Carcinogen
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{{wiktionary|carcinogen}}
[[Image:GHS carcinogen sign.svg|thumb|right|200px|The hazard symbol for carcinogenic chemicals in the [[Globally Harmonized System]].]]
The term '''carcinogen''' refers to any substance, [[radionuclide]] or radiation that is an agent directly involved in the promotion of [[cancer]] or in the facilitation of its propagation. This may be due to ability to damage the [[genome]] 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 ray]]s and [[alpha particle]]s, which they emit. Common examples of carcinogens are inhaled [[asbestos]], certain [[dioxins]], and [[tobacco]] smoke.
[[Cancer]] is 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 [[DNA]] directly in [[cell (biology)|cell]]s, which interferes with [[biology|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]] B<sub>1</sub>, which is produced by the [[fungus]] ''[[Aspergillus flavus]]'' growing on stored [[cereal|grain]]s, [[nut (fruit)|nut]]s and [[peanut butter]], is an example of a potent, naturally-occurring [[microbe|microbial]] carcinogen. Certain viruses such as [[Hepatitis B]] and [[human papilloma virus]]es 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]], [[1,2-Dibromoethane|EDB]], [[asbestos]], and the waste rock of [[oil shale]] mining have all been classified as carcinogenic.{{Fact|date=April 2007}} As far back as the 1930s, [[Industry|industrial]] smoke and [[tobacco]] smoke were identified as sources of dozens of carcinogens, including [[benzopyrene]], tobacco-specific [[nitrosamine]]s such as [[nitrosonornicotine]], and reactive [[aldehyde]]s such as [[formaldehyde]] — which is also a hazard in [[embalming]] and making [[plastic]]s. [[Vinyl chloride]], from which [[PVC]] is manufactured, is a carcinogen and thus a hazard in PVC production.
[[Co-carcinogen]]s 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-soluble]] so 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 [[nucleophilic attack|attacks]] them. For example, some [[alkene]]s are [[Toxication|toxicated]] by human enzymes to produce an [[Electrophile|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 [[radionuclide]]s as carcinogens, although the nature of the emitted [[Radioactive decay|radiation]] ([[alpha particle|alpha]], [[beta particle|beta]], or [[gamma ray|gamma]], and the energy), its consequent capacity to cause [[ionization]] in 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.
For example, [[Thorotrast]], a (incidentally-radioactive) [[suspension (chemistry)|suspension]] previously used as a [[Radiocontrast|contrast]] medium in [[x-ray]] diagnostics, is a potent human carcinogen known because of its retention within various [[organ (biology)|organ]]s 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 spectrum]] are 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 [[Food irradiation|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 radiation]] produced inside nuclear reactors can make substances radioactive.)
==Carcinogens in prepared food==
Cooking food at high temperatures, for example [[grilling]] 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]]).<ref>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.</ref> Charring of food resembles [[coking]] and tobacco [[pyrolysis]], and produces similar carcinogens. There are several carcinogenic pyrolysis products, such as polynuclear aromatic hydrocarbons, which are converted by human [[enzyme]]s into [[epoxide]]s, which attach permanently to DNA. Pre-cooking meats in a [[microwave oven]] for 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.<ref>[http://www.cancer.gov/cancertopics/factsheet/Risk/heterocyclic-amines National Cancer Institute, 2004 analysis and recommendations]</ref>
Reports from the Food Standards Agency have found that the known animal carcinogen [[acrylamide]] is generated in fried or overheated [[carbohydrate]] foods (such as [[french fries]] and [[potato chips]]).<ref>{{cite web |title=Acrylamide |url= http://www.food.gov.uk/safereating/chemsafe/acrylamide_branch/}} </ref> Studies are underway at the [[US Food and Drug Administration|FDA]] and [[Europe]]an regulatory agencies to assess its potential risk to humans. The charred residue on [[barbecue]]d meats has been identified as a carcinogen, along with many other [[tar]]s.
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 [[carcinoma]]s, and has a high activity of detoxifying enzymes.
==Classification of carcinogens==
Carcinogens can be classified as genotoxic or nongenotoxic. [[Genotoxin]]s 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 light]] and [[ionizing radiation]]. Certain viruses can also act as carcinogens by interacting with DNA.
[[Nongenotoxin]]s do not directly affect DNA but act in other ways to promote growth. These include hormones and some organic compounds.<ref>{{cite news | first= | last= | coauthors= | title=The Gale Encyclopedia of Cancer: A guide to Cancer and its Treatments, Second Edition. Page no. 137 }}</ref>
== [[International Agency for Research on Cancer|IARC]] classification of carcinogens ==
*[[List of IARC Group 1 carcinogens|Group 1]]: the agent (mixture) is definitely carcinogenic to humans. The exposure circumstance entails exposures that are carcinogenic to humans.
*[[List of IARC Group 2A carcinogens|Group 2A]]: the agent (mixture) is probably carcinogenic to humans. The exposure circumstance entails exposures that are probably carcinogenic to humans.
*[[List of IARC Group 2B carcinogens|Group 2B]]: the agent (mixture) is possibly carcinogenic to humans. The exposure circumstance entails exposures that are possibly carcinogenic to humans.
*[[List of IARC Group 3 carcinogens|Group 3]]: the agent (mixture or exposure circumstance) is not classifiable as to its carcinogenicity to humans.
*[[List of IARC Group 4 carcinogens|Group 4]]: the agent (mixture) is probably not carcinogenic to humans.
Further details can be found in the [http://monographs.iarc.fr/ IARC Monographs].
==Procarcinogen== <!--Procarcinogen redirects here-->
A procarcinogen is a [[precursor]] to a carcinogen. One example is [[nitrites]], for example when taken in by the diet. They are not carcinogenic themselves, but turn into [[nitrosamines]] in the body, which are carcinogenic.<ref> [http://www.lactospore.com/glossary.htm Web definitions for Procarcinogen]</ref>
==Notes==
{{reflist}}
== See also ==
* [[International Agency for Research on Cancer]]
* [[Mutagen]]
* [[Teratogen]]
* [[Warburg hypothesis]]
== External links ==
* [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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[[Category:Carcinogens|*]]
[[Category:Oncology]]
[[Category:Radiation health effects]]
[[Category:Toxicology]]
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