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Carcinogens and How They Are Brought to Light

Peer Review Status: Holden Comprehensive Cancer Center, Cancer Information Service
First Published: December 2004
Last Revised: January 2005

Cancer is caused by a cell's genetic blueprint being abnormal. This abnormality could be inherited from relatives or it could be caused by exposures to substances outside the body, such as radiation chemicals or viruses. These substances that can cause changes to the cell are called carcinogens. Carcinogen and carcinogenic refer to a substance that causes cancer. Carcinogens do not cause cancer in every case, every time. Substances classified as carcinogens may have different levels of cancer-causing ability. Some might cause cancer only after a very long exposure at high concentration. The risk for developing cancer depends on many factors, which includes the length and intensity of exposure to the carcinogen and the person's genetic makeup.

How do we know if something is a carcinogen?

• Laboratory studies with cell cultures and animals
• Epidemiologic studies--which look at the factors that may affect the occurrence of cancer in humans

Scientists combine the data from both types of studies to make a well-studied estimate about a substance's ability to cause cancer. If the evidence is conclusive, the substance may be considered a probable carcinogen.

The most widely used system for classifying carcinogens comes from the International Agency for Research on Cancer (IARC). IARC is part of the World Health Organization and has evaluated the cancer-causing ability of about 900 substances in the past 30 years. These substances are then placed into one of 5 groups:

• 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

In the United States, the National Toxicology Program (NTP) releases the Report on Carcinogens every two years. The NTP is formed from several government groups, including the National Institutes of Health, the Centers for Disease Control and Prevention and the Food and Drug Administration.

The Report on Carcinogens identifies two groups of substances:

• Known to be human carcinogens
• Reasonably anticipated to be human carcinogens

Points to remember about these lists:

• The IARC and NTP act independently but have studied many of the same agents; so many probable or suspected carcinogens appear on both lists. When one substance appears on one list but not the other, it does not mean there is a controversy. It could mean that one agency has not yet studied the substance.
• These lists are not all-inclusive. They will include only the agents that have been studied.
• Most substances on the list are linked only with specific kinds of cancer, not all kinds of cancer. These substances do not cause cancer in all people. Many times a substance causes cancer when the person is exposed to it in a certain way. Some substances cause cancer in very small amounts. Others will cause cancer after an intense exposure over a long period of time.
• Some substances on the lists are not necessarily to be avoided. For instance, chemotherapy drugs are listed as causing cancer. But taking the drug may be the only way to treat another type of cancer. The risks and benefits have to be weighed and discussed with a physician.