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Standard IARC classification

Compounds or physical factors assessed by IARC (International Agency for Research on Cancer) are classified in four groups based on the existing scientific evidence for carcinogenicity.

Group 1: "Carcinogenic to humans" There is enough evidence to conclude that it can cause cancer in humans.
IARC definition and list of compounds

Group 2A: "Probably carcinogenic to humans" There is strong evidence that it can cause cancer in humans, but at present it is not conclusive.
IARC definition and list of compounds

Group 2B: "Possibly carcinogenic to humans" There is some evidence that it can cause cancer in humans but at present it is far from conclusive.
IARC definition and list of compounds

Group 3: "Unclassifiable as to carcinogenicity in humans" There is no evidence at present that it causes cancer in humans.
IARC definition and list of compounds

Group 4: "Probably not carcinogenic to humans" There is strong evidence that it does not cause cancer in humans.
IARC definition and compound listed

Standard IARC classification categorization descriptions

Group 1: "The agent (mixture) is carcinogenic to humans . The exposure circumstance entails exposures that are carcinogenic to humans."

"This category is used when there is sufficient evidence of carcinogenicity in humans. Exceptionally, an agent (mixture) may be placed in this category when evidence of carcinogenicity in humans is less than sufficient but there is sufficient evidence of carcinogenicity in experimental animals and strong evidence in exposed humans that the agent (mixture) acts through a relevant mechanism of carcinogenicity."

Examples include asbestos, benzene and ionizing radiation.
List of agents evaluated as group 1 to date. 

Group 2 (A and B): "This category includes agents, mixtures and exposure circumstances for which, at one extreme, the degree of evidence of carcinogenicity in humans is almost sufficient, as well as those for which, at the other extreme, there are no human data but for which there is evidence of carcinogenicity in experimental animals. Agents, mixtures and exposure circumstances are assigned to either group 2A (probably carcinogenic to humans) or group 2B (possibly carcinogenic to humans) on the basis of epidemiological and experimental evidence of carcinogenicity and other relevant data."

Group 2A: "The agent (mixture) is probably carcinogenic to humans . The exposure circumstance entails exposures that are probably carcinogenic to humans."

"This category is used when there is limited evidence of carcinogenicity in humans and sufficient evidence of carcinogenicity in experimental animals. In some cases, an agent (mixture) may be classified in this category when there is inadequate evidence of carcinogenicity in humans and sufficient evidence of carcinogenicity in experimental animals and strong evidence that the carcinogenesis is mediated by a mechanism that also operates in humans. Exceptionally, an agent, mixture or exposure circumstance may be classified in this category solely on the basis of limited evidence of carcinogenicity in humans."

Examples include diesel engine exhaust, Formaldehyde and PCBs.
List of agents evaluated as group 2A to date. 

Group 2B: "The agent (mixture) is possibly carcinogenic to humans. "

"The exposure circumstance entails exposures that are possibly carcinogenic to humans.

This category is used for agents, mixtures and exposure circumstances for which there is limited evidence of carcinogenicity in humans and less than sufficient evidence of carcinogenicity in experimental animals. It may also be used when there is inadequate evidence of carcinogenicity in humans but there is sufficient evidence of carcinogenicity in experimental animals. In some instances, an agent, mixture or exposure circumstance for which there is inadequate evidence of carcinogenicity in humans but limited evidence of carcinogenicity in experimental animals together with supporting evidence from other relevant data may be placed in this group."

Examples include glass wool, styrene and gasoline exhaust.
List of agents evaluated as group 2B to date. 

Group 3: "The agent (mixture) is unclassifiable as to carcinogenicity in humans. "

"This category is used most commonly for agents, mixtures and exposure circumstances for which the evidence of carcinogenicity is inadequate in humans and inadequate or limited in experimental animals. Exceptionally, agents (mixtures) for which the evidence of carcinogenicity is inadequate in humans but sufficient in experimental animals may be placed in this category when there is strong evidence that the mechanism of carcinogenicity in experimental animals does not operate in humans.

Agents, mixtures and exposure circumstances that do not fall into any other group are also placed in this category."

Examples include anthracene, caffeine and fluorescent lighting.
List of agents evaluated as group 3 to date. 

Group 4: "The agent (mixture) is probably not carcinogenic to humans."

"This category is used for agents or mixtures for which there is evidence suggesting lack of carcinogenicity in humans and in experimental animals. In some instances, agents or mixtures for which there is inadequate evidence of carcinogenicity in humans but evidence suggesting lack of carcinogenicity in experimental animals, consistently and strongly supported by a broad range of other relevant data, may be classified in this group."

The only agent in that group is: Caprolactam (see Group 4 to date )

Source: IUPAC Glossary of Terms Used in Toxicology Classification of carcinogenicity 

Translation(s):

Español: Clasificación estándar de la IARC
Français: Classification du CIRC (IARC)

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