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Environmental Carcinogens and Cancer Risk

Does any exposure to a known carcinogen always result in cancer?

Any substance that causes cancer is known as a carcinogen. But simply because a substance has been designated as a carcinogen does not mean that the substance will necessarily cause cancer. Many factors influence whether a person exposed to a carcinogen will develop cancer, including the amount and duration of the exposure and the individual’s genetic background. Cancers caused by involuntary exposures to environmental carcinogens are most likely to occur in subgroups of the population, such as workers in certain industries who may be exposed to carcinogens on the job.

How can exposures to carcinogens be limited?

In the United States, regulations have been put in place to reduce exposures to known carcinogens in the workplace. Outside of the workplace, people can also take steps to limit their exposure to known carcinogens, such as quitting smoking, limiting sun exposure, limiting alcohol drinking, or, for those of the appropriate age, having HPV and HBV vaccination. See Risk Factors for Cancer for more information about known and suspected carcinogens.

Who decides which environmental exposures cause cancer in humans?

Two organizations—the National Toxicology Program (NTP), an interagency program of the U.S. Department of Health and Human Services (HHS), and the International Agency for Research on Cancer (IARC), the cancer agency of the World Health Organization—have developed lists of substances that, based on the available scientific evidence, are known or are reasonably anticipated to be human carcinogens.

Specifically, the NTP publishes the Report on Carcinogens every few years. This congressionally mandated publication identifies agents, substances, mixtures, or exposures (collectively called “substances”) in the environment that may cause cancer in humans. The 2021 edition lists 63 known human carcinogens and includes descriptions of the process for preparing the science-based report and the criteria used to list a substance as a carcinogen.    

IARC also produces science-based reports on substances that can increase the risk of cancer in humans. Since 1971, the agency has evaluated more than 1,000 agents, including chemicals, complex mixtures, occupational exposures, physical agents, biological agents, and lifestyle factors. Of these, more than 500 have been identified as carcinogenic, probably carcinogenic, or possibly carcinogenic to humans.

IARC convenes expert scientists to evaluate the evidence that an agent can increase the risk of cancer. The agency describes the principles, procedures, and scientific criteria that guide the evaluations. For instance, agents are selected for review based on two main criteria: (a) there is evidence of human exposure and (b) there is some evidence or suspicion of carcinogenicity.

How does the NTP decide whether to include a substance on its list of known human carcinogens?

As new potential carcinogens are identified, they are evaluated scientifically by the NTP’s Board of Scientific Counselors and the NTP Director. Next, a draft Report on Carcinogens monograph is prepared, which is reviewed by other scientific experts as needed, the public, and other federal agencies. The draft monograph is then revised as necessary and released for additional public comment and peer review by a dedicated panel of experts. Lastly, a finalized monograph and recommendation for listing is sent to the HHS Secretary for approval.

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