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What are the four different categories into which agents are classified

Posted Date:2025/5/27

What are the four different categories into which agents are classified?

Group 1: The agent is carcinogenic to humans

This category is used when there is sufficient evidence of carcinogenicity in humans. In other words, there is convincing evidence that the agent causes cancer in humans. The evaluation is usually based on the results of epidemiological studies showing development of cancer in exposed humans. Agents can also be classified in Group 1 on the basis of sufficient evidence of carcinogenicity in experimental animals supported by strong evidence in exposed humans that the agent exhibits one or more of the recognized key characteristics of human carcinogens.

Group 2

This category includes agents with a range of evidence for carcinogenicity in humans and in experimental animals. At one extreme of the range are agents with positive but not conclusive evidence in humans. At the other extreme are agents for which evidence in humans is not available but for which there is sufficient evidence of carcinogenicity in experimental animals. There are two subcategories, which indicate different levels of evidence.

Group 2A: The agent is probably carcinogenic to humans

This category is used when there is limited evidence of carcinogenicity in humans and either sufficient evidence of carcinogenicity in experimental animals or strong mechanistic evidence, showing that the agent exhibits key characteristics of human carcinogens. Limited evidence of carcinogenicity means that a positive association has been observed between exposure to the agent and cancer but that other explanations for the observations (technically termed “chance”, “bias”, or “confounding”) could not be ruled out with reasonable confidence. This category may also be used when there is inadequate evidence regarding carcinogenicity in humans but both sufficient evidence of carcinogenicity in experimental animals and strong mechanistic evidence in human cells or tissues.

Group 2B: The agent is possibly carcinogenic to humans

This category is generally used when only one of the following evaluations has been made by the Working Group:

• limited evidence of carcinogenicity in humans

• sufficient evidence of carcinogenicity in experimental animals

• strong mechanistic evidence, showing that the agent exhibits key characteristics of human

carcinogens.

Group 3: The agent is not classifiable as to its carcinogenicity to humans

This category is used most commonly when the evidence of carcinogenicity in humans is inadequate, the evidence of carcinogenicity in experimental animals is limited (or inadequate), and the mechanistic evidence is limited (or inadequate). Limited evidence of carcinogenicity in experimental animals means that the available information suggests a carcinogenic effect but is not conclusive.