Cancer Causes & Risk Factors Carcinogens and How They Cause Cancer By Lisa Fayed Lisa Fayed is a freelance medical writer, cancer educator and patient advocate. Learn about our editorial process Lisa Fayed Medically reviewed by Medically reviewed by Douglas A. Nelson, MD on April 16, 2020 linkedin Douglas A. Nelson, MD, is a board-certified oncologist and hematologist who previously served for 13 years as a physician in the US Air Force. Learn about our Medical Review Board Douglas A. Nelson, MD Updated on April 22, 2020 Print A carcinogen is any substance or agent that causes cancer. It does so by altering the cellular metabolism or by damaging DNA in our cells, interfering with normal cellular processes. The identification of substances in the environment that cause people to become ill with cancer helps in prevention efforts. Zero Creatives/Cultura/Getty Images Carcinogens can be chemical substances, viruses, or even medications and radiation therapies used to treat cancer. While a carcinogen or a combination of carcinogens can cause cancer, it may not always be the only reason since the tendency to develop cancer can be inherited. How Carcinogens Cause Cancer Carcinogenic substances can cause cancer in the following different ways: By directly damaging the DNA in cells leading to mutations (disrupting the normal process of cells) By not affecting the DNA directly, but instead causing cells to divide at a faster rate than normal, which can increase the chances that DNA changes and mutations will occur. The cell’s DNA can be damaged by a wide range of substances and exposures, including: Lifestyle: what you eat, if you smoke, lack of physical activity Natural exposure: to ultraviolet light, radon gas, infectious agents Medical treatment: radiation and chemotherapy, hormones, immunosuppressants Workplace exposure: some jobs have increased exposure to industrial chemicals or products Household exposure: cleaning products, paints, herbicides, and pesticides, etc. Pollution: outdoor air pollution or even secondhand tobacco smoke Some carcinogens do not directly cause cancer but can lead to cancer. Some carcinogens cause cancer if you have long-term, high levels of exposure. How your body reacts to this level of exposure, the length, the time, and the intensity of the exposure, combined with your genetic makeup, will determine the risk of developing cancer. Classification of Carcinogens Carcinogens are classified by The International Agency for Research on Cancer (IARC). The IARC is part of the World Health Organization (WHO) and its main goal is to determine the cancer-causing potential of different substances and classify carcinogens accordingly. Carcinogens are classified into one of the following 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 Known Substances That Are Carcinogenic to Humans It can be difficult to test and classify substances as carcinogenic to humans as it is not ethical to test to see if something can cause cancer by exposing people to it. Therefore researchers have only been able to classify a little over 100 substances as “carcinogenic to humans.” Some of the most common substances and exposures known as being carcinogenic to humans include (there are many more): Alcoholic beverages Arsenic and inorganic arsenic compounds Asbestos (all forms) and mineral substances (such as talc or vermiculite) that contain asbestos Benzene Cadmium and cadmium compounds Coal, indoor emissions from household combustion Engine exhaust, diesel Epstein-Barr virus (infection with) Estrogen postmenopausal therapy Formaldehyde Helicobacter pylori (infection with) Hepatitis B virus (chronic infection with) Hepatitis C virus (chronic infection with) Human immunodeficiency virus type 1 (HIV-1) (infection with) Human papillomavirus (HPV) (infection with certain types) Ionizing radiation (all types) Iron and steel founding (workplace exposure) Leather dust Mineral oils, untreated or mildly treated MOPP and other combined chemotherapy including alkylating agents Nickel compounds Outdoor air pollution Paint (workplace exposure as a painter) Processed meat (consumption of) Radon Rubber manufacturing industry Shale oils Silica dust, crystalline, in the form of quartz or cristobalite Solar radiation Tobacco, smokeless Tobacco smoke, secondhand Tobacco smoking Ultraviolet (UV) radiation and ultraviolet-emitting tanning devices Vinyl chloride Wood dust X- and Gamma-radiation Was this page helpful? Thanks for your feedback! Limiting processed foods and red meats can help ward off cancer risk. These recipes focus on antioxidant-rich foods to better protect you and your loved ones. Sign up and get your guide! Sign Up You're in! Thank you, {{form.email}}, for signing up. There was an error. Please try again. What are your concerns? Other Inaccurate Hard to Understand Submit Article Sources Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. American Cancer Society. Known and Probably Human Carcinogens. World Health Organization. Cancer Prevention.