecting public health. Moreover, wealth creates health: poor people have shorter life expectancy than healthy people. When money and resources are wasted on trivial problems, society's wealth, and hence health, are harmed.

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Last updated: October 28, 1998

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