Carcinogenicity defined by the International Agency for Research into Cancer
| Group | Definition | Used when |
|---|---|---|
| 1 | Carcinogenic to humans | Evidence is sufficient |
| 2A | Probably carcinogenic to humans | Limited evidence in humans, and sufficient evidence in experimental animals, |
| 2B | Possibly carcinogenic to humans | Limited evidence in humans, and absence of sufficient evidence in experimental animals, or inadequate evidence in humans or human data non-existent and sufficient evidence in experimental animals |
| 3 | Not classifiable as to carcinogenicity to humans | Not classifiable to any other group |
| 4 | Probably not carcinogenic to humans | Evidence suggests a lack of carcinogenicity in humans and in experimental animals |