Cancer Scares Over Trivia : Natural Carcinogens in Food Outweigh Traces in Our Water
The following commentary was adapted from testimony before the California Senate Committee on Toxics and Public Safety Management.
The carcinogens currently being found in California water supplies, such as in Silicon Valley, are present in extraordinarily tiny amounts that, except in rare cases, are trivial relative to the background level of carcinogens in nature. Therefore, I am convinced that such water pollution is irrelevant as a cause of human cancer.
The fallacy in our approach to such pollution is the belief that carcinogens are rare and that they are mostly man-made chemicals. Quite the contrary is the case; every meal has many natural carcinogens. My estimate is that more than 99.99% of the carcinogens we ingest are from natural or traditional sources such as cigarettes, alcohol and chemicals formed by cooking food.
Man-made toxic chemicals in food and water are almost always present in the parts per billion (ppb) range. One part per billion, (one person in all of China) is an extraordinarily small amount. The carcinogens in some common beverages occur in much higher concentrations.
Coffee contains the natural carcinogens hydrogen peroxide and methylglyoxal, each at about 4,000 ppb. Tap water contains the carcinogen chloroform at 83 ppb (U.S. average) due to chlorination. Cola drinks contain the carcinogen formaldehyde at 7,900 ppb. Even human blood averages about 3,000 ppb in formaldehyde from normal metabolism. Beer contains the carcinogens formaldehyde (700 ppb), and alcohol (50 million ppb, or 5%). Alcohol consumption is a known cause of 3% of human cancer in this country, and pure ethyl alcohol is a carcinogen in rats.
Milk contains a high percentage of fat. High fat consumption has been linked to human colon and breast cancer (though calcium from milk may be an important anti-carcinogen). Fruit juices may have carcinogenic mold toxins. The primary way we identify carcinogens is by testing chemicals at enormous doses in animals. The potency of carcinogens varies more than a million-fold. For example, aflatoxin, a mold carcinogen that is present in small amounts in peanut butter (2 ppb U.S. average), or in corn products such as tortillas, requires about a million times smaller dose to cause the same incidence of cancer in test animals as trichloroethylene, which was the main contaminant in Silicon Valley wells. Calculating a possible hazard to humans from a cancer test on rats must take into account a chemical’s potency as well as the daily human dose. My colleagues and I have done such a study and these are some examples of our findings.
The level of identified carcinogens in contaminated well water only rarely involves a possible hazard more than that of ordinary chlorinated tap water. Of 35 private wells shut down in Silicon Valley because of their supposed carcinogenic hazard found in an Environmental Protection Agency study, only two were of greater possible hazard than ordinary chlorinated tap water, and the most polluted well (2,800 ppb trichloroethylene) is still at least 1,000 times less of a possible hazard than an equal volume of cola, beer or wine. This is because trichloroethylene is an extremely weak carcinogen. It is comparable to saccharin and only 10 times more potent than alcohol, which is present at about 50 million ppb in a beer. Given that we drink only about one or two quarts of tap water per day, the trace amounts of man-made pollution typically found are an insignificant hazard.
Man-made pesticide residues in our food are about 100 ppb on the average; most of these residues are noncarcinogenic. DDT and its metabolite DDE, however, are carcinogens in animals. The possible DDT-DDE hazard of the average U.S. daily intake equals that of the chloroform in one glass of tap water and is insignificant compared to natural carcinogens in our diet. Even an occasional highly DDT-DDE or PCB-contaminated fish (100 times the average level) would contribute a possible hazard that is comparable to the average peanut butter sandwich and is very small compared to other common minimal risks, such as a glass of beer.
Our diet contains natural pesticides in amounts at least 10,000 times greater than residues of man-made pesticides. Natural pesticides are toxic chemicals, which are present in all plants, usually making up 5% to 10% of a plant’s weight. There is an enormous variety of them, though only a few are present in each plant species. The function of these chemicals is to protect the plants against fungi, insects and animal predators. Thus a major aspect of evolution of plants is chemical warfare. There has been relatively little research in the toxicology or carcinogenicity of these compounds, and so very few of the large number present in the human diet have been tested in animals.
A fair percentage of those few that have been tested have turned out to be carcinogens in rats or mice. They include estragole (in basil), safrole (in herbs), symphitine (in comfrey tea), psoralens (in parsley and celery), hydrazines (in mushrooms), and allyl isothiocyanate (in mustard). The possible carcinogenic hazard of nature’s pesticides completely overshadows the traces of man-made pesticide residues found in the daily diet. Plants also contain anti-carcinogens and valuable nutrients, so I believe that even these possible hazards seem too small to worry about, particularly in view of the difficulties of predicting risks to humans from tests in rats.
All calculations of human risk based on rat and mouse cancer tests, both from natural and man-made carcinogens, are hypothetical. Thus they should be viewed with a great deal of skepticism, unlike known human carcinogens, such as smoking and alcohol. Smoking causes 400,000 deaths from cancer, heart disease and other maladies every year; alcoholic beverages cause 100,000 deaths each year.
There also are many new reasons for being skeptical of uncritical low-dose extrapolation of risk to humans from animal data obtained by feeding enormous doses of carcinogens, which are too complicated to explain here. This is reinforced by the studies of cancer epidemiologists who are making considerable progress in understanding the smoking, dietary, hormonal, viral and occupational contributions to human cancer, but who are finding remarkably little solid evidence for any significant contribution from pollution.
Since we now know that carcinogens are common, not rare (more than half the chemicals tested in rodents were judged carcinogens), we must ignore the trivia if we are to deal effectively with the important causes of cancer. We might possibly eliminate every trace of man-made carcinogens from our water or food supply, but it would cost an enormous amount, be of minimal relevance to the causes of human cancer and distract health workers from real, more important cancer risks.
What we do need are some sensible regulations about pollution. We should also remember that life expectancy increases every year. Age-adjusted death rates for stomach, uterus and liver cancer have been decreasing for years; other major cancers, such as colon, breast and prostate have not increased significantly; only lung cancer--which is almost always due to smoking--has increased dramatically.
