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The International Agency for Research on Cancer (IARC) is a cross-governmental agency under the World Health Organization, mainly responsible for the study of the etiology of cancer. IARC proposed that carcinogens are substances that can cause the occurrence and increase of malignant tumors, and chemicals that induce benign tumors under certain conditions can also be considered chemical carcinogens. The IARC carcinogens list is a reference indicator of carcinogenicity, which includes more than 1000 known carcinogens. With the emergence of new research data, IARC will also adjust and update the list from time to time.

IARC has carried out carcinogens classification for many years, and has now expanded the scope of classification to chemicals, chemical mixtures, environmental exposures, physical factors, biological factors and lifestyles, etc., mainly from the evidence of human carcinogenicity reports (epidemiological investigations and case reports), laboratory animal carcinogenicity data and biological mechanism research data for carcinogens classification.

In 2019, the IARC special report changed the carcinogens classification, simplifying the original four categories and five groups (Class 1, Class 2A, Class 2B, Class 3 and Class 4) into three categories and four groups (Class 1, Class 2A, Class 2B and Class 3). The specific classification definitions and classification criteria are shown in the following table:

Category	Definition	Classification criteria
Class 1	Carcinogenic to humans	1. Sufficient evidence of carcinogenicity in humans 2. important carcinogenic characteristics and sufficient evidence of carcinogenicity in experimental animals
Category 2A	Very likely to cause cancer in humans	Perform at least two of the following evaluations, including at least one involving the human body and human cells or tissues: 1. Limited evidence of carcinogenicity in humans 2. Sufficient evidence of carcinogenicity in laboratory animals 3. Strong evidence of having significant carcinogenic characteristics This category includes substances or mixtures that have a high likelihood of causing cancer in humans, have sufficient evidence of carcinogenicity in animal experiments, and have limited experimental evidence of carcinogenicity in humans despite theoretical carcinogenicity.
Category 2B	Possibly carcinogenic to humans	This category includes one of the following evaluations: 1. Limited evidence of carcinogenicity in humans 2. Sufficient evidence of carcinogenicity in animal experiments 3. Strong evidence of having significant carcinogenic characteristics (whether exposed to humans or human cells)
Category 3	Carcinogenicity to humans not yet classified	1. Factors that do not fall into any of the above categories are usually placed in this category. 2. insufficient evidence of carcinogenicity in both animal experiments and humans. 3. strong evidence that the substance has a carcinogenic mechanism in animal experiments but does not work in humans, or the evidence in humans is insufficient.

For unclassified items, no determination of non-carcinogenicity or overall safety should be inferred. The classification categories in the list only indicate the hazard intensity of the substance, but do not confirm the likelihood of risk, and are for reference only.

Regulations and standards

►Preventable Exposures Associated With Human Cancers (Cogliano et al., 2011)
►IARC Monographs Q&A

Regulatory authorities

►The International Agency for Research on Cancer (IARC)

How to obtain the list?

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Search on the official website: https://monographs.iarc.who.int/list-of-classifications
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