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The low down on carcinogens

known human carcinogens, including alcoholic beverages, will not necessarily cause cancer. But the following are a few of the more recognizable substances considered known human carcinogens.


Coal, indoor emissions from household combustion


Mineral oils, untreated or mildly treated

Neutron radiation

Nickel compounds

Shale oils

•Tobacco smoke and smokeless tobacco

Are there different classifications of carcinogens?

The International Agency for Research on Cancer has spent the last 30 years evaluating the cancer-causing potential of more than 900 likely substances and exposures. That research has enabled the IARC to devise a system for classifying carcinogens as follows:

Group 1: Carcinogenic to humans

Group 2A: Probably carcinogenic to humans

Group 2B: Possibly carcinogenic to humans

Group 3: Unclassifiable as to carcinogenicity in humans

Group 4: Probably not carcinogenic to humans

Because testing carcinogens is so difficult (it is unethical to test on humans), many of the carcinogens studied by the IARC are classified as being a probable, possible or unknown risk. Slightly more than 100 of the candidates studied by the IARC are classified as Group 1 carcinogens.

It's important to note that, in addition to the IARC rating system, other agencies and groups, including the National Toxicology Program and the U.S. Environmental Protection Agency, have developed their own rating systems to determine the cancer-causing potential of certain substances and exposures.

For more information on carcinogens, including a list of known human carcinogens, visit the American Cancer Society at

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