Known Human Carcinogens List: IARC and NTP Classifications

Author: Ian C. Langtree - Writer/Editor for Disabled World (DW)
Published: 11 Oct 2014 - Updated: 29 Sep 2026
Publication Type: Informative

Table of Contents:
Synopsis - Definition - Overview - FAQs - Insights, Updates - Related Content

Synopsis

This information brings together the main classification lists of cancer-causing agents in one place, including the International Agency for Research on Cancer Group 1 and Group 2A agents from the IARC Monographs and the known and reasonably anticipated human carcinogens identified in the U.S. National Toxicology Program Report on Carcinogens. The lists cover a wide range of exposures, from industrial chemicals such as benzene, vinyl chloride, and formaldehyde to viruses like hepatitis B and HPV, radiation sources including radon and UV light, and everyday habits such as smoking and drinking alcohol. It also notes that the 14th Report on Carcinogens grew to 243 listings and makes the important point that a listing signals a cancer hazard, not a guarantee of cancer, since dose, length of exposure, and personal susceptibility all play a role. The reference is useful for workers, caregivers, and health-conscious consumers, and it has particular value for seniors and people with disabilities, who may have weaker immune systems, spend more time indoors where radon and household combustion fumes build up, or take long-term medications that appear on these lists.

At a Glance

• 1 - Shiftwork that disrupts the body's circadian rhythm is classified as probably carcinogenic to humans.
• 2 - Pentachlorophenol, a wood preservative used on utility poles and fence posts, has been tied to a higher risk of non-Hodgkin lymphoma.
• 3 - Tamoxifen is listed as a Group 1 carcinogen, yet it also lowers the risk of cancer in the opposite breast for breast cancer patients.
• 4 - Ortho-toluidine, used to make rubber chemicals, pesticides, and dyes, was upgraded to a known human carcinogen after studies linked it to bladder cancer.

Topic Definition

Human Carcinogen

A human carcinogen is any substance, mixture, radiation source, infectious agent, or exposure circumstance shown to cause cancer in people. Carcinogens work by damaging DNA directly, by triggering chronic inflammation, or by disrupting the normal processes that control how cells grow and divide, and the effects often take years or decades to appear. Health agencies sort carcinogens by the strength of the evidence rather than by how powerful they are: the International Agency for Research on Cancer places agents in Group 1 when there is sufficient evidence of cancer in humans and in Group 2A when they are probably carcinogenic, while the U.S. National Toxicology Program lists them as either known or reasonably anticipated to be human carcinogens. Carcinogens can be natural or synthetic, and they include tobacco smoke, asbestos, radon, ultraviolet radiation, alcohol, benzene, and viruses such as hepatitis B and human papillomavirus. Whether exposure leads to cancer depends on the dose, how long the exposure lasts, and individual factors such as genetics and overall health.

Overview

Recently Added Carcinogens

Four substances have been added to the U.S. Department of Health and Human Services 14th Report on Carcinogens, a science-based document that identifies chemical, biological, and physical agents that are considered cancer hazards for people living in the United States. The new report now includes 243 listings.

• Ortho-toluidine, used to make rubber chemicals, pesticides, and dyes have been reevaluated and are now listed as a known human carcinogen.

Three substances have been added as reasonably anticipated to be human carcinogens. These include:

• 1-bromopropane, used as a cleaning solvent and spray adhesive;
• cumene used to make phenol and acetone, and also found in fuel products and tobacco smoke;
• wood preservative mixture pentachlorophenol.

"Identifying substances in our environment that can make people vulnerable to cancer will help in prevention efforts," said Linda Birnbaum, Ph.D., director of the National Institute of Environmental Health Sciences (NIEHS) and the National Toxicology Program (NTP). "This report provides a valuable resource for health regulatory and research agencies, and it empowers the public with information people can use to reduce exposure to cancer-causing substances."

The Report on Carcinogens is a congressionally mandated report prepared for the HHS Secretary by NTP. The report identifies agents, substances, mixtures, or exposures in two categories: known to be a human carcinogen and reasonably anticipated to be a human carcinogen.

A listing in the report indicates a cancer hazard but does not mean that a substance will cause cancer. Many factors, including the amount and duration of exposure and an individual's susceptibility to a substance, can affect whether a person will develop cancer.

Also see our list of List of Ingredients and Additives in Cigarettes.

Jump-To:

• Carcinogenic to Humans - Full List
• Known to be Human Carcinogens
• Probably Carcinogenic to Humans
• Reasonably Anticipated to be Human Carcinogens

Substance Added as a Known Human Carcinogen

Since 1983, ortho-toluidine has been listed in the Report on Carcinogens as reasonably anticipated to be a human carcinogen. However, new cancer studies led NTP to reevaluate and reclassify ortho-toluidine. It is now added to the category of known human carcinogen, based on studies in humans showing it causes urinary bladder cancer. Ortho-toluidine is a synthetic chemical produced in other countries and imported into the United States by several companies in high volumes. It is primarily used to make rubber chemicals, pesticides, and dyes. It is also used in some consumer and medical products. People are mainly exposed through the workplace by skin contact and inhalation when using ortho-toluidine. People can also be exposed outside the workplace through sources such as tobacco smoke.

3 Substances Added to the New Report as Reasonably Anticipated to Be a Human Carcinogen

• The chemical 1-bromopropane is a colorless pale yellow liquid used as a solvent in many commercial industries. It is used as a cleaner for optics, electronics, and metals and a solvent for aerosol-applied adhesives such as those used in foam cushion manufacturing. It is also used in dry cleaning and solvent sprays for aircraft maintenance. Workers in certain occupations may be more exposed to 1-bromopropane than the general population. No human studies were identified that evaluated the relationship between human cancer and exposure specifically to 1-bromopropane. However, inhalation exposure to 1-bromopropane in rodents caused tumors in several organs, including the skin, lungs, and large intestine.
• Cumene is a flammable, volatile liquid with a gasoline-like odor. It is a natural component of coal tar and petroleum in tobacco smoke. It is used primarily to make acetone and phenol. People are mainly exposed to cumene through the environment and workplaces that use or produce cumene. It can be found in emissions from petroleum products. Inhalation exposure to cumene caused lung tumors in male and female mice and liver tumors in female mice. No human studies were identified that looked at the relationship between human cancer and exposure specifically to cumene.
• Pentachlorophenol and by-products of its synthesis are complex mixtures of chemicals used as wood preservatives. Because virtually everyone exposed to pentachlorophenol is also exposed to its synthesis by-products, they were evaluated together. In the United States, pentachlorophenol has been regulated since the 1980s as a restricted-use pesticide. It is used industrially for treating utility poles, wood pilings, fence posts, and lumber or timber for construction. Most exposure has occurred in settings where workers treat or come in contact with treated lumber. People may also be exposed to this mixture from breathing contaminated air, dust, or contact with contaminated soil. Exposure to this mixture was associated with an increased risk of non-Hodgkin lymphoma in human studies. It also caused tumors in the liver and other organs in mice.

References

• International Agency for Research on Cancer (IARC). Agents Classified by the IARC Monographs, Volumes 1 - 100. 2011.
• US Department of Health and Human Services. Public Health Service, National Toxicology Program. Report on Carcinogens, Twelfth Edition. 2011.
• List of Ingredients and Additives in Cigarettes - Disabled World - https://www.disabled-world.com/medical/pharmaceutical/addiction/cigarette-ingredients.php

Frequently Asked Questions

What does IARC Group 2B mean?

Group 2B means an agent is possibly carcinogenic to humans. There is limited evidence of cancer in humans and less than sufficient evidence in animal studies, so it is a weaker category than Group 1 or Group 2A and often reflects the need for more research.

Does a Group 1 classification mean a substance is as dangerous as tobacco?

No. IARC groups describe how strong the evidence is that something can cause cancer, not how much cancer it causes. Two agents in Group 1 can carry very different levels of risk depending on the dose and how often people are exposed.

Is processed meat considered a carcinogen?

Yes. In 2015 IARC classified processed meat such as bacon, ham, and sausages as a Group 1 carcinogen based on its link to colorectal cancer, and it classified red meat as Group 2A, probably carcinogenic to humans.

Are PFAS forever chemicals carcinogenic?

In 2023 IARC classified PFOA, a common PFAS chemical once used in nonstick and water-resistant products, as a Group 1 carcinogen, and it classified the related chemical PFOS as possibly carcinogenic in Group 2B.

Is aspartame a carcinogen?

In 2023 IARC placed aspartame in Group 2B, possibly carcinogenic to humans, based on limited evidence. At the same time, the WHO and FAO expert committee on food additives kept the acceptable daily intake at 40 milligrams per kilogram of body weight.

How can I test my home for radon?

Radon test kits are sold at hardware stores and online, and professional testing is also available. The U.S. Environmental Protection Agency recommends fixing a home if radon levels reach 4 picocuries per liter or higher, usually with a sealed vent and fan system.

How long does it take for cancer to develop after exposure to a carcinogen?

The delay, called the latency period, varies widely by agent and cancer type. Mesothelioma from asbestos often appears 20 to 50 years after exposure, while some blood cancers linked to benzene or radiation can develop within a few years.

Do workers have legal protection from carcinogen exposure on the job?

In the United States, the Occupational Safety and Health Administration sets exposure limits for many carcinogens such as asbestos, benzene, and formaldehyde. Employers must provide hazard information, training, and protective equipment, and workers who develop job-related cancer may qualify for workers compensation.

Insights, Analysis, and Developments

Editorial Note: What stands out in these lists is how many carcinogens are part of ordinary life rather than rare industrial accidents. Sunlight, tanning beds, outdoor air pollution, diesel exhaust, wood dust, hot mate, and emissions from high-temperature frying all appear alongside better-known threats like asbestos and tobacco. Several entries are tied to specific jobs, including painters, hairdressers and barbers, rubber industry workers, and people in petroleum refining, which makes workplace ventilation and protective gear a real cancer prevention issue. Some important medications are listed too, including cyclosporine and azathioprine, which many transplant recipients and people with autoimmune conditions rely on, so these are cases where doctors weigh cancer risk against clear medical benefit instead of simply avoiding the drug. Knowing where these hazards show up in homes, jobs, and daily routines gives people practical ways to cut exposure where they can and to discuss unavoidable risks with their health care providers.

Author Credentials: Ian is the founder and Editor-in-Chief of Disabled World, a leading resource for news and information on disability issues. With a global perspective shaped by years of travel and lived experience, Ian is a committed proponent of the Social Model of Disability, a transformative framework developed by disabled activists in the 1970s that emphasizes dismantling societal barriers rather than focusing solely on individual impairments. His work reflects a deep commitment to disability rights, accessibility, and social inclusion. To learn more about Ian's background, expertise, and accomplishments, visit his full biography.