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Known and Probable Carcinogens
(Including Industrial Processes, Occupational Exposures, Infectious Agents, Chemicals, and Radiation)

What Is a Carcinogen?

Cancer is caused by abnormalities in a cell’s DNA (its genetic blueprint). Abnormalities may be inherited from parents, or they may be caused by outside exposures to the body such as chemicals, radiation, or even infectious agents. Some carcinogens do not act on DNA directly, but cause cancer in other ways, such as causing cells to divide at a faster rate. Substances that can cause changes that can lead to cancer are called carcinogens.

Carcinogens do not cause cancer in every case, all the time. Substances classified 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 will depend on many factors, including the length and intensity of exposure to the carcinogen and the person’s genetic makeup.

How Do We Determine if Something Is a Carcinogen?

Scientists get much of their data about whether or not something might be carcinogenic from laboratory (cell culture and animal) studies. Although it isn’t possible to predict with absolute certainty which substances will be carcinogenic to humans based on animal studies alone, virtually all known human carcinogens that have been adequately tested in lab animals produce cancer in these animals. In many cases, carcinogens are first found to cause cancer in lab animals and are later found to cause cancer in people. Because there are far too many substances (natural and manmade) to test each one in lab animals, scientists use knowledge about chemical structure, other types of lab tests, and information about the extent of human exposure to select chemicals for testing.

Most studies of potential carcinogens in lab animals expose the 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.

Another important way to identify carcinogens is through epidemiologic studies, which look at the factors that might affect the occurrence of cancer in human populations. While 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. And it is usually many years (often decades) between exposure to a carcinogen and the development of cancer. Therefore, it can be very difficult to single out any particular exposure as having a definite link to cancer.

By combining data from both types of studies, scientists are able 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.

How Are Carcinogens Classified?

The most widely used system for classifying carcinogens comes from the International Agency for Research on Cancer (IARC), which is part if the World Health Organization (WHO). The IARC has evaluated the cancer-causing potential of about 900 likely candidates in the last 30 years, 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, most of the agents are of probable, possible, or unknown risk. Only about 90 are classified as "carcinogenic to humans."

In the United States, the National Toxicology Program (NTP) releases the Report on Carcinogens every two years. The NTP is formed from parts of several different 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 Report on Carcinogens (RoC) identifies two 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.

Below are the lists of known and probable human carcinogens from both groups.

Some Important Points to Know About These Lists

  • 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 are not necessarily all-inclusive - they include only those substances that have been evaluated by the agencies.

  • Most of the agents on the list 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 respective Web sites).

  • The lists themselves say nothing about the relative carcinogenicity of the agents. 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 as opposed to touching). 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.

  • It is also important to keep in mind that not all carcinogens are to be avoided at all costs. The lists include many commonly used medicines, particularly some hormones and drugs used to treat cancer. Tamoxifen, for example, increases the risk of certain kinds of uterine cancer, but decreases the risk of recurrence (return) of breast cancer, which may be more important for some women. If you have questions about a medicine you are taking that appears on one of these lists, be sure to ask your doctor.

Known Human Carcinogens

International Agency for Research on Cancer (IARC)

"Carcinogenic to Humans" (Group 1)

Agents and Groups of Agents
Aflatoxins (naturally occurring mixtures of)
4-Aminobiphenyl
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)
Asbestos
Azathioprine
Benzene
Benzidine
Beryllium and beryllium compounds
N
,N-Bis(2-chloroethyl)-2-naphthylamine (Chlornaphazine)
Bis(chloromethyl)ether and chloromethyl methyl ether (technical-grade)
1,4-Butanediol dimethanesulfonate (Busulphan; Myleran)
Cadmium and cadmium compounds
Chlorambucil
1-(2-Chloroethyl)-3-(4-methylcyclohexyl)-1-nitrosourea (Methyl-CCNU; Semustine)
Chromium [VI] compounds
Cyclophosphamide
Cyclosporin
Diethylstilbestrol
Epstein-Barr virus
Erionite
Estrogen therapy, postmenopausal
Estrogens, nonsteroidal (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)
Ethylene oxide
Etoposide in combination with cisplatin and bleomycin
[Gamma Radiation: see X- and Gamma (g)-Radiation]
Helicobacter pylori
(infection with)
Hepatitis B virus (chronic infection with)
Hepatitis C virus (chronic infection with)
Herbal remedies containing plant species of the genus Aristolochia
Human immunodeficiency virus type 1 (infection with)
Human papillomavirus type 16
Human papillomavirus type 18
Human T-cell lymphotropic virus type I
Melphalan
8-Methoxypsoralen (Methoxsalen) plus ultraviolet A radiation
MOPP and other combined chemotherapy including alkylating agents
Mustard gas (Sulfur mustard)
2-Naphthylamine
Neutrons
Nickel compounds
Opisthorchis viverrini
(infection with)
Oral contraceptives, combined (Note: There is also conclusive evidence that these agents have a protective effect against cancers of the ovary and endometrium)
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
(infection with)
Silica, crystalline (inhaled in the form of quartz or cristobalite from occupational sources)
Solar radiation
Talc containing asbestiform fibers
Tamoxifen (Note: There is also conclusive evidence that this agent (tamoxifen) reduces the risk of contralateral breast cancer)
2,3,7,8-Tetrachlorodibenzo-para-dioxin
Thiotepa
Thorium-232 and its decay products, administered intravenously as a colloidal dispersion of thorium-232 dioxide
Treosulfan
Vinyl chloride
X- and Gamma (g)-Radiation

Mixtures
Alcoholic beverages
Analgesic mixtures containing phenacetin
Betel quid with tobacco
Coal-tar pitches
Coal-tars
Mineral oils, untreated and mildly treated
Salted fish (Chinese-style)
Shale-oils
Soots
Tobacco products, smokeless
Tobacco smoke
Wood dust

Exposure Circumstances
Aluminum production
Auramine, manufacture of
Boot and shoe manufacture and repair
Coal gasification
Coke production
Furniture and cabinet making
Hematite mining (underground) with exposure to radon
Iron and steel founding
Isopropanol manufacture (strong-acid process)
Magenta, manufacture of
Painter (occupational exposure as a)
Rubber industry
Strong inorganic acid mists containing sulfuric acid (occupational exposure to)

National Toxicology Program (NTP) 10th Report on Carcinogens

"Known to Be Human Carcinogens"

Aflatoxins
Alcoholic Beverage Consumption
4-Aminobiphenyl
Analgesic Mixtures Containing Phenacetin
Arsenic Compounds, Inorganic
Asbestos
Azathioprine
Benzene
Benzidine
Beryllium and Beryllium Compounds
1,3-Butadiene
1,4-Butanediol Dimethylsulfonate (Myleran ®)
Cadmium and Cadmium Compounds
Chlorambucil
1-(2-Chloroethyl)-3-(4-methylcyclohexyl)-1-nitrosourea (MeCCNU)
bis(Chloromethyl) Ether and Technical-Grade Chloromethyl Methyl Ether
Chromium Hexavalent Compounds
Coal Tar Pitches
Coal Tars
Coke Oven Emissions
Cyclophosphamide
Cyclosporin A (Ciclosporin)
Diethylstilbestrol
Dyes Metabolized to Benzidine
Environmental Tobacco Smoke
Erionite
Estrogens, Steroidal
Ethylene Oxide
Melphalan
Methoxsalen with Ultraviolet A Therapy (PUVA)
Mineral Oils (Untreated and Mildly Treated)
Mustard Gas
2-Naphthylamine
Nickel Compounds
Radon
Silica, Crystalline (Respirable Size)
Smokeless Tobacco
Solar Radiation
Soots
Strong Inorganic Acid Mists Containing Sulfuric Acid
Sunlamps or Sunbeds, Exposure to
Tamoxifen
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD); "Dioxin"
Thiotepa
Thorium Dioxide
Tobacco Smoking
Vinyl Chloride
Ultraviolet Radiation, Broad Spectrum UV Radiation
Wood Dust

Probable Carcinogens

International Agency for Research on Cancer (IARC)

"Probably Carcinogenic to Humans" (Group 2A)

Agents and Groups of Agents
Acrylamide
Adriamycin
Androgenic (anabolic) steroids
Aristolochic acids (naturally occurring mixtures of)
Azacitidine
Benz[a]anthracene
Benzidine-based dyes
Benzo[a]pyrene
Bischloroethyl nitrosourea (BCNU)
1,3-Butadiene
Captafol
Chloramphenicol
a-Chlorinated toluenes (benzal chloride, benzotrichloride, benzyl chloride) and benzoyl chloride (combined exposures)
1-(2-Chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU)
4-Chloro-ortho-toluidine
Chlorozotocin
Cisplatin
Clonorchis sinensis
(infection with)
Dibenz [a,h]anthracene
Diethyl sulfate
Dimethylcarbamoyl chloride
1,2-Dimethylhydrazine
Dimethyl sulfate
Epichlorohydrin
Ethylene dibromide
N
-Ethyl-N-nitrosourea
Etoposide
Formaldehyde
Glycidol
Human papillomavirus type 31
Human papillomavirus type 33
IQ (2-Amino-3-methylimidazo[4,5-f]quinoline)
Kaposi's sarcoma herpesvirus/human herpesvirus 8
5-Methoxypsoralen
4,4´-Methylene bis(2-chloroaniline) (MOCA)
Methyl methanesulfonate
N-Methyl-N´-nitro-N-nitrosoguanidine (MNNG)
N-Methyl-N-nitrosourea
Nitrogen mustard
N-Nitrosodiethylamine
N-Nitrosodimethylamine
Phenacetin
Procarbazine hydrochloride
Styrene-7,8-oxide
Teniposide
Tetrachloroethylene
ortho
-Toluidine
Trichloroethylene
1,2,3-Trichloropropane
Tris(2,3-dibromopropyl) phosphate
Ultraviolet radiation A
Ultraviolet radiation B
Ultraviolet radiation C
Vinyl bromide
Vinyl fluoride

Mixtures

Creosotes (from coal-tars)
Diesel engine exhaust
Hot mate
Non-arsenical insecticides (occupational exposures in spraying and application of)
Polychlorinated biphenyls

Exposure Circumstances

Art glass, glass containers and pressed ware (manufacture of)
Hairdresser or barber (occupational exposure as a)
Petroleum refining (occupational exposures in)
Sunlamps and sunbeds (use of)

National Toxicology Program (NTP) 10th Report on Carcinogens

"Reasonably Anticipated to Be Human Carcinogens"

Acetaldehyde
2-Acetylaminofluorene
Acrylamide
Acrylonitrile
Adriamycin® (Doxorubicin Hydrochloride)
2-Aminoanthraquinone
o-Aminoazotoluene
1-Amino-2-methylanthraquinone
2-Amino-3-methylimidazo[4,5-f]quinoline (IQ)
Amitrole
o-Anisidine Hydrochloride
Azacitidine (5-Azacytidine®, 5-AzaC)
Benz[a]anthracene
Benzo[b]fluoranthene
Benzo[j]fluoranthene
Benzo[k]fluoranthene
Benzo[a]pyrene
Benzotrichloride
Bromodichloromethane
2,2-bis-(Bromoethyl)-1,3-propanediol (Technical Grade)
Butylated Hydroxyanisole (BHA)
Carbon Tetrachloride
Ceramic Fibers (Respirable Size)
Chloramphenicol
Chlorendic Acid
Chlorinated Paraffins (C12 , 60% Chlorine)
1-(2-Chloroethyl)-3-cyclohexyl-1-nitrosourea
bis(Chloroethyl) nitrosourea
Chloroform
3-Chloro-2-methylpropene
4-Chloro-o-phenylenediamine
Chloroprene
p-Chloro-o-toluidine and p-Chloro-o-toluidine Hydrochloride
Chlorozotocin
C.I. Basic Red 9 Monohydrochloride
Cisplatin
p-Cresidine
Cupferron
Dacarbazine
Danthron (1,8-Dihydroxyanthraquinone)
2,4-Diaminoanisole Sulfate
2,4-Diaminotoluene
Dibenz[a,h]acridine
Dibenz[a,j]acridine
Dibenz[bi>a,h]anthracene
7
H-Dibenzo[c,g]carbazole
Dibenzo[a,e]pyrene
Dibenzo[a,h]pyrene
Dibenzo[a,i]pyrene
Dibenzo[a,l]pyrene
1,2-Dibromo-3-chloropropane
1,2-Dibromoethane (Ethylene Dibromide)
2,3-Dibromo-1-propanol
tris(2,3-Dibromopropyl) Phosphate
1,4-Dichlorobenzene
3,3’-Dichlorobenzidine and 3,3’-Dichlorobenzidine Dihydrochloride
Dichlorodiphenyltrichloroethane (DDT)
1,2-Dichloroethane (Ethylene Dichloride)
Dichloromethane (Methylene Chloride)
1,3-Dichloropropene (Technical Grade)
Diepoxybutane
Diesel Exhaust Particulates
Diethyl Sulfate
Diglycidyl Resorcinol Ether
3,3’-Dimethoxybenzidine
4-Dimethylaminoazobenzene
3,3’-Dimethylbenzidine
Dimethylcarbamoyl Chloride
1,1-Dimethylhydrazine
Dimethyl Sulfate
Dimethylvinyl Chloride
1,6-Dinitropyrene
1,8-Dinitropyrene
1,4-Dioxane
Disperse Blue 1
Dyes Metabolized to 3,3’-Dimethoxybenzidine
Dyes Metabolized to 3,3’-Dimethylbenzidine
Epichlorohydrin
Ethylene Thiourea
di(2-Ethylhexyl) Phthalate
Ethyl Methanesulfonate
Formaldehyde (Gas)
Furan
Glasswool (Respirable Size)
Glycidol
Hexachlorobenzene
Hexachlorocyclohexane Isomoers
Hexachloroethane
Hexamethylphosphoramide
Hydrazine and Hydrazine Sulfate
Hydrazobenzene
Indeno[1,2,3-cd]pyrene
Iron Dextran Complex
Isoprene
Kepone® (Chlordecone)
Lead Acetate
Lead Phosphate
Lindane and Other Hexachlorocyclohexane Isomers
2-Methylaziridine (Propylenimine)
5-Methylchrysene
4,4’-Methylenebis(2-chloroaniline)
4-4’-Methylenebis(N,N-dimethyl)benzenamine
4,4’-Methylenedianiline and 4,4’-Methylenedianiline Dihydrochloride
Methyleugenol
Methyl Methanesulfonate
N-Methyl-N’-nitro-N-nitrosoguanidine
Metronidazole
Michler’s Ketone [4,4’-(Dimethylamino)benzophenone]
Mirex
Nickel (Metallic)
Nitrilotriacetic Acid
o-Nitroanisole
6-Nitrochrysene
Nitrofen (2,4-Dichlorophenyl-p-nitrophenyl ether)
Nitrogen Mustard Hydrochloride
2-Nitropropane
1-Nitropyrene
4-Nitropyrene
N-Nitrosodi-n-butylamine
N-Nitrosodiethanolamine
N-Nitrosodiethylamine
N-Nitrosodimethylamine
N-Nitrosodi-n-propylamine
N-Nitroso-N-ethylurea
4-(N-Nitrosomethylamino)-1-(3-pyridyl)-1-butanone
N-Nitroso-N-methylurea
N-Nitrosomethylvinylamine
N-Nitrosomorpholine
N-Nitrosonornicotine
N-Nitrosopiperidine
N-Nitrosopyrrolidine
N-Nitrososarcosine
Norethisterone
Ochratoxin A
4,4’-Oxydianiline
Oxymetholone
Phenacetin
Phenazopyridine Hydrochloride
Phenolphthalein
Phenoxybenzamine Hydrochloride
Phenytoin
Polybrominated Biphenyls (PBBs)
Polychlorinated Biphenyls (PCBs)
Polycyclic Aromatic Hydrocarbons (PAHs)
Procarbazine Hydrochloride
Progesterone
1,3-Propane Sultone
beta-Propiolactone
Propylene Oxide
Propylthiouracil
Reserpine
Safrole
Selenium Sulfide
Streptozotocin
Styrene-7,8-oxide
Sulfallate
Tetrachloroethylene (Perchloroethylene)
Tetrafluoroethylene
Tetranitromethane
Thioacetamide
Thiourea
Toluene Diisocyanate
o-Toluidine and o-Toluidine Hydrochloride
Toxaphene
Trichloroethylene
2,4,6-Trichlorophenol
1,2,3-Trichloropropane
Ultraviolet A Radiation
Ultraviolet B Radiation
Ultraviolet C Radiation
Urethane
Vinyl Bromide
4-Vinyl-1-cyclohexene Diepoxide
Vinyl Fluoride

Additional Resources

International Agency for Research on Cancer (IARC)
Centre International de Recherche sur le Cancer (CIRC)
150 Cours Albert Thomas
F-69372 Lyon Cedex 08
France
Internet Address: www.iarc.fr

National Toxicology Program
Report on Carcinogens (RoC)
P.O. Box 12233, MD EC-14
Research Triangle Park, NC 27709
Internet Address: http://ntp-server.niehs.nih.gov/default.html

References

Monograph: Overall Evaluations of Carcinogenicity to Humans. International Agency for Research on Cancer (IARC). 2002. Available online at http://193.51.164.11/monoeval/crthall.html. Accessed February 2003.

10th Report On Carcinogens. U.S. Department of Health and Human Services. Public Health Service - National Toxicology Program. 2002. Available online at http://ehp.niehs.nih.gov/roc/toc10.html. Accessed February 2003.

Revised 9/12/03

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