Progress in identifying the preventable causes of human cancer: the experience of the IARC Monographs program

For 54 years, the Monographs program of the International Agency for Research on Cancer has evaluated the carcinogenicity of agents suspected of causing cancer in humans, classifying them into categories according to strength of the scientific evidence. More than 15 years ago, all Group 1 agents (carcinogenic to humans, the highest strength-of-evidence category) were re-evaluated. Since then, 24 additional agents have been classified for the first time in Group 1. Here, we summarize the evidence for the 135 Group 1 agents, emphasizing those recently classified and providing a synthetic overview of the evidence that contributed to the classification. All but two of these newly identified Group 1 agents were found to have sufficient evidence for one or more cancer types in humans, most commonly cancers of lung, hematolymphoid tissue, skin, and urinary bladder. Perfluorooctanoic acid and "dioxin-like" polychlorinated biphenyls were classified in Group 1 with less than sufficient evidence for cancer in humans. For nearly all Group 1 agents, there was also sufficient evidence for cancer in experimental animals, strong mechanistic evidence, or both, illustrating the robust biological plausibility of Group 1 classifications. This survey of the known human carcinogens confirms that there are few established preventable causes for 3 of the 4 most common cancer types (breast, prostate, and colorectum) and for reproductive cancers in both women and men. Over the last 15 years, the number of known human carcinogens has increased by >20%, reflecting the continued advancement of evidence and implying the existence of as yet unrecognized, preventable causes of cancer.