This page was reviewed under our medical and editorial policy by
Mustafa Raoof, MD, MS, Surgical Oncologist, City of Hope | Duarte.
This page was reviewed on June 1, 2023.
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).
Cancer is caused by mutations (changes) in the DNA of cells. Carcinogens may lead to these changes in different ways:
The likelihood that exposure to a carcinogen leads to cancer is affected by several factors, such as:
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:
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.
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.
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:
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: