Cancer is the second leading cause of death in the United States. Understandably, Americans are very concerned and fearful about cancer and would welcome sound information on how to reduce the risk of the various forms of this disease. In an attempt to fill that need, some 20 years ago Congress passed a law requiring the Secretary of the Department of Health and Human Services (DHHS) to publish a biennial report telling American consumers what is known or suspected to cause human cancer. The DHHS delegated that responsibility to the National Toxicology Program (NTP).
Within the medical specialty of epidemiology the study of the causes and distribution of human disease in a population there is the sub-specialty of cancer epidemiology. There are thick text books and a multitude of professional journals which are replete with information on what causes cancer. Were a government agency to distill this information on causation and risk factors for cancer in a consumer-friendly, practical way, it would allow Americans to take a hands-on approach to reducing cancer risk. Ultimately such information could lead to a substantial decrease in cancer mortality.
Unfortunately, however, the "cancer causation" lists as prepared by the National Toxicology Program are misleading, out of context, irrelevant to 99% of Americans and, in short, are worse than useless. One might legitimately argue that the NTP "cancer list" actually does more harm than good by a) overwhelming us with "background noise" about cancer causation thus diverting our attention from the real, preventable causes of cancer; b)deterring us from using pharmaceuticals which promote overall good health, despite the fact that they may also present a small risk of cancer; and c)providing fodder for mischief by those who wish to terrorize us about the alleged cancer risk of trace levels of synthetic environmental chemicals.
There are five major defects in the NTP cancer list.
First, the lists of "substances, mixtures or exposures.... known to be human carcinogens" and its counterpart list of exposures "reasonably anticipated to be human carcinogens" are prepared primarily by committees of toxicologists not cancer epidemiologists.
Toxicology is a noble profession, and contributes to our understating of the safety and efficacy of various chemicals in our lives, including food additives, drugs and cosmetics. Toxicologists, however, do not study people; the subjects of their studies are laboratory animals. On the other hand, cancer epidemiologists do study human populations. For that reason alone, advice on what does and might cause cancer should be coming from the National Cancer Institute, not the National Toxicology Program.
Second, the vast majority of chemical substances listed on the "known" and "maybe" lists have zero relevance to the lives of the overwhelming majority of Americans. For example, ethylene oxide (a chemical used to make other chemicals and also used in the health care industry to sterilize medical devices) and cadmium (used in the manufacture of batteries) are listed as "known" carcinogens. We are also told that tetrafluoroethylene (used in the production of polymers) and trichlorethylene (used as a degreaser for metal parts) are "likely" to be human carcinogens. Almost none of us will ever have any occupational exposure to these chemicals, and the fraction of a percent of those who do have such exposure will not be at increased risk of cancer because exposure levels in the workplace are by law exceedingly low.
Third, and most offensive, the NTP groups the major preventable causes of cancer together with these purely hypothetical risks. "Tobacco Smoking" is slipped in there on the "known" list along with cadmium, crystalline silica, 1,3 Butadene and "dyes metabolized to benzidine". There is no clue in the NTP "cancer causation guide" that these chemicals account for approximately 0% of cancer causation and tobacco use accounts for nearly 40% of cancer mortality annually in the U.S. The NTP list must warm the heart of the cigarette manufacturers.
Fourth, the NTP list is one-dimensional. It makes no attempt to separate out any possible benefits of the listed chemicals, as well as purported cancer risk and it shockingly makes no reference to dose of exposure as it relates to risk. For example, the NTP lists tamoxifen as a "known carcinogen," because it increases the risk of uterine cancer, even while acknowledging the conclusive data that this same drug can reduce the rate of new and recurrent breast cancers in high risk women. Similarly, the NTP is proposing listing on its cancer causation list steroidal estrogen the hormone used in postmenopausal hormone replacement therapy and birth control pills because these preparations have been associated with endometrial cancer, and to a lesser extent, breast cancer. Another type of conjugated estrogen used to alleviate menopausal symptoms is already on the carcinogen list. Tragically, putting estrogens on the same list as tobacco as a "carcinogen" will inevitably dissuade women from using these forms of estrogen replacement, thus denying them enormous health benefits ,including protection from heart disease, osteoporosis and perhaps even Alzheimers disease.
"Consumption of alcoholic beverages" makes the "known cause of cancer" list but NTP never mentions what dose. How much alcohol is "known" to cause cancer? One glass of wine a day? It is known that moderate alcohol consumption, in the absence of smoking cigarettes, has never been shown to be a risk factor for cancer.
Fifth, NTP's long list of exposures "likely" to cause cancer is based on the premise that human exposure to trace levels of synthetic chemicals which cause cancer in high dose in laboratory animals presents a human cancer risk. Simply put, the NTP premise for "likely" carcinogens is that a mouse is a little man. NTP believes it is a "prudent, protective step" to assume that something will cause cancer in humans if it causes cancer at high dose in the laboratory. This assumption is at variance with the reality that a myriad of naturally occurring chemicals in food including safrole in table pepper and hydrazines in mushrooms have been shown to cause cancer in laboratory animals. But apparently being a "natural" chemical affords some degree of immunity from the NTP cancer classification.
Giving Americans the facts about how they can reduce their cancer risk is a good idea. But Congress should assign that task to cancer epidemiologists, not toxicologists, and should demand that the presentation of risks be relevant, prioritized and set in a scientific context.