The causes and prevention of cancer: the role of environment.

Abstract

The idea that synthetic chemicals such as DDT are major contributors to human cancer has been inspired, in part, by Rachel Carson's passionate book, Silent Spring. This chapter discusses evidence showing why this is not true. We also review research on the causes of cancer, and show why much cancer is preventable. Epidemiological evidence indicates several factors likely to have a major effect on reducing rates of cancer: reduction of smoking, increased consumption of fruits and vegetables, and control of infections. Other factors are avoidance of intense sun exposure, increases in physical activity, and reduction of alcohol consumption and possibly red meat. Already, risks of many forms of cancer can be reduced and the potential for further reductions is great. If lung cancer (which is primarily due to smoking) is excluded, cancer death rates are decreasing in the United States for all other cancers combined. 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 frequency of positive results 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 greatly exaggerates risks. Scientists must determine mechanisms of carcinogenesis for each substance and revise acceptable dose levels as understanding advances. The vast bulk of chemicals ingested by humans is natural. For example, 99.99% of the pesticides we eat are naturally present in plants to ward off insects and other predators. Half of these natural pesticides tested at the MTD are rodent carcinogens. Reducing exposure to the 0.01% 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 them more expensive by reducing synthetic pesticide use will increase cancer. 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 (19/28) are rodent carcinogens. 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 rather that animal cancer tests and worst-case risk assessment, build in enormous safety factors and should not be considered true risks. The reason humans can eat the tremendous variety of natural chemical "rodent carcinogens" is that 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 ability to cause cancer in high-dose rodent tests. The idea that there is an epidemic of human cancer caused by synthetic industrial chemicals is false. In addition, there is a steady rise in life expectancy in the developed countries. Linear extrapolation from the maximum tolerated dose in rodents to low level exposure in humans has led to grossly exaggerated mortality forecasts. Such extrapo