A Pesticide Information Project of Cooperative Extension Offices of Cornell University, Michigan State University, Oregon State University, and University of California at Davis. Major support and funding was provided by the USDA/Extension Service/National Agricultural Pesticide Impact Assessment Program.
Scientists do not yet understand exactly how cancer occurs or why some chemicals seem to cause cancer and others do not.
Chemicals which are known to cause cancer are called carcinogens and the process of cancer development is called carcinogenesis. Up to now, scientists have identified about two dozen chemicals or occupational exposures which appear to be definitely carcinogenic to humans. Some of the most familiar are tobacco smoke and asbestos. In addition, there are a number of chemicals which cause cancer in animals and are suspected of being human carcinogens. Since not all chemicals have been tested at present, it is possible that the number of known and suspect human carcinogens will increase in the future.
It must be remembered, however, that as with all toxic effects, the dose or amount of exposure is critical. Just as a small enough amount of cyanide will not lead to death, smoking one cigarette will not lead to lung cancer. Thus, in order to decide on the risk that a particular carcinogen poses, it is important to determine how much of the chemical will cause how many cases of cancer in a specified population. This value can then be compared to what is considered an acceptable risk. Currently, the generally accepted increase in risk of cancer is one additional cancer in one million people. A few exceptions to this criterion are made in the cases of food additives, including pesticides that are considered as food additives, where no amount of carcinogen is allowed (the Delaney Clause, as documented in the Federal Food, Drug and Cosmetic Act) and drinking water where a goal of zero contamination for carcinogens has been set.
Through epidemiological studies among industrial worker populations, it was possible to show that asbestos is linked to lung cancer, vinyl chloride to a rare form of liver cancer, and benzene to leukemia. There have also been suggestions that pesticide exposure to farmers might lead to cancer but the results are not clear cut and there is still much controversy about the epidemiological studies which have been performed on these populations. Even in well documented cases, it is not possible to use epidemiology to establish the exact risk of exposure to specific levels (concentrations) of these chemicals.
As a result, the acceptable exposure levels (published in the Federal Register for each carcinogen) usually represent what is called the "worst case" exposure. An assumption made in the calculation of worst case exposure levels is that humans will be exposed to the same concentration of the chemical every day of their lives for seventy years. As a result, the published acceptable risk level does not necessarily represent the "safe level" but rather a target level with the expectation that the true risk to exposure is less than the published value. Remember that the exposure criteria are guidelines for the protection of sensitive elements of the population and are calculated with many factors of uncertainty (the relationship of animal toxicity to human toxicity for instance).
In light of these considerations, it is not possible to determine the exact cancer risk for any human population, much less any individual. Public policy makers have tried to use worst-case analyses to be as protective of human health as possible. To minimize cancer, regulations have been designed to reduce population exposure to known human carcinogens as much as possible. In the case of known animal carcinogens, minimizing exposure is also a regulatory goal. However, since many of these chemicals are also quite beneficial to society, there are questions as to how much exposure reduction can be achieved without eliminating the benefits of these chemicals. Achieving a balance of risk and benefit is especially difficult when the uncertainties involved in determining the actual risk to humans is considered.
At present, there are a number of pesticides known to be animal carcinogens. None have been shown absolutely to be human carcinogens. Exposure to pesticides which are probable human carcinogens can be minimized through proper protective equipment and proper storage, use and disposal of these pesticides. These measures not only protect the pesticide applicator but also the general public, which consumes foods treated with pesticides or spends time in buildings treated with these chemicals.