Human Carcinogens

The International Agency for Research on Cancer (IARC) Monographs (www.monographs.iarc.fr) identify environmental and occupational causes of human cancer. Sometimes called the WHO “Encyclopedia of Carcinogens,” the IARC Monographs are critical reviews and evaluations of the weight of the evidence that an agent can increase the risk of cancer in humans. Since the program’s inception in 1971, over 1000 agents have been evaluated, including individual chemicals, complex mixtures, physical agents, biological agents, personal habits, and occupational exposures.

The agents are classified as “carcinogenic to humans” (Group 1), “probably carcinogenic to humans” (Group 2A), “possibly carcinogenic to humans” (Group 2B), “not classifiable as to their carcinogenicity to humans” (Group 3), or as “probably not carcinogenic to humans” (Group 4). This classification, based on all published scientific literature, reflects the strength of the evidence derived from epidemiological studies in humans, cancer bioassays in experimental animals, and in-vivo and in-vitro studies on the mechanisms of carcinogenicity. Evidence from studies in humans and animals is considered to be sufficient, limited, inadequate, or suggesting lack of carcinogenicity. Data from mechanistic studies are considered as providing strong, moderate, or weak evidence for a given mechanism. To date, 120 agents have been classified in Group 1, the vast majority on the basis of sufficient evidence from epidemiological studies that the agent can cause cancer at one or several sites in humans. Some important risk factors known to cause cancer in humans have however not been covered in the IARC Monographs program, notably genetic traits, reproductive status, and some nutritional factors. Other factors, such as weight control or physical activity, have been evaluated by the IARC Handbooks for their preventive effects.

The main figure shows, for each organ or group of organs in the human body, which agent(s) can cause an increased risk of cancer at a given site. (Figure 1)

Over 40 agents have more than one target organ site, with up to 17 sites for tobacco smoking and 14 sites for X-radiation and gamma-radiation. (Figure 2) Some agents have been classified in Group 1 with less than sufficient evidence from epidemiological studies, often on the basis of sufficient evidence of carcinogenicity in experimental animals and strong evidence in exposed humans that the agent acts through a relevant mechanism of carcinogenicity. It is noteworthy that a few agents have been shown to cause cancer in the offspring of the person exposed.