Introduction to Carcinogens


Many people worry about substances or exposures in their environment that may cause cancer. As part of the American Cancer Society's role in informing and educating people about cancer and its possible causes, this document provides lists of substances and exposures that are known or suspected to cause cancer. The lists below have been developed by two highly respected agencies -- the International Agency for Research on Cancer (IARC) and the US National Toxicology Program (NTP). To help put these lists into context, some related information is included on how these and other agencies and groups test and classify possible carcinogens.

The American Cancer Society does not keep detailed information on each of the exposures on these lists. Anyone looking for more in-depth information on a particular item on these lists should refer to the agencies in the "Additional resources" section of this document.

What is a carcinogen?

Cancer is caused by changes (mutations) in a cell's DNA -- its genetic "blueprint". Some of these changes may be inherited from our parents, while others may be caused by outside exposures, which are often referred to as environmental factors. Environmental factors can include a wide range of exposures, such as lifestyle factors (nutrition, tobacco use, physical activity, etc.), naturally occurring exposures (ultraviolet light, radon, infectious agents, etc.), medical treatments (chemotherapy, radiation, and immune system-suppressing drugs used after organ transplants, etc.), workplace and household exposures, and pollution.

Substances and exposures that can lead to cancer are called carcinogens. Some carcinogens do not act on DNA directly, but lead to cancer in other ways. For example, they may cause cells to divide at a faster than normal rate, which could increase the chances that DNA changes will occur.

Carcinogens do not cause cancer in every case, all the time. Substances labeled as carcinogens may have different levels of cancer-causing potential. Some may cause cancer only after prolonged, high levels of exposure. And for any particular person, the risk of developing cancer depends on many factors, including how they are exposed to a carcinogen, the length and intensity of the exposure, and the person's genetic makeup.

How do researchers determine if something is a carcinogen?

Testing substances or exposures to see if they can cause cancer is often difficult. It is not ethical to test a substance by exposing people to it and seeing if they get cancer from it. Therefore, scientists must resort to other types of tests, which may not always provide clear answers.

Lab studies

Scientists get much of their data about whether something might cause cancer from lab studies in cell cultures and animals. Because there are far too many substances (natural and man-made) to test each one in lab animals, scientists use knowledge about chemical structure, other types of lab tests, information about the extent of human exposure, and other factors to select chemicals for testing. For example, they can often get an idea about whether a substance might cause a problem by looking at its chemical structure and comparing it to similar chemicals that have been better studied.

Although it isn't possible to predict with certainty which substances will cause cancer in humans based on lab studies alone, virtually all known human carcinogens that have been adequately tested produce cancer in lab animals. In many cases, carcinogens are first found to cause cancer in lab animals and are later found to cause cancer in people.

Most studies of potential carcinogens expose the lab animals to doses that are higher than common human exposures. This is so that cancer risk can be detected in relatively small groups of animals. For most carcinogens, it is assumed that those that cause cancer at larger doses in animals will also cause cancer in people. Although it isn't always possible to know the relationship between exposure dose and risk, it is reasonable for public health purposes to assume that lowering human exposure will reduce risk.

Epidemiologic (population-based) studies

Another important way to identify carcinogens is through epidemiologic studies, which look at human populations to determine which factors might be linked to cancer. Although these studies also provide useful information, they also have their limitations. Humans do not live in a controlled environment. People are exposed to numerous substances at any one time, including those they encounter at work, school, or home; in the food they eat; and the air they breathe. It's very unlikely they know exactly what they've been exposed to or that they would be able to remember all of their exposures if asked by a researcher. And there are usually many years (often decades) between exposure to a carcinogen and the development of cancer. Therefore, it can be very hard to single out any particular exposure as having a definite link to cancer.

By combining data from both types of studies, scientists do their best to make an educated assessment of a substance's cancer-causing ability. When the available evidence is compelling but not felt to be conclusive, the substance may be considered to be a probable carcinogen. But in some cases there simply isn't enough information to be certain one way or the other.

For more information on how possible carcinogens are studied and classified, see the separate American Cancer Society document, Environmental and Occupational Cancer Risk Factors: Overview.

Who determines how carcinogens are classified?

Several agencies (national and international) are responsible for determining the cancer-causing potential of different substances.

International Agency for Research on Cancer

The International Agency for Research on Cancer (IARC) is part of the World Health Organization (WHO). Its major goal is to identify causes of cancer. The most widely used system for classifying carcinogens comes from the IARC. In the past 30 years, the IARC has evaluated the cancer-causing potential of more than 900 likely candidates, placing them 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
Perhaps not surprisingly, based on how hard it can be to test these candidate carcinogens, most are listed as being of probable, possible, or unknown risk. Only a little over 100 are classified as "carcinogenic to humans."

National Toxicology Program

The US National Toxicology Program (NTP) is formed from parts of several different US government agencies, including the National Institutes of Health (NIH), the Centers for Disease Control and Prevention (CDC), and the Food and Drug Administration (FDA). In the United States, the NTP releases the Report on Carcinogens about every 2 years.

