Environmental Factors

  • Environmental factors (including smoking, diet, and sun exposure) probably account for three quarters of all cancer cases in the United States. For most people, the risks from carcinogens in tobacco smoke and from nutritional factors, including obesity and physical inactivity, have a larger effect on personal cancer risk than do pollutants in food, drinking water, and air. However, for both voluntary and involuntary exposures, the degree of cancer hazard depends on the concentration, intensity, and duration of exposure. Substantial increases in risk have been demonstrated in occupational settings where workers have been exposed to high concentrations of certain chemicals, metals, and other exposures, as well as among radiation victims, and patients treated with drugs or therapies later found to be carcinogenic.
  • Even low-dose exposures that pose only a small risk to individuals can represent significant public health hazards if the exposures are widespread (for example, secondhand tobacco smoke). Strong regulatory control and continuing attention to safe occupational practices, drug testing, and consumer product safety play an important role in minimizing these risks.
  • For most potential carcinogens, data are only available from high dose experiments in animals or highly exposed occupational groups. To use such information to set human safety standards, regulators must extrapolate from animals to humans and from high-dose to low-dose conditions. Because both extrapolations involve much uncertainty, as does the effect of mixtures of chemicals and of especially susceptible subgroups of the population, risk assessment generally makes conservative assumptions to err on the side of safety. For cancer safety standards, only increased risks of one case or less per million persons over a lifetime are usually acceptable.
  • Safety standards developed in this way for chemical or radiation exposures are the basis for federal regulatory activities at the Food and Drug Administration, the Environmental Protection Agency, and the Occupational Safety and Health Administration. The application of laws and procedures by which standards are implemented and risks are controlled is called risk management.
  • Various chemicals (for example, benzene, asbestos, vinyl chloride, arsenic, aflatoxin) show definite evidence of human carcinogenicity; others are considered probable human carcinogens based on evidence from animal experiments (for example, chloroform, formaldehyde, polychlorinated biphenyls [PCBs], polycyclic aromatic hydrocarbons). Often in the past, direct evidence of human carcinogenicity has come from studies of workplace conditions involving sustained, high-dose exposures. Occasionally, risks are greatly increased when particular exposures occur together (for example, asbestos exposure and cigarette smoking).
  • Only high-frequency radiation-ionizing radiation (IR) and ultraviolet (UV) radiation-has been proven to cause human cancer. Exposure to sunlight (UV radiation) causes almost all cases of basal and squamous cell skin cancer and is a major cause of skin melanoma. Disruption of the earth's ozone layer by atmospheric chemical pollution (the "ozone hole") may lead to rising levels of UV radiation.
  • Evidence that high-dose IR (x-rays, radon, etc.) causes cancer comes from studies of atomic bomb survivors, patients receiving radiotherapy, and certain occupational groups (for example, uranium miners). Virtually any part of the body can be affected by IR, but especially bone marrow and the thyroid gland. Diagnostic medical and dental x-rays are set at the lowest dose levels possible to minimize risk without losing image quality. Radon exposures in homes can increase lung cancer risk, especially in cigarette smokers; remedial actions may be needed if radon levels are too high.
  • Many kinds of pesticides (insecticides, herbicides, etc.) are widely used in producing and marketing our food supply. Although high doses of some of these chemicals cause cancer in experimental animals, the very low concentrations found in some foods are generally well within established safety levels. However, environmental pollution by slowly degraded pesticides such as DDT, a result of past agricultural practices, can lead to food chain bioaccumulation and to persistent residues in body fat. Continued research regarding pesticide use is essential for maximum food safety, improved food production through alternative pest control methods, and reduced pollution of the environment. In the meantime, pesticides play a valuable role in sustaining our food supply. When properly controlled, the minimal risks they pose are greatly overshadowed by the health benefits of a diverse diet rich in foods from plant sources, and the great reduction in prevalence of insect-borne diseases.
  • Household hazardous waste can threaten human health through air, water, and soil pollution. Although many toxic chemicals contained in such wastes can be carcinogenic at high doses, most community exposures appear to involve very low or negligible dose levels. The clean up of existing landfills and close control of toxic material disposal in the future are essential to ensure healthy living conditions in our industrialized society.

*Source: American Cancer Society, August 2001.