Misconceptions about the Causes of Cancer

The major avoidable causes of cancer are: 1) smoking, which accounts for 27% of cancer deaths in Canada and 80-90% of lung cancer deaths; 2) dietary imbalances, which account for about an other third, e.g., lack of sufficient amounts of dietary fruits and vegetables. 3) chronic infections, mostly in developing countries; and 4) hormonal factors, which are influenced primarily by life style. This list may surprise readers who have come to think that synthetic chemicals like pesti cide residues and water pollutants are major causes.

There is no cancer epidemic except for cancer of the lung due to smoking. (Cancer is actu ally many diseases, and the causes differ for cancers at different target sites.) Since 1971, over all can cer mortality rates in Canada (exclud ing lung can cer) have declined 17% in women and 5% in men. Regulatory policy that focuses on traces of synthetic chemicals is based on miscon cep tions about animal cancer tests. Current research indicates that it is not rare for substances to cause cancer in laboratory rodents in the standard high-dose experiments. Half of all chemicals tested, whether occur ring naturally or pro duced synthetically, are "carcinogens"; there are high-dose effects in rodent cancer tests that are not relevant to low-dose human exposures and which may contribute to the high proportion of chemicals that test positive.

The focus of regulatory policy is on synthetic chemi cals, but 99.9% of the chemicals humans ingest are natural. For example, more than 1000 naturally occurring chemicals have been de scribed in coffee: 30 have been tested and 21 have been found to be carcinogenic in rodents in high-dose tests. Plants in the human diet contain thousands of natural "pesticides" produced by plants to protect themselves from in sects and other predators: 72 have been tested and 38 have been found to give cancer to rodents. Thus, exposure to synthetic rodent carcinogens is small compared to the natural background of rodent carcinogens. High dose rodent cancer tests need to be re-evaluated by viewing results from this perspective.

There is no convincing evidence that synthetic chemical pollutants are important as a cause of human cancer. Regulations targeted to eliminate low levels of synthetic chemi cals are enor mously expensive: the United States Environmental Protection Agency has estimated that envi ron mental regulations cost $140 billion/year in the United States. Others have estimated that the me dian toxic control program costs 146 times more per hypothetical life-year saved than the me dian medical in tervention. Attempting to reduce low hypothetical risks has other costs as well: if re ducing syn thetic pesticides makes fruits and vegetables more expensive, thereby decreasing consumption, then the cancer rate will likely increase. The prevention of cancer will come from knowledge obtained from biomedical research, education of the public, and lifestyle changes made by individuals. A re-examination of priorities in cancer prevention, both public and pri vate, seems called for.

In this study we highlight nine misconceptions about pollution, pesticides, and the causes of cancer. We briefly present the scientific evidence that undermines each misconception. The nine misconceptions are listed in the Table of Contents and an extensive bibliography is pro vided. Phrases in italics in the text are defined in the glossary.