A carcinogen is a substance that can cause cancer in humans or animals. Carcinogens bring about molecular and biochemical disturbances in cells, resulting in dedifferentiation (the loss of cells' morphological and functional specializations, such that they behave like immature cells capable of resuming cell division) and uncontrolled growth (neoplasia).
Some common substances that are known to be carcinogenic are asbestos, pesticides, lead, cadmium, arsenic , benzene, polyvinyl chlorides (PVC), soot, crystallized silica, glass wool (often a component of fiberglass), tobacco smoke, and smokeless tobacco. Most of these chemical carcinogens are called "procarcinogens," requiring metabolic conversion into "ultimate carcinogens" capable of damaging deoxyribonucleic acid ( DNA , the genetic material in cells). Ultraviolet radiation (UVA, UVB, and UVC) from the Sun is also carcinogenic and can lead to different types of skin cancer.
For a substance to be declared a carcinogen there must exist sufficient evidence of a relationship between exposure to that substance and cancer in humans or animals. Because the use of human test subjects is deemed unethical, testing is done on animals (e.g., mice and rats) and on animal and human cell cultures (specialized nutrient-rich growth media); the data are then extrapolated to humans. An agent's potential carcinogenicity in humans is also determined from what is known of that agent's effect at the molecular level (i.e., damage to DNA/protein) or from anecdotal evidence(e.g., UV exposure and cancer).
The potency of a carcinogen is expressed as the dose rate that, when administered chronically throughout the standard life span of a test species, will reduce the probability of the population remaining tumorless for that period by 50 percent. Called the TD50 value, the rate is measured in milligrams per kilogram [mg/kg] body weight per day. Hence, the lower the TD50 value, the more potent the carcinogen. Caution is necessary in extrapolating TD50 values from animal systems to humans, however, because the pathway through which that substance becomes carcinogenic may be absent in humans.
Pohanish, Richard P., and Sittig, Marshall, eds. (2002). Handbook of Toxic and Hazardous Chemicals and Carcinogens. Norwich, NY: William Andrew Inc.
"National Toxicological Program Report on Carcinogens." National Toxicology Program. Updated December 11, 2002. Available at http://ntp-server.niehs.nih.gov/NewHomeRoC/AboutRoC.html .
"Safety and Health Topics: Carcinogens." U.S. Department of Labor, Occupational Safety & Health Administration. Available at http://www.osha.gov/SLTC/carcinogens .