Carcinogen

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 inhaled asbestos and tobacco smoke.

Carcinogenity

Cancer is a disease where damaged cells of the patient's body do not undergo programmed cell death, but divide in an uncontrolled manner. Carcinogens may increase the risk of getting cancer by altering cellular metabolism or damaging DNA directly in cells, which interferes with biological processes, and induces the uncontrolled, malignant division ultimately leading to the formation of tumors. Usually DNA damage, if too severe to repair, leads to programmed cell death, but if the programmed cell death pathway is damaged, then the cell cannot prevent itself from becoming a cancer cell.

There are many natural carcinogens. 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. 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.

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 which PVC is manufactured, is a carcinogen.

Co-carcinogens are chemicals which do not separately cause cancer, but do so in specific combinations.

DNA is nucleophilic, therefore soluble carbon electrophiles are carcinogenic, because DNA attacks them. For example, some alkenes are toxicated by human enzymes to produce an electrophilic epoxide. DNA attacks the epoxide, and is bound permanently to it. This is the mechanism behind the carcinogenity of benzopyrene in tobacco smoke, other aromatics, aflatoxin and mustard gas.

Radiation

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.

Carcinogens in prepared food

Cooking protein-rich food at high temperatures, for example broiling or barbecuing meats, can lead to the formation of minute quantities of many potent carcinogens that are comparable to those found in cigarette smoke (i.e., benzopyrene).^[1] Charring of food resembles coking and tobacco pyrolysis and produces similar carcinogens. They are converted to by human enzymes into epoxides, which attach permanently to DNA. Pre-cooking meats in a microwave oven for 2-3 minutes before broiling can help minimize the formation of these carcinogens.

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.

Nevertheless, the fact that the food contains minute quantities doesn't necessarily mean that there is a significant hazard. The gastrointestinal tract sheds its outer layer continuously to protect itself from carcinomas, and has a high activity of detoxifying enzymes. The lungs are not protected in this manner, therefore smoking is much more hazardous.

IARC classification of carcinogens

• Group 1: the agent (mixture) is definitely carcinogenic to humans. The exposure circumstance entails exposures that are carcinogenic to humans.
• Group 2A: the agent (mixture) is probably carcinogenic to humans. The exposure circumstance entails exposures that are probably carcinogenic to humans.
• Group 2B: the agent (mixture) is possibly carcinogenic to humans. The exposure circumstance entails exposures that are possibly carcinogenic to humans.
• Group 3: the agent (mixture or exposure circumstance) is not classifiable as to its carcinogenicity to humans.
• Group 4: the agent (mixture) is probably not carcinogenic to humans.

Further details can be found in the IARC Monographs.

Notes

See also

• International Agency for Research on Cancer
• Mutagen
• Teratogen

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