Carcinogens are agents that can cause cancer. Carcinogenic agents include various chemicals, radiation, and viruses. Although exposure to carcinogens can initiate cancer, the conditions that promote the disease process are not fully understood. Chemical carcinogens can be found in tobacco smoke, effluents from industry, environmental pollutants from automobiles, and fumes from factories and refineries. Physical carcinogens include ionizing radiation from X-rays and radioactive materials, as well as ultraviolet rays from sunlight. Some viruses are thought to cause cancer in animals, including humans, and are frequently referred to as oncogenic viruses. It is also possible that recurring irritation to a part of the body might be carcinogenic. Some cancers appear to be heritable in the sense that a predisposition exists. However, in such cases other carcinogenic influences appear to be necessary to trigger the development of cancer.
Many chemical substances that are known mutagens (induce mutations in genes) are also carcinogenic. Since Percivall Pott (1714-1788) called attention to the relationship between coal tar and chimney-sweeps' cancer of the scrotum, chemicals ranging from simple compounds such as formaldehyde to complex alkaloids have been indicted as carcinogens. A test devised by the American biochemist Bruce Ames has shown that about 90 percent of organic compounds that are known to cause cancer are also mutagens. The Ames test uses a bacterial system (Salmonella) as a model system to assess the production of single-gene mutations. More complicated in vitro tests using cultured animal cells have been used to measure single-gene mutations and chromosomal aberrations, such as changes in chromosome number, deletions or duplications within individual chromosomes. Unfortunately, some chromosomal abnormalities prevent cell growth and cannot be assessed in such systems. Therefore, such tests are ineffective in assessing somatic mutations, which are probably a significant cause of cancer and other serious human diseases.
Residents of industrial societies are probably exposed to more than 50,000 different chemicals in daily life. About 20 percent of the compounds that have been subjected to the Ames test appear to be mutagenic. However, most of the industrial chemicals in the environment have not been tested. Although it is impossible to calculate total exposure to widely used mutagenic chemicals, scientists generally agree that environmental carcinogens and mutagens present a definite public health risk.
In addition to their unavoidable exposure to carginogenic agents, many people voluntarily subject themselves to known carcinogens and mutagens, such tobacco and alcohol. Cigarette smoking is the most important environmental factor associated with lung cancer. The National Cancer Institute/National Toxicology Program (NCI/NTP) and other institutions have established the Carcinogenic Potency Database (CPDB) to serve as a resource for collecting and disseminating the results of chronic, long-term animal cancer tests. It provides a standardized, accessible database that includes information concerning experiments on carcinogenesis.
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