A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes.

Cancer is caused by changes in a cell’s DNA – its genetic “blueprint.” Some of these changes may be inherited from our parents, while others may be caused by outside exposures, which are often referred to as environmental factors. Environmental factors can include a wide range of exposures, such as:

Lifestyle factors (nutrition, tobacco use, physical activity, etc.)Naturally occurring exposures (ultraviolet light, radon gas, infectious agents, etc.)Medical treatments (radiation and medicines including chemotherapy, hormone drugs, drugs that suppress the immune system, etc.)Workplace exposuresHousehold exposuresPollution

Carcinogens do not cause cancer in every case, all the time. Substances labeled as carcinogens may have different levels of cancer-causing potential. Some may cause cancer only after prolonged, high levels of exposure. And for any particular person, the risk of developing cancer depends on many factors, including how they are exposed to a carcinogen, the length and intensity of the exposure, and the person’s genetic makeup.

The International Agency for Research on Cancer (IARC) is part of the World Health Organization (WHO). Its major goal is to identify causes of cancer. The most widely used system for classifying carcinogens comes from the IARC. In the past 30 years, the IARC has evaluated the cancer-causing potential of more than 900 likely candidates, placing them into one of the following groups:

Group 1: Carcinogenic to humansGroup 2A: Probably carcinogenic to humansGroup 2B: Possibly carcinogenic to humansGroup 3: Unclassifiable as to carcinogenicity in humansGroup 4: Probably not carcinogenic to humans