Carcinogens and How They Cause Cancer

Cancer is a disease of abnormal DNA and abnormal cell growth. When cancer starts to grow, cells with abnormal DNA do not 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 how they can cause cancer.

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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 the ways 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 to develop is more indirect. When damage occurs to a cell's DNA, the body has to perform repair processes to try to fix the 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 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. Carcinogens also can cause cancer from 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 is an intergovernmental agency that puts substances into one of five categories 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 to them in research just to see if they cause 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


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 it sometimes can't because of the problems carcinogens have caused. 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 single 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, the intergovernmental agency that ranks carcinogens, there are 121 known carcinogens to humans.

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.
  1. Barnes JL, Zubair M, John K, Poirier MC, Martin FL. Carcinogens and DNA damageBiochem Soc Trans. 2018;46(5):1213-1224. doi:10.1042/BST20180519.

  2. American Cancer Society. Determining if something is a carcinogen.

  3. International Agency for Research on Cancer. Preamble to the IARC monographs on the identification of carcinogenic hazards to humans (amended January 2019).

  4. Carpenter DO, Bushkin-Bedient S. Exposure to chemicals and radiation during childhood and risk for cancer later in lifeJournal 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 Julie Scott, MSN, ANP-BC, AOCNP
Julie is an Adult Nurse Practitioner with oncology certification and a healthcare freelance writer with an interest in educating patients and the healthcare community.

Originally written by Lisa Fayed