Many people worry that substances or exposures in their environment 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). 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 gas, 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 calledcarcinogens. 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 give clear answers.
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 studied more.
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 much higher than common human exposures. This is so that cancer risk can be detected in relatively small groups of animals. It is not always clear if the results from animal studies will be the same for people if they are normally exposed to a substance. For example, the effects seen with very high doses of a substance may not be the same at much lower doses or if the route of exposure is different. And the bodies of lab animals and humans don't always process substances in the same way.
But for safety reasons, it is usually assumed that exposures that cause cancer at larger doses in animals may also cause cancer in people. It isn't always possible to know the relationship between exposure dose and risk, but it is reasonable for public health purposes to assume that lowering human exposure will reduce risk.
Studies in people
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 many substances at any given 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 definitely link any particular exposure 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 evidence is conclusive, the substance is labeled as a carcinogen. 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.
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 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). The NTP updates itsReport on Carcinogens(RoC) every few years.
TheReport on Carcinogensidentifies 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), the Food and Drug Administration (FDA), and the National Cancer Institute 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 American Cancer Society's role
The American Cancer Society (ACS) contributes in many ways to evaluating how environmental factors affect a person's likelihood of developing cancer, including:
Conducting epidemiologic research on the causes of cancer
Funding laboratory and epidemiologic research at universities and other institutions that study environmental causes of cancer
Advocating for environmental health on a local, state, and federal level
Informing the public about environmental factors that affect cancer risk and how to decrease their risk of developing cancer
In most cases, the ACS does not directly evaluate the potential cancer-causing ability of particular substances or exposures. Instead, the ACS looks to national and international organizations such as the NTP and IARC, whose mission is to evaluate environmental cancer risks based on evidence from laboratory and human research studies.
Some important points about the IARC and NTP lists here
The IARC and NTP act independently but have studied many of the same agents; therefore many known or suspected carcinogens appear on both lists. But because an agent appears on one and not on the other does not necessarily mean there is a controversy, as one agency may not have evaluated it.
These lists include only those agents that have been evaluated by the agencies. While these agencies tend to focus on substances and exposures most likely to cause cancer, there are many others that have not been studied fully yet.
Most of the agents on the lists are connected only with certain kinds of cancer, not all types. For more detailed information, refer to the specific monographs or reports published by the agencies (available on their Web sites).
The lists themselves say nothing about how likely it is that an agent will cause cancer. Carcinogens do not cause cancer at all times, under all circumstances. Some may only be carcinogenic if a person is exposed in a certain way (for example, ingesting it as opposed to touching it). Some may only cause cancer in people who have a certain genetic makeup. Some of these agents may lead to cancer after only a very small exposure, while others might require intense exposure over many years. Again, you should refer to the agencies' reports for specifics.
Even if a substance or exposure is known or suspected to cause cancer, this does not necessarily mean that it can or should be avoided at all costs. For example, exposure to ionizing radiation is known to cause cancer, with increased risks even at low levels of exposure. Yet there is no way to completely prevent exposure to cosmic radiation from the sun or natural sources of radiation such as radon in soil. Although looking at the list below can tell you whether or not something may increase your risk of cancer, it is important to try to get an idea of how much it might increase your risk. It is also important to know what your risk is to begin with. Many factors can enter into this, including your age, gender, family history, and lifestyle factors (tobacco and alcohol use, weight, diet, physical activity level, etc.). As noted above, the type and extent of exposure to a substance may also play a role. You should consider the actual amount of increased risk when deciding if you should limit or avoid an exposure.
As an example, these lists include many commonly used medicines, particularly some hormones and drugs used to treat cancer. For example, tamoxifen increases the risk of certain kinds of uterine cancer but can be very useful in treating some breast cancers, which may be more important for some women. If you have questions about a medicine that appears on one of these lists, be sure to ask your doctor.
Known human carcinogens
International Agency for Research on Cancer
Group 1: Carcinogenic to humans
Agents and groups of agents
Arsenic and arsenic compounds (Note: This evaluation applies to the group of compounds as a whole and not necessarily to all individual compounds within the group)
Estrogen-progestogen oral contraceptives (combined) (Note: There is also convincing evidence in humans that these agents confer a protective effect against cancer in the endometrium and ovary)
Estrogens, non-steroidal (Note: This evaluation applies to the group of compounds as a whole and not necessarily to all individual compounds within the group)
Estrogens, steroidal (Note: This evaluation applies to the group of compounds as a whole and not necessarily to all individual compounds within the group)
Estrogen therapy, postmenopausal
Ethanol in alcoholic beverages
Etoposide in combination with cisplatin and bleomycin
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 papilloma virus (HPV) types 16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59 and 66 (Note: The HPV types that have been classified as carcinogenic to humans can differ by an order of magnitude in risk for cervical cancer)
Human T-cell lymphotropic virus type I (HTLV-1)
8-Methoxypsoralen (Methoxsalen) plus ultraviolet A radiation
MOPP and other combined chemotherapy including alkylating agents
Mustard gas (Sulfur mustard)
N'-Nitrosonornicotine (NNN) and 4-(N-Nitrosomethylamino)-1-(3-pyridyl)-1-butanone (NNK)
[Oral contraceptives, combined estrogen-progestogen: see Estrogen-progestogen oral contraceptives (combined)]
Oral contraceptives, sequential
Phosphorus-32, as phosphate
Plutonium-239 and its decay products (may contain plutonium-240 and other isotopes), as aerosols
Radioiodines, short-lived isotopes, including iodine-131, from atomic reactor accidents and nuclear weapons detonation (exposure during childhood)
Radionuclides, alpha-particle-emitting, internally deposited (Note: Specific radionuclides for which there is sufficient evidence for carcinogenicity to humans are also listed individually as Group 1 agents)
Radionuclides, beta-particle-emitting, internally deposited (Note: Specific radionuclides for which there is sufficient evidence for carcinogenicity to humans are also listed individually as Group 1 agents)
Radium-224 and its decay products
Radium-226 and its decay products
Radium-228 and its decay products
Radon-222 and its decay products
Schistosoma haematobium(flatworm; infection with)
Silica, crystalline (inhaled in the form of quartz or cristobalite from occupational sources)
Talc containing asbestiform fibres
Tamoxifen (Note: There is also conclusive evidence that tamoxifen reduces the risk of contralateral breast cancer)
Thorium-232 and its decay products, administered intravenously as a colloidal dispersion of thorium-232 dioxide
X- and Gamma-radiation
Aflatoxins (naturally occurring mixtures of)
Betel quid with tobacco
Betel quid without tobacco
Household combustion of coal, indoor emissions from
Mineral oils, untreated and mildly treated
Phenacetin, analgesic mixtures containing
Plants containing aristolochic acid
Salted fish (Chinese-style)
Arsenic in drinking-water
Boot and shoe manufacture and repair
Furniture and cabinet making
Hematite mining (underground) with exposure to radon
Involuntary smoking (exposure to secondhand or 'environmental' tobacco smoke)
*Inclusion on this list does not imply endorsement by the American Cancer Society.
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International Agency for Research on Cancer (IARC). Monograph: Overall Evaluations of Carcinogenicity to Humans. 2009. Accessed at http://monographs.iarc.fr/ENG/Classification/crthall.php on May 13, 2010.
US Department of Health and Human Services. Public Health Service, National Toxicology Program. Report on Carcinogens, Eleventh Edition. 2005. Accessed at http://ntp.niehs.nih.gov/ntp/roc/toc11.html on May 13, 2010.
Last Medical Review: 02/17/2011
Last Revised: 02/17/2011