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This page was updated on May 25, 2021.

Carcinogen overview

A carcinogen is anything that causes cancer. Carcinogens may be environmental elements such as chemicals, metals or gases, or substances that come into contact with the body through eating, drinking or smoking. Some infections as well as certain medical procedures and treatments are also considered carcinogenic.

Some well-known carcinogens include:

Carcinogens don’t cause cancer in every person who comes into contact with them, and not every cancer is caused by carcinogens.

Limiting exposure to carcinogens when possible is important for lowering the risk of cancer—quitting tobacco use or limiting alcohol consumption are some examples. Vaccines and treatments may reduce the risk of cancer from infectious carcinogens, such as human papillomavirus (HPV). Environmental and occupational safety regulations also help workers and communities avoid dangerous exposure to carcinogens.

However, some carcinogens may be unavoidable. Ultraviolet (UV) rays from the sun are carcinogenic, but it’s not really possible or even healthy to always avoid the sun. Because it may take many years to study the effects of a given substance on the body, other carcinogens may not yet be known. Experts are always studying possible carcinogens in the lab and population studies.

National, international and federal organizations and research groups help to label chemicals and substances as carcinogens. Among others, these include the International Agency for Research on Cancer (IARC) and the U.S. Department of Health and Human Services’ National Toxicology Program (NTP).

How carcinogens cause cancer

Cancer is caused by mutations (changes) in the DNA of cells. Carcinogens may lead to these changes in different ways:

  • Some carcinogens cause the mutations directly.
  • Other carcinogens cause cells to change in some way, such as dividing faster than they normally would, which over time increases the chances of a mutation happening.

The likelihood that exposure to a carcinogen leads to cancer is affected by several factors, such as:

  • How much exposure the person has had and for how long
  • How the exposure happened (for example, through the skin vs. inhalation)
  • Age and genetic predisposition to certain cancers

Carcinogens you should know about

Lifestyle factors

Making certain lifestyle changes may drastically reduce the amount of carcinogens a person comes into contact with. The National Cancer Institute (NCI) recommends avoiding or limiting exposure to the following factors to reduce the risk of cancer.

Tobacco: The leading cause of cancer and death from cancer is tobacco. Smoking tobacco (cigarettes, cigars and pipes) may cause lung, larynx, mouth, esophagus, throat, bladder, kidney, liver, stomach, pancreas, colorectal and cervical cancers, as well as acute myeloid leukemia. Chewing tobacco and snuff may increase the risk of mouth, esophagus and pancreatic cancers. Any amount of tobacco use is considered risky, but quitting—even among people who have been diagnosed with cancer—may reduce the risk of death.

Alcohol: The more alcohol you drink, the higher the risk of certain cancers, such as head and neck cancer, esophageal cancer, liver cancer, breast cancer and colorectal cancer. Studies have shown that moderation is key. The U.S. government’s “2015-2020 Dietary Guidelines for Americans” publication recommends up to one drink per day for women and up to two drinks per day for men to keep the cancer risk from rising.

Sunlight: UV radiation from sunlight, as well as from tanning beds, damages skin and may lead to skin cancer. Wearing a wide-brim hat, covering up with clothing, applying sunscreen, and limiting time spent in the sun may reduce the risk.

There’s been speculation that certain foods or minerals are implicated in cancer risk, such as the black char on grilled meat, acrylamide in french fries and other foods, fluoride and artificial sweeteners. However, human studies haven’t shown a direct link between cancer and a particular food, beverage or nutrient—except for alcohol.


Certain infectious agents—such as viruses, bacteria or parasites—may cause cancer or increase the risk of developing cancer. The NCI lists these causes:

  • Epstein-Barr virus (EBV)
  • Hepatitis B virus and hepatitis C virus (HBV and HCV)
  • Human immunodeficiency virus (HIV)
  • Human papillomaviruses (HPVs)
  • Human T-cell leukemia/lymphoma virus type 1 (HTLV-1)
  • Kaposi sarcoma-associated herpesvirus (KSHV)
  • Merkel cell polyomavirus (MCPyV)
  • Helicobacter pylori (H. pylori)
  • Opisthorchis viverrini
  • Schistosoma haematobium

Vaccines may prevent hepatitis B and HPV. Other medicines that either prevent or treat HIV, H. pylori and other infections may also lower a person’s cancer risk.

Medical procedures and treatments

Certain medical procedures and treatments are known carcinogens.

