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Ontario Cancer Facts

The International Agency for Research on Cancer’s carcinogen classifications support cancer prevention

Jul 2020


  • The International Agency for Research on Cancer (IARC) classifies potential carcinogens, which can lead to changes in government, industry, and public health policies and practices to support cancer prevention.
  • IARC updated its classification process in 2019 by switching from 5 classification categories to 4, increasing its use of experimental studies, and developing a more streamlined evaluation process.
  • Ontario Health (Cancer Care Ontario) uses the IARC classifications to identify carcinogens that affect people in Ontario.


Identifying a carcinogenic hazard is an important first step in cancer prevention. The International Agency for Research on Cancer (IARC) – a specialized agency of the World Health Organization – plays a key role in this step because it reviews scientific evidence to identify and classify causes of cancer.

Impact of IARC classifications

When IARC determines that something causes cancer, the classification can trigger many actions by government, industry, public health organizations and advocacy groups :

  • Some products may be banned, restricted or phased out.
  • Manufacturing processes or workplace practices could be changed.
  • New policies could be implemented.

For example, Ontario introduced restrictions on tanning bed use by youth after IARC announced that using tanning beds increases the risk of melanoma.,

IARC’s Monograph Program

IARC’s Monograph Program classifies many potentially cancer-causing agents, including:

  • industrial chemicals (e.g., benzene)
  • complex mixtures (e.g., diesel engine exhaust)
  • occupational exposures (e.g., asbestos)
  • biological agents (e.g., human papillomavirus)
  • lifestyle factors (e.g., tobacco smoking)
  • exposure circumstances (e.g., shiftwork)

The IARC classifications describe the likelihood that an agent causes cancer, rather than the strength of the carcinogen. For example, when IARC first classified processed meat as “carcinogenic to humans,” which is the same classification as tobacco smoking, many news stories wrongly claimed that “bacon is as bad as cigarettes.” However, the IARC classification does not mean that the level of risk is the same – it simply indicates that both agents cause cancer.

How IARC evaluates carcinogens

IARC updated the way it classifies carcinogens in 2019. The key changes include:

  • a more streamlined evaluation process
  • a switch from 5 classification categories to 4
  • a larger role for experimental studies in the classification

However, overall the process remains the same. Agents are recommended to IARC for review, and chosen based on whether there is some evidence of human exposure and cancer hazard. Each evaluation is done by a Working Group of scientists from around the world, chosen for their expertise and lack of any conflicts of interest. These scientists carefully consider 3 types of data to make classification decisions:

  • studies of cancer in humans
  • animal studies
  • experimental studies of how an agent may cause cancer (also called mechanistic studies)

The final evaluation is done by integrating the 3 evidence streams. IARC uses the 4 classification categories described below and as of spring 2020, it has reviewed 1,017 agents. More detail on the classification process is available in the IARC Monographs Preamble.

Carcinogenic to humans (Group 1) – Agents in this group are known causes of cancer. Very strong, clear evidence is needed for an agent to be classified in Group 1. If the evidence from studies of cancer in humans is strong enough, the agent is automatically placed into Group 1. Otherwise, a combination of strong animal studies and mechanistic evidence in humans is needed.

Probably carcinogenic to humans (Group 2A) – Agents generally need to fulfill 2 of the following 3 requirements to be placed in Group 2A: some evidence of cancer in humans, strong animal evidence and strong mechanistic evidence.

Possibly carcinogenic to humans (Group 2B) – Agents that only have some evidence of cancer in humans or strong animal evidence or strong mechanistic evidence are placed in this category.

Not classifiable (Group 3) – More research is needed on the agents in this group to determine whether they cause cancer.

Agents classified by the IARC Monographs
Group 1 – carcinogenic to humans (120 agents)
  • Asbestos
  • Eating processed meat
  • Tobacco smoking
  • UV-emitting tanning devices
Group 2A – probably carcinogenic to humans (83 agents)
  • Eating red meat
  • Glyphosate (a common pesticide)
  • Night shiftwork
Group 2B – possibly carcinogenic to humans (314 agents)
  • Chloroform
  • Gasoline engine exhaust
  • Radiofrequency electromagnetic fields (cell phones)
Group 3 – not classifiable as to its carcinogenicity to humans (500 agents)
  • Acetaminophen
  • Caffeine
  • Coal dust

Source: International Agency for Research on Cancer [Internet]. Lyon: Agents Classified by the IARC Monographs, Volumes 1–125 [updated 2020 Feb 18; cited 2020 Mar 5].

Notes: Before 2019, IARC also used to include “Group 4 (probably not carcinogenic to humans).” However, because IARC only reviews agents if there is some evidence that they may cause cancer, this group was rarely used and has been removed.

How Ontario Health (Cancer Care Ontario) uses IARC classifications

Ontario Health (Cancer Care Ontario) used the IARC evaluations to identify cancer-causing substances that have a large impact in Ontario for its reports on cancer burden:

Read the reports to learn about cancers caused by common infections and exposures at work or in the environment, as well as policy changes that could help prevent these cancers.


  1. Samet JM. Expert review under attack: glyphosate, talc, and cancer. Am J Public Health. 2019;109(7):976–8. Available from:
  2. Ontario Ministry of Health and Ministry of Long-Term Care. The Skin Cancer Prevention Act (Tanning Beds), 2013. Available from:
  3. McWhirter JE, Byl S, Green A, et al. Availability of tanning salons in Ontario relative to indoor tanning policy (2001–2017). Prev Med Rep. 2018;12:40–5. International Agency for Research on Cancer. Preamble to the IARC Monographs, amended January 2019. Available from:
  4. Preamble to the IARC Monographs, amended January 2019. Available from:
  5. Samet JM, Chiu WA, Cogliano V, et al. The IARC Monographs: Updated Procedures for Modern and Transparent Evidence Synthesis in Cancer Hazard Identification. J Natl Cancer Inst. 2020;112(1):30–7. Available from: