Groups 1, 2A, 2B and 3

The four groups

• Group 1: carcinogenic to humans. Convincing evidence, usually from studies in people.
• Group 2A: probably carcinogenic to humans. For example, limited evidence in people together with sufficient evidence in animals or strong evidence about the mechanism.
• Group 2B: possibly carcinogenic to humans. Weaker or less consistent evidence than for the groups above.
• Group 3: not classifiable. Most often used when the evidence is inadequate or limited.

This is a simplified summary. Each group is assigned from an overall assessment that combines evidence from studies in people, in experimental animals and on mechanisms, in several possible combinations; the full criteria are set out in the IARC Monographs Preamble.

Same group, different risk

IARC states plainly that the classification does not indicate the level of risk. Two agents in Group 1 can carry very different cancer risks; they share a group only because the evidence that each can cause cancer is equally convincing.

Who decides

Each evaluation is carried out by a Working Group of independent international experts, who review the published evidence against criteria set out in the Monographs’ Preamble. A separate Advisory Group recommends which agents to evaluate next. The full, current list of classified agents is published by IARC.