IARC classifications
The International Agency for Research on Cancer sorts every agent it studies into one of five groups, based on how strong the evidence is that the agent can cause cancer under some conditions. Below is what each group actually means — in plain language.
Carcinogenic to humans
Strong evidence that this can cause cancer in humans. Examples: smoking, alcohol, UV radiation.
See all Group 1 factors →Probably carcinogenic
Likely to cause cancer. Evidence in humans is limited, but strong in animals or other studies.
See all Group 2A factors →Possibly carcinogenic
Some evidence exists but it is limited or mixed. Less certain than Group 2A.
See all Group 2B factors →Not classifiable
Not enough reliable evidence yet to say whether this causes cancer. This does NOT mean it is safe — it means science has not answered the question.
See all Group 3 factors →Probably not carcinogenic
Strong evidence that this is unlikely to cause cancer. Used very rarely.
See all Group 4 factors →Important reminder
IARC classifications describe the strength of evidence that an agent can cause cancer under some conditions — not the size of the risk to any individual, not the dose required, and not how likely everyday exposure is. A Group 1 classification does not mean an exposure is unavoidable or catastrophic; a Group 3 classification does not mean an agent is safe.