Cancer Causes & Risk Factors Carcinogens and How They Cause Cancer By Julie Scott, MSN, ANP-BC, AOCNP Julie Scott, MSN, ANP-BC, AOCNP LinkedIn Oncology Certified Nurse Practitioner and freelance healthcare writer with over a decade of medical oncology and hematology experience. Learn about our editorial process Updated on May 17, 2022 Medically reviewed Verywell Health articles are reviewed by board-certified physicians and healthcare professionals. These medical reviewers confirm the content is thorough and accurate, reflecting the latest evidence-based research. Content is reviewed before publication and upon substantial updates. Learn more. by Gagandeep Brar, MD Medically reviewed by Gagandeep Brar, MD Twitter Gagandeep Brar, MD, is a board-certified hematologist and medical oncologist in Los Angeles, California. Learn about our Medical Expert Board Print Cancer is a disease of abnormal DNA and abnormal cell growth. When cancer starts to grow, the cell with abnormal DNA doesn't behave normally. The cells may grow and divide too rapidly, or may not die when they are supposed to. Many risk factors are associated with the development of cancer, one of them being exposure to carcinogens. A carcinogen is a substance that has the potential to cause cancer. This article will review carcinogens and the mechanism in which they can cause cancer. Zero Creatives/Cultura/Getty Images How Carcinogens Cause Cancer Carcinogens can be chemical substances, viruses, or even medications or radiation therapies used to treat cancer. Carcinogenic substances can cause cancer in different ways. One of how carcinogens can lead to cancer is through direct damage to the DNA inside of a cell. The damage done by the carcinogen directly to the DNA then causes that DNA to become abnormal and not continue to function normally. This eventually leads to the development of cancer. Another way that carcinogens can cause cancer development is more indirect. When damage occurs to a cell's DNA, there are repair processes that the body has to try to fix that damage. Sometimes the damage caused by the carcinogen interferes with the ability of the repair work, which has caused mutations in the function of those cells. The body is then unable to repair the damage, leading to cancer development. People can be exposed to carcinogens in a wide variety of ways, including: Lifestyle choices: Foods, smoking habits, lack of physical activity Natural exposures: Ultraviolet light, radon gas, and infectious agents Medical treatments: Radiation, chemotherapy, hormones, immunosuppressants Workplace exposures: Exposure to industrial chemicals or products while at work Household exposures: cleaning products, paints, herbicides, pesticides 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. Your risk of developing cancer depends on when, how often, and how long you are exposed to a carcinogen. Your genetic makeup also plays a role. Classification of Carcinogens Carcinogens are classified by The International Agency for Research on Cancer (IARC). The IARC puts substances into one of five groups based on the strength of the evidence that they can cause cancer in humans. Category Classification Human Evidence Animal Evidence Group 1 Carcinogenic to humans Sufficient Sufficient Group 2A Probably carcinogenic to humans Limited Sufficient Group 2B Possibly carcinogenic to humans Limited or inadequate None or inadequate Group 3 Carcinogenicity not classifiable None or inadequate Inadequate or limited Group 4 Probably not carcinogenic Suggests not carcinogenic Suggests not carcinogenic Keep in mind that these categories only indicate how strong the evidence is that a substance causes cancer. Substances in the same category can differ in how much they increase cancer risk. Known Substances That Are Carcinogenic to Humans Sometimes it can be difficult to test and classify substances as carcinogenic to humans, and it isn't ethical to expose people just to see if it causes cancer. Therefore researchers have only been able to classify a little over 100 substances as “carcinogenic to humans.” Some of the most common group 1 carcinogens are: Alcoholic beverages Aluminum production Arsenic and inorganic arsenic compounds Asbestos (all forms) and mineral substances (such as talc or vermiculite) that contain asbestos Coal, indoor emissions from household combustion Engine exhaust, diesel Epstein-Barr virus (infection with) Estrogen postmenopausal therapy 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) Outdoor air pollution Radon Tobacco, smokeless Tobacco smoke, secondhand Tobacco smoking Ultraviolet (UV) radiation and ultraviolet-emitting tanning beds Summary Carcinogens are substances that can possibly cause cancer due to the damage they cause to DNA inside the cells. DNA tries to repair itself, but sometimes can't because of the problems carcinogens have caused, and ultimately, cancer grows. The IARC evaluates the carcinogenicity of substances and classifies them based on scientific evidence. More than 100 substances are considered carcinogenic to humans. Frequently Asked Questions How long do carcinogens stay in the body? There is no one specific answer to this question. Some carcinogens leave the body quickly but can cause damage that is long lasting. Other carcinogens may remain in the body for a longer period of time. How many known carcinogens are there? According to the IARC, who rates carcinogens, there are 121 known carcinogens to humans. Originally written by Lisa Fayed 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 4 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. Barnes JL, Zubair M, John K, Poirier MC, Martin FL. Carcinogens and DNA damage. Biochem Soc Trans. 2018;46(5):1213-1224. doi:10.1042/BST20180519. American Cancer Society. Determining if something is a carcinogen. International Agency for Research on Cancer. Preamble to the IARC monographs on the identification of carcinogenic hazards to humans (amended January 2019). Carpenter DO, Bushkin-Bedient S. Exposure to chemicals and radiation during childhood and risk for cancer later in life. Journal of Adolescent Health. 2013;52(5):S21-S29. doi:10.1016/j.jadohealth.2013.01.027 Additional Reading American Cancer Society. Known and Probably Human Carcinogens. World Health Organization. Cancer Prevention. By clicking “Accept All Cookies”, you agree to the storing of cookies on your device to enhance site navigation, analyze site usage, and assist in our marketing efforts. Cookies Settings Accept All Cookies