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There are two main ways in which carcinogens Carcinogens are classified. The main classification is produced by the International Agency for Research on Cancer, the other by and the National Toxicology Program.

The [[International Agency for Research on Cancer]] (IARC), which is part if of the [[World Health Organization]], produces the most widely used system <ref name="ACS"/> for classifying carcinogens in its ''Monographs'', (published since 1971.<ref>http://monographs.iarc.fr/index.php</ref> ). Substances are classified by IARC as to their carcinogenic hazard to humans: ''Established'', ''Probable'', ''Possible'', ''Unclassifiable'', and ''Not probable''.<ref>IARC: [http://monographs.iarc.fr/ENG/Classification/index.php Classifications]</ref> The group qualifiers ''probable'' and ''possible'' have no [[quantitative]] significance but are used as descriptors of different levels of evidence of human carcinogenicity.<ref>Group [http://monographs.iarc.fr/ENG/Preamble/currentb6evalrationale0706.php definitions].</ref>

The [[National Toxicology Program]](NTP), part of the [[U.S. Department of Health and Human Services]], classifies carcinogens in two categories: ''Known'', and ''Reasonably anticipated''.<ref name="NTP">National Toxicology Progam: [http://ntp.niehs.nih.gov/index.cfm?objectid=03CA6383-9766-1F64-6637241FE0114FE9 Questions & Answers about the RoC].</ref> Since 1978, it has produced a list of carcinogenic material along with federal regulations limiting exposure.<ref name="NTP"/> ==Examples==The following are common-place examples of carcinogens:*[[Alcohol|Alcoholic]] beverages*[[Asbestos]]*Asphalt fumes*[[Chloroform]]*Diesel exhaust*[[Estrogen]]*Gasoline*Nickel*[[Solar radiation]]*[[Tobacco]] products*[[X-Radiation]]