The Report on Carcinogens (RoC) identifies 2 groups of agents:

"Known to be human carcinogens"
"Reasonably anticipated to be human carcinogens"
Unlike the IARC's list, the RoC does not list substances that have been studied and found not to be carcinogens.

Environmental Protection Agency

The US Environmental Protection Agency (EPA) maintains the Integrated Risk Information System (IRIS), an electronic database that contains information on human health effects from exposure to various substances in the environment. The EPA uses a rating system similar to that of IARC when describing the cancer-causing potential of a substance:

Group A: Carcinogenic to humans
Group B: Likely to be carcinogenic to humans
Group C: Suggestive evidence of carcinogenic potential
Group D: Inadequate information to assess carcinogenic potential
Group E: Not likely to be carcinogenic to humans
Other agencies and groups

Other federal agencies, such as the CDC's National Institute for Occupational Safety and Health (NIOSH) and the Food and Drug Administration (FDA), may comment on whether a substance or exposure may cause cancer and/or what levels of exposure to a particular substance might be considered acceptable.

Some state agencies also keep lists of known or probable carcinogens. For example, the California Environmental Protection Agency (CalEPA) maintains a list of "chemicals known to the state to cause cancer or reproductive toxicity." (Much of this list is based on the IARC and NTP lists below.)

The list of carcinogensKnown carcinogens

The following are a few examples of the most commonly encountered carcinogenic materials. For a full list of the latest known carcinogens review the report from the US Department of Health and Human Services (HHS) report on known carcinogens.

Formaldehyde: It's the same stuff you used when you dissected the frog in seventh grade and is also commonly used as a preservative in many household products, glue in particleboard, and in plywood furniture.

Paradichlorobenzene: This probable carcinogen is found in toilet bowl cleaners and can cause harm to the central nervous system.

Perchloroethylene (or 1-1-1 trichloroethane solvents): These chemicals are commonly found in dry cleaning fluid, spot removers, and carpet cleaners.

Pesticides: Used to control bugs and other vermin, they are loaded with carcinogens, including sodium 2, 4-dichlorophenoxyacetate. Overexposure has been associated with lymphoma and leukemia.
Tobacco smoke: A well-known carcinogen that can harm you even if you don't smoke but are simply exposed to it.
Toronto Smog: A well-known fact that the air we breathed has numerous carcinogens.

Consumer products

Deodorant, a bar of soap, toothpaste, hair spray, detergent... All of these products can contain carcinogens and many do. While each product may only contain a small amount of cancer-causing agents, most of us use these products every day.

Making an informed, healthy choice starts by becoming aware of these products and choosing to use products made by companies that do not use harmful ingredients.

The list below consists of common consumer products that contain carcinogenic materials. This is just a start -- please add to it and share information about other products that you know of so we can all live more healthy lives.

Bath and beauty products

Dove Beauty Bar: It's 99% water, but watch out for that other 1%. It includes quaternium 15 and formaldehyde, known carcinogens, as well as irritants to the skin, eyes, and mucous membranes.

Johnson's Baby Shampoo: Contains carcinogens quaterium 15, FD&C RED 40, which can cause dermatitis.

Crest Tarter Control Toothpaste: This best selling toothpaste contains saccharin and phenol fluoride.

Talcum powder: Talc, the main ingredient, is a carcinogen that increases the risk of ovarian cancer. Use corn starch instead.

Cover Girl Replenishing Natural Finish Make Up (foundation): This makeup includes BHA, talc, titanium dioxide, triethanolamine. These interact with nitrites to form nitrosamines and lanolin, which is often contaminated with DDT and other carcinogenic pesticides.

Household cleaning products

Tide & Cheer Laundry Detergent: Our favorite detergent contains trisodium nitrilotriacetate, a carcinogen.

Lysol Disinfectant: While it makes the air sweet smelling, it contains the dioxin.

Food products

Oscar Meyer beef hot dogs: Labeled ingredients in this American favorite include nitrite, which interacts with meat amines to form nitrosamines. Tests have also found other carcinogens such as benzene hexachloride, dacthal, dieldrin, DDT, heptachlor, hexachlorobenzene, and lindane. If you have to eat hot dogs, look for ones without nitrates in them.
Whole milk: Certain containers contain DDT, dieldrin, heptachlor, hexachlorobenzene, recombinant bovine growth hormone and Igf-1. All of these increase the chances of getting breast, colon and prostate cancers. Look for RBGH-free organic milk.

Pet products

Zodiac flea collars: These dog collars include the labeled carcinogen propoxur. Try Trader Joe’s herbal flea collars instead.

Other products

Carpets: Some carpets are made or finished with petrolatum-based chemicals. These chemicals can "outgas" into the home. Petrolatum is believed to a human carcinogen.

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