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A carcinogen is any substance capable of causing cancer.[1] Carcinogens do not always cause cancer. Risk factors include the length and intensity of exposure and the genetic makeup of the individual.[2]


Laboratory studies are the primary source for identifying carcinogens. In these studies, it is assumed that if a large dose (usually higher than common human exposure) of a substance causes cancer in cell cultures or animals it will also cause cancer in humans.[2]

Epidemiologic studies are another importance source of information about carcinogens. These focus on the interactions of human populations with the substances found in their environments. It is difficult to identify carcinogens through these studies because of their uncontrolled nature, but they aid scientists in making educated assessments of the cancer-causing ability of suspected carcinogens.[2]


Carcinogens are classified by the International Agency for Research on Cancer and the National Toxicology Program.

International Agency for Research on Cancer

The International Agency for Research on Cancer (IARC), of the World Health Organization, produces the most widely used system[2] for classifying carcinogens in its Monographs (published since 1971[3]). Substances are classified by IARC as to their carcinogenic hazard to humans: Established, Probable, Possible, Unclassifiable, and Not probable.[4] The group qualifiers probable and possible have no quantitative significance but are used as descriptors of different levels of evidence of human carcinogenicity.[5]

National Toxicology Program

The National Toxicology Program (NTP), of the U.S. Department of Health and Human Services, classifies carcinogens in two categories: Known, and Reasonably anticipated.[6] Since 1978, it has produced a list of carcinogenic material along with federal regulations limiting exposure.[6]

Commonplace examples

The following are common-place examples of carcinogens:

Disputed examples


  1. Compact Oxford English Dictionary.
  2. 2.0 2.1 2.2 2.3 American Cancer Society: Known and Probable Carcinogens
  4. IARC: Classifications
  5. Group definitions.
  6. 6.0 6.1 National Toxicology Progam: Questions & Answers about the RoC.
  7. Kasprzak KS, Sunderman FW Jr, Salnikow K. Nickel carcinogenesis
  8. Feds label estrogen carcinogen

External links