For example, X-rays, positron emission tomography (PET) scans, computed tomography (CT) scans and radiation therapy all carry some cancer risk due to the radiation that has the potential to damage cells. Typically, the levels are low enough to not present a significant risk.

Hormone therapy to treat menopause may also increase the risk of certain cancers, such as breast and endometrial cancer. The hormone estrogen, contained in these therapies, is a known carcinogen.

In the case of both tests and treatments, oftentimes the benefits may outweigh the cancer risks.

Environmental and occupational factors

Carcinogens may be present in the environment where people live or work. While environmental and occupational safety regulations exist to keep people safe from these carcinogens, those who work or live near certain industries may still face increased risks of cancer.

According to the NTP’s 14th Report on Carcinogens in 2016, the following 27 substances in the environment are among the most likely carcinogens to affect human health, says the National Cancer Institute:

  • Aflatoxins, a fungus present in the agriculture of certain crops
  • Aristolochic acids, found in certain herbal supplements
  • Arsenic, a naturally occurring substance that may be a by-product of industrial processes, leading to contaminated groundwater; also found in tobacco smoke
  • Asbestos, fibrous minerals found in home and commercial insulation
  • Benzene, a chemical found in gasoline fumes, tobacco smoke (first and secondhand) and industrial emissions
  • Benzidine, a chemical that was used for dyes and other manufacturing processes
  • Beryllium, a metal associated with coal-fired power plants
  • 1,3-Butadiene, a gas associated with the manufacture of synthetic rubber products such as tires.
  • Cadmium, a natural element found in the earth that is used to make batteries, pigments, metal coatings and plastics
  • Coal tars and coal-tar pitches, substances derived from coal that are used in products such as asphalt, roofing and paving materials, as well as some treatments for skin disorders
  • Coke-oven emissions, gases and other substances emitted from plants that heat coal and produce coke, and that workers in the aluminum, steel, graphite, electrical and construction industries may also be exposed to
  • Crystalline silica (respirable size), a natural mineral found in sand, stone and soil; it poses risks when inhaled as tiny, airborne particles produced in certain industrial processes or in household materials such as cleansers, cosmetics, paints and more
  • Erionite, a fibrous mineral that is hazardous when particles are airborne
  • Ethylene oxide, a highly flammable gas that is used in various industrial processes, including producing antifreeze, or as a pesticide or sterilizing agent
  • Formaldehyde, a strong-smelling, flammable chemical used as a preservative in medical laboratories, mortuaries and other settings, as well as a material in the production of building materials
  • Hexavalent chromium compounds, a metallic compound used in industries like electroplating, welding and chromate painting
  • Mineral oils, untreated and mildly treated—a liquid by-product of the production of gasoline and other petroleum products from crude oil
  • Nickel compounds, metallic compounds containing nickel are produced and used in many industrial settings, such as mining, smelting, welding, casting and grinding.
  • Radon, a gas, present in nearly all air, that’s part of the normal decay of radioactive elements in the ground, the highest concentration of which is found in uranium mines; test kits may be used to test the levels of radon inside homes and other buildings
  • Tobacco smoke (environmental tobacco smoke), a combination of the smoke released from burning tobacco and the smoke exhaled by the person smoking it
  • Soot, the by-product of incomplete burning of products such as wood, fuel oil and household refuse; chimney sweeps, heating-unit service personnel, brick masons, building demolition personnel and horticulturists may all be regularly exposed to soot, as well as the general public through the use of fireplaces and furnaces
  • Strong inorganic acid mists containing sulfuric acid, a corrosive liquid used in various manufacturing processes such as copper smelting
  • Thorium, a naturally occurring radioactive metal that was once widely used in medical radiology in a contrast agent called Thorotrast, as well as in household items such as lantern mantles and ceramic glazes
  • Trichloroethylene, a synthetic liquid used in industry to make refrigerants and as a degreasing solvent for metal equipment, as well as in commercial dry cleaning and some household products such as cleaning products, paint removers and spot cleaners; it may be found in air, water and soil near where it’s produced/used
  • Vinyl chloride, a gas used in the production of polyvinyl chloride (PVC), a material used to make pipes, wire and cable coatings, and packaging materials; people who work at facilities producing or using PVC may be exposed through inhalation, as well as the general population who lives around those facilities
  • Wood dust, the particles created when wood is cut or shaped.

These aren’t the only known or likely carcinogens. The NTP names 62 human carcinogens and 186 anticipated human carcinogens. For a more complete picture of carcinogens, visit:

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