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Ontario Cancer Facts

Putting Carcinogens into Context: Understanding the International Agency for Research on Cancer’s Ratings

Feb 2013

Increasingly, when a new substance is identified as a carcinogen, it becomes a major news story. For example, when radio frequency electromagnetic fields were classified as a possible carcinogen, it raised many questions about whether cell phones are safe to use. Understanding the range of classifications and the meanings behind them can help put these stories in context. 

The Monograph Program of the International Agency for Research on Cancer (IARC)—a specialized agency of the World Health Organization that promotes cancer research worldwide—is responsible for rating many potentially carcinogenic agents, including chemicals, complex mixtures, occupational exposures, physical and biological substances, lifestyle factors and exposure circumstances (e.g., shiftwork). IARC uses 5 classification categories ranging from carcinogenic to humans (Group 1) to probably not carcinogenic to humans (Group 4), which describe whether or not an agent causes cancer (as opposed to the strength of the carcinogen or level of exposure at which cancer risk increases).  

The table below shows some examples of agents in each of the 5 categories. Very few agents fall into Group 4 (there is only 1 to date) because anything in this category requires a great deal of strong evidence showing that it does not cause cancer in humans and animals over the full range of likely human exposures. Another reason that Group 4 is so sparse is that agents that come to the IARC Monograph Program often have at least some evidence of carcinogenicity in humans or animals that has prompted researchers to review them.

Agents Classified by the IARC Monographs 

Group 1 - carcinogenic to humans
(120 agents)
  • Alcoholic beverages
  • Asbestos
  • Consumption of processed meat
  • Cyclophosphamide
  • Diesel engine exhaust
  • Hepatitis B
  • Outdoor air pollution
  • Solar radiation
  • Tobacco smoke
  • UV-emitting tanning devices
Group 2A - probably carcinogenic to humans
(81 agents)
  • Anabolic steroids
  • Consumption of red meat
  • DDT
  • Shiftwork involving circadian disruption
Group 2B - possibly carcinogenic to humans
(299 agents)
  • Chloroform
  • Gasoline engine exhaust
  • Radiofrequency electromagnetic fields (cell phones)


Group 3 - not classifiable as to its carcinogenicity to humans
(502 agents)


  • Acetaminophen
  • Caffeine
  • Chlorinated drinking water
  • Cholesterol
Group 4 - probably not carcinogenic to humans
(1 agent)
  • Caprolactam (used to make nylon)

Source: International Agency for Research on Cancer [Internet]. Lyon: Agents Classified by the IARC Monographs, Volumes 1–120 - List of classifications by Group. [updated 2018 Mar 29; cited 2018 Apr 5]. 

Each evaluation is done by a Working Group made up of 20 to 30 scientists from around the world with no conflicts of interest. They are chosen for their expertise and carefully consider a variety of data to make classification decisions: exposure data (how and where people are exposed to the agent, and at what levels), studies of cancer in humans (epidemiology), studies of cancer in experimental animals, and ways the agent may cause cancer. 

Sub-groups within the Working Group start by doing a preliminary default evaluation to determine whether the evidence in humans and animals is sufficient, limited, or inadequate, as shown in the figure, or suggests a lack or carcinogenicity (omitted from the figure because it rarely occurs). The evidence is assessed based on the number of studies showing that an agent causes cancer, the strength of this causal relationship and the quality of the studies. Strong proof of a causal relationship is needed to reach the sufficient evidence level. Similarly, there must be very strong negative evidence across the range of exposures likely to occur in humans to draw the conclusion that there is evidence suggesting a lack of carcinogenicity. Limited evidence implies that there is some proof of an association between the agent and cancer, but not enough to make the claim that the agent actually causes cancer. Inadequate evidence is a neutral category in which no conclusions can be drawn about a causal relationship. 

Preliminary Default Evaluation 

  Cancer in Experimental Animals: Sufficient  Cancer in Experimental Animals: Limited Cancer in Experimental Animals: Inadequate
Cancer in Humans: Sufficient Group 1 Group 1 Group 1
Cancer in Humans: Limited Group 2A Group 2B Exceptionally: Group 2A Group 2B Exceptionally: Group 2A
Cancer in Humans: Inadequate Group 2B Group 3 Group 3

Source: International Agency for Research on Cancer [Internet].
Lyon: Preamble to the IARC Monographs [posted 2006 Jan 23; updated 2015 Sept 4].





Next comes the final evaluation, which is made when the preliminary default evaluation is combined with relevant data on the ways in which the agent may cause cancer. For example, an agent with inadequate evidence in humans but sufficient evidence in animals might be upgraded from Group 2B to Group 2A if studies showed that the way it causes cancer in animals could also happen in humans. Of the 1003 agents reviewed to date, IARC has identified 120 Group 1 carcinogens, 81 Group 2A probable carcinogens, and 299 Group 2B possible carcinogens.