The term carcinogen refers to any substance,
radionuclideor radiation that is an agent directly involved in the promotion of canceror in the fatation of its propagation. This may be due to ability to damage the genomeor to the disruption of cellular metabolicprocesses. Several radioactive substances are considered carcinogens, but their carcinogenic activity is attributed to the radiation, for example gamma rays and alpha particles, which they emit. Common examples of carcinogens are inhaled asbestos, certain dioxins, and tobaccosmoke. Canceris a disease where damaged cells of the patient's body do not undergo programmed cell death, but their growth is no longer controlled and their metabolism is altered. Carcinogens may increase the risk of getting cancer by altering cellular metabolism or damaging DNAdirectly 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.
AflatoxinB1, 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 Band human papilloma viruses have been found to cause cancer in humans. The first one shown to cause cancer in animals is Rous sarcoma virus, discovered in 1910 by Peyton Rous. Benzene, kepone, EDB, asbestos, and the waste rock of oil shalemining have all been classified as carcinogenic.Fact|date=April 2007 As far back as the 1930s, industrial smoke and tobaccosmoke 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 embalmingand making plastics. Vinyl chloride, from which PVCis manufactured, is a carcinogen and thus a hazard in PVC production. Co-carcinogens are chemicals that do not separately cause cancer, but do so in specific combinations.
After the carcinogen enters the body, the body makes an attempt to eliminate it through a process called
biotransformation. The purpose of these reactions is to make the carcinogen more water-solubleso that it can be removed from the body. But these reactions can also convert a less toxic carcinogen into a more toxic one.
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 benzopyrenein tobacco smoke, other aromatics, aflatoxin and mustard gas.
CERCLAidentifies all radionuclides as carcinogens, although the nature of the emitted radiation (alpha, beta, or gamma, and the energy), its consequent capacity to cause ionizationin tissues, and the magnitude of radiation exposure, determine the potential hazard. Carcinogenity of radiation depends of the type of radiation, type of exposure and penetration. For example, alpha radiation has low penetration and is not a hazard outside the body, but are carcinogenic when inhaled or ingested.
Thorotrast, a (incidentally-radioactive) suspension previously used as a contrast medium in x-raydiagnostics, is a potent human carcinogen known because of its retention within various organs and persistent emission of alpha particles. Marie Curie, one of the pioneers of radioactivity, died of cancer caused by radiation exposure during her experiments.
Not all types of electromagnetic radiation are in fact carcinogenic. Low-energy waves on the
electromagnetic spectrumare generally not, including radio waves, microwave radiation, infrared radiation, and visible light. Higher-energy radiation, including ultraviolet radiation(present in sunlight), x-rays, and gamma radiation, generally "is" carcinogenic, if received in sufficient doses.
Substances or foods irradiated with electrons or electromagnetic radiation (such as microwave, X-ray or gamma) are not carcinogenic. No "radiation" remains, just like no light remains in a lens. (In contrast, non-electromagnetic
neutron radiationproduced inside nuclear reactors can produce secondary radiation by making bombarded substances radioactive.)
Carcinogens in prepared food
Cooking food at high temperatures, for example
grillingor barbecuingmeats, can lead to the formation of minute quantities of many potent carcinogens that are comparable to those found in cigarette smoke (i.e., benzopyrene). [Wei Zheng, Deborah R Gustafson, Rashmi Sinha, James R Cerhan, "et al." "Well-done meat intake and the risk of breast cancer." "Journal of the National Cancer Institute". Oxford: Nov 18, 1998.Vol. 90, Iss. 22; pg. 1724, 6 pgs.] Charring of food resembles cokingand tobacco pyrolysis, and produces similar carcinogens. There are several carcinogenic pyrolysis products, such as polynuclear aromatic hydrocarbons, which are converted by human enzymes into epoxides, which attach permanently to DNA. Pre-cooking meats in a microwave ovenfor 2-3 minutes before grilling shortens the time on the hot pan, and removes heterocyclic amine (HCA) precursors, which can help minimize the formation of these carcinogens. [ [http://www.cancer.gov/cancertopics/factsheet/Risk/heterocyclic-amines National Cancer Institute, 2004 analysis and recommendations] ]
Reports from the Food Standards Agency have found that the known animal carcinogen
acrylamideis generated in fried or overheated carbohydratefoods (such as french friesand potato chips). [cite web |title=Acrylamide |url= http://www.food.gov.uk/safereating/chemsafe/acrylamide_branch/ ] 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 does not 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.
Carcinogens in cigarettes
Tobacco smokecontains over 4000 chemical compounds, many of which are carcinogenic or otherwise toxic. [http://quitsmoking.about.com/cs/nicotineinhaler/g/carbonmonoxide.htm]
Classification of carcinogens
Carcinogens can be classified as genotoxic or nongenotoxic.
Genotoxins cause irreversible genetic damage or mutations by binding to DNA. Genotoxins include chemical agents like N-Nitroso-N-Methylurea(MNU) or non-chemical agents such as ultraviolet lightand ionizing radiation. Certain viruses can also act as carcinogens by interacting with DNA. Nongenotoxins do not directly affect DNA but act in other ways to promote growth. These include hormones and some organic compounds. [cite news | first= | last= | coauthors= | title=The Gale Encyclopedia of Cancer: A guide to Cancer and its Treatments, Second Edition. Page no. 137 ]
= 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 [http://monographs.iarc.fr/ IARC Monographs] .
A procarcinogen is a
precursorto a carcinogen. One example is nitrites, for example when taken in by the diet. They are not carcinogenic themselves, but turn into nitrosaminesin the body, which are carcinogenic. [ [http://www.lactospore.com/glossary.htm Web definitions for Procarcinogen] ]
International Agency for Research on Cancer
* [http://ntp.niehs.nih.gov/index.cfm?objectid=03C9B512-ACF8-C1F3-ADBA53CAE848F635 U.S. National Toxicology Program's Report on Carcinogens]
* [http://www.scorecard.org/health-effects/chemicals.tcl?short_hazard_name=cancer&all_p=t Recognized Carcinogens]
* [http://www.cancer.org/docroot/PED/content/PED_1_3x_Known_and_Probable_Carcinogens.asp?sitearea=PED American Cancer Society]
* [http://www.cancer-carcinogen.info Information on Carcinogens]
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