Table 2.
Integration of streams of evidence in reaching overall classifications
| Stream of evidence |
Basis of overall evaluation | Classification based on strength of evidence | ||
|---|---|---|---|---|
| Cancer in humans* | Cancer in experimental animals | Mechanistic evidence | ||
| Sufficient | Not necessary | Not necessary | Cancer in humans | Carcinogenic to humans (group 1) |
| Limited or inadequate | Sufficient | Strong: key characteristics of carcinogens, from exposed humans | Cancer in experimental animals and mechanistic evidence | |
| Limited | Sufficient | Not necessary | Cancer in humans and cancer in experimental animals | Probably carcinogenic to humans (group 2A) |
| Inadequate | Sufficient | Strong: key characteristics of carcinogens, from human cells or tissues | Cancer in experimental animals and mechanistic evidence | |
| Limited | Less than sufficient | Strong: key characteristics of carcinogens | Cancer in humans and mechanistic evidence | |
| Limited or inadequate | Not necessary | Strong: the agent belongs to a mechanistic class of agents for which one or more members have been classified in group 2A or 1 | Mechanistic evidence | |
| Limited | Less than sufficient | Limited or inadequate | Cancer in humans | Possibly carcinogenic to humans (group 2B) |
| Inadequate | Sufficient | Not necessary | Cancer in experimental animals | |
| Inadequate | Less than sufficient | Strong: key characteristics of carcinogens | Mechanistic evidence | |
| Limited | Sufficient | Strong: the mechanism of carcinogenicity in experimental animals does not operate in humans† | Cancer in humans and mechanistic evidence | |
| Inadequate | Sufficient | Strong: the mechanism of carcinogenicity in experimental animals does not operate in humans† | Mechanistic evidence | Not classifiable as to its carcinogenicity to humans (group 3) |
| All other situations not listed above | ||||
*
Highest strength of evidence for any cancer site(s).
†
The “strong evidence that the mechanism of carcinogenicity in experimental animals does not operate in humans” must specifically be for the tumor sites supporting the classification of “sufficient evidence in experimental animals.”