Science and the Environment
Huge expenditures on tiny hypothetical risks do not improve public health.
SEPTEMBER 01, 1993 by BRUCE AMES
Bruce Ames is Professor of Biochemistry and Molecular Biology at the University of California, Berkeley, where he is also the director of the Environmental Health Sciences Center.
It is popular these days to espouse an apocalyptic vision of the future of our planet. Pollution is being blamed for global warming and ozone depletion, pesticides for cancer. Yet these and many other purported environmental causes are based on weak or bad science. The reality is that the future of the planet has never been brighter. With the bankruptcy of Communism, a hopeful world is on the path to democracy, free markets, and greater prosperity. Science and technology develop in a free society, and free markets bring wealth, which is associated with both better health and lower birth rates. Scientific advances and free markets can also lead to technologies that minimize pollution for the lowest cost. A market for pollution rights is desirable—polluting shouldn’t be free—and is much more effective than a bureaucratic monopoly. In my scenario for the future, I would like to see environmentalism based on scientific evidence and directed at solving real problems rather than phantoms.
An example of this problem is the public misconception that pollution is a significant contributor to cancer and that cancer rates are soaring. As life expectancy continues to increase in industrialized countries, cancer rates (unadjusted for age) also increase; however, the age- adjusted cancer death rate in the United States for all cancers combined (excluding lung cancer from smoking) has been steady or decreasing since 1950. Decreasing since 1950 are primarily stomach, cervical, uterine, and rectal cancers. Increasing are primarily lung cancer (which is due to smoking, as are 30 percent of all U.S. cancer deaths), melanoma (possibly due to sunburn), and non-Hodgkin’s lymphoma. Cancer is fundamentally a degenerative disease of old age, although external factors can increase cancer rates (cigarette smoking in humans) or decrease them (eating more fruits and vegetables).
A second misconception is that high-dose animal cancer tests tell us the significant cancer risks for humans. Approximately half of all chemicals—whether natural or synthetic—that have been tested in standard animal cancer tests have turned out to be carcinogenic. These standard tests of chemicals are conducted chronically, at near-toxic doses—the maximum tolerated dose—and evidence is accumulating that it may be the high dose itself, rather than the chemical per se that is the risk factor for cancer. (This is because high doses can cause chronic wounding of tissues or other effects that lead to chronic cell division, which is a major risk factor for cancer.) At the very low levels of chemicals to which humans are exposed through water pollution or synthetic pesticide residues, such increased cell division does not occur. Thus, they are likely to pose no or minimal cancer risks.
The third misconception is that human exposures to carcinogens and other toxins are nearly all due to synthetic chemicals. On the contrary, the amount of synthetic pesticide residues in plant foods are insignificant compared to the amount of natural pesticides produced by plants themselves. Of all dietary pesticides, 99.99 percent are natural: They are toxins produced by plants to defend themselves against fungi and animal predators. Because each plant produces a different array of toxins, we estimate that on average Americans ingest roughly 5,000 to 10,000 different natural pesticides and their breakdown products. Americans eat an estimated 1,500 milligrams of natural pesticides per person per day, which is about 10,000 times more than they consume of synthetic pesticide residues. By contrast, the FDA found the residues of 200 synthetic chemicals, including the synthetic pesticides thought to be of greatest importance, average only about 0.09 milligram per person per day.
The fourth misconception is that synthetic toxins pose greater carcinogenic hazards than natural toxins. On the contrary, the proportion of natural chemicals that is carcinogenic when tested in both rats and mice is the same as for synthetic chemicals—roughly half. All chemicals are toxic at some dose, and 99.99 percent of the chemicals we ingest are natural.
The fifth misconception is that the toxicology of man-made chemicals is different from that of natural chemicals. Humans have many general natural defenses that make us well buffered against normal exposures to toxins, both natural and synthetic. DDT is often viewed as the typically dangerous synthetic pesticide. However, it saved millions of lives in the tropics and made obsolete the pesticide lead arsenate, which is even more persistent and toxic, although all natural. While DDT was unusual with respect to bioconcentration, natural pesticides also bioconcentrate if they are fat soluble. Potatoes, for example, naturally contain fat soluble neurotoxins detectable in the bloodstream of all potato eaters. High levels of these neurotoxins have been shown to cause birth defects in rodents.
The sixth misconception is that correlation implies causation. The number of storks in Germany has been decreasing for decades. At the same time, the German birth rate also has been decreasing. Aha! Solid evidence that storks bring babies! Cancer clusters in small areas are expected to occur by chance alone, and there is no persuasive evidence from either epidemiology or toxicology that pollution is a significant cause of cancer for the general population.
There are tradeoffs involved in eliminating pesticides. Plants need chemical defenses—either natural or synthetic—in order to survive pest attack. One consequence of disproportionate concern about synthetic pesticide residues is that some plant breeders are currently developing plants to be more insect-resistant and inadvertently are selecting plants higher in natural toxins. A major grower recently introduced a new variety of highly insect-resistant celery into commerce. The pest-resistant celery contains 6,200 parts per billion (ppb) of carcinogenic (and mutagenic) psoralens instead of the 800 ppb normally present in celery. The celery is still on the market.
Synthetic pesticides have markedly lowered the cost of plant foods, thus making them more available to consumers. Eating more fruits and vegetables is thought to be the best way to lower risks from cancer and heart disease, other than giving up smoking; our vitamins, anti-oxidants, and fiber come from plants and are important anti-carcinogens. Thus, eliminating essential pesticides is likely to increase cancer rates. Huge expenditure of money and effort on tiny hypothetical risks does not improve public health. Rather, it diverts our resources from real human health hazards, and it hurts the economy.