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The term carcinogen refers to any substance, radionuclide or radiation which is an agent directly involved in the promotion of cancer or in the facilitation of its propagation. This may be due to genomic instability or to the disruption of cellular metabolic processes. Several radioactive substances are considered carcinogens, but their carcinogenic activity is attributed to the radiation, for example gamma rays or alpha particles, which they emit. Common examples of carcinogens are asbestos and tobacco smoke.
Carcinogens may cause cancer by altering cellular metabolism or damaging DNA directly in cells, which interferes with normal biological processes. Aflatoxin B1, which is produced by the fungus Aspergillus flavus growing on stored grains, nuts and peanut butter, is an example of a potent, naturally-occurring microbial carcinogen.
Cooking protein-rich food at high temperatures, for example broiling or barbecuing meats, can lead to the formation of many potent carcinogens that are comparable to those found in cigarette smoke (i.e., benzo[a]pyrene). Pre-cooking meats in a microwave oven for 2-3 minutes before broiling can help minimize the formation of these carcinogens.
Benzene, kepone, EDB, asbestos, and the waste rock of oil-shale mining have all been classified as carcinogenic. As far back as the 1930s, industrial and tobacco smoke were identified as sources of dozens of carcinogens, including benzopyrene, tobacco-specific nitrosamines such as nitrosonornicotine, and reactive aldehydes such as formaldehyde — which is also a hazard in embalming and making plastics. Vinyl chloride from PVC is a carcinogen. Certain viruses such as Hepatitis B and human papilloma viruses have been found to cause cancer in humans. The first one shown to cause cancer in animals was Rous sarcoma virus, discovered in 1910 by Peyton Rous.
CERCLA identifies all radionuclides as carcinogens, although the nature of the emitted radiation (alpha, beta, or gamma, and the energy), its consequent capacity to cause ionization in tissues, and the magnitude of radiation exposure, determine the potential hazard. For example, Thorotrast, an (incidentally-radioactive) suspension previously used as a contrast medium in x-ray diagnostics, is thought by some to be the most potent human carcinogen known because of its retention within various organs and persistent emission of alpha particles. Both Wilhelm Röntgen and Marie Curie died of cancer caused by radiation exposure during their experiments.
Recent reports have found that the known animal carcinogen acrylamide is generated in fried or overheated carbohydrate foods (such as french fries and potato chips). Studies are underway at the FDA and European regulatory agencies to assess its potential risk to humans. The charred residue on barbecued meats has been identified as a carcinogen, along with many other tars.
Co-carcinogens are chemicals which do not separately cause cancer, but do so in specific combinations.