IARC: Probably carcinogenic? | © Billion Photos @ Shutterstock

How relevant is “probably carcinogenic”?

IARC in the public eye

About IARC

Nowadays it is a news, when the International Agency for Research on Cancer (IARC) assesses a substance as “probably carcinogenic to humans”. In the past years this was the case for e. g. UV radiation, diesel, red meat and glyphosate.

IARC is subordinated to the WHO. Its task is to assess risks from environmental substances. This task is obviously not easy to fullfill. Since the early 1980’s it has been known that people can be exposed to hazardous substances from food and experts started dealing with the question, how risk assessments should be conducted. The “Red Book” of the US National Research Council from 1983 has still significant influence. The central problem is that risks emerging from carcinogenic substances can hardly ever be determined. The information, that is necessary to completely assess a risk, is not available and it cannot be deliverd with the existing scientific tools. It is more the rule than an exception that the human exposure to a substance cannot be determined and that it is not proven whether the substance causes cancer in humans.

4 steps of risk assessment

A risk assessment consists of four steps:

  1. Hazard Identification = A substance is identified as a hazard. Its characteristics are described.
  2. Hazard Characterisation = The hazard is described, especially the dose-response-relationship, ideally based on epidemiological data and results from animal experiments.
  3. Exposure Assessment = The exposure of the average people is assessed and vulnerable populations are identified.
  4. Risk Characterisation = In this section the open questions should be answered, e. g.: Who is affected? How often/to what extent? Which health impairments can result from the exposure? Are they reversible? Is the causal relation evident? Is it possible to control the risk?

Results of the risk assessments

Until now (16. September 2016) IARC published 996 risk assessments. The results are according to the following scheme:

Group 1        The substance is “Carcinogenic to humans”“This category is used when there is sufficient evidence of carcinogenicity in humans.”

Group 2 a     The substance is “Probably carcinogenic to humans”. “This category is used when there is limited evidence of carcinogenicity in humans and sufficient evidence of carcinogenicity in experimental animals.”

Group 2 b     The substance is “Possibly carcinogenic to humans”. “This category is used for agents for which there is limited evidence of carcinogenicity in humans and less than sufficient evidence of carcinogenicity in experimental animals.”

Group 3        The substance is “Not classifiable as to its carcinogenicity to humans”. “This category is used most commonly for agents for which the evidence of carcinogenicity is inadequate in humans and inadequate or limited in experimental animals.”

Group 4         The substance is “Probably not carcinogenic to humans”. “This category is used for agents for which there is evidence suggesting lack of carcinogenicity in humans and in experimental animals.”

In 996 risk assessments 118 substances were classified as “carcinogenic to humans”, 79 as “probably carcinogenic”, 291 as “possibly carcinogenic” and one as “probably not carcinogenic to humans“. 507 substances were not classifiable.

Needless controversies about the category “probably carcinogenic”

In the past years, it created public awareness and raised controversial debates when substances were classified as “probably carcinogenic”. The term sounds clunky and is not appropriate for headlines in the media. In particular the term suggests a lot of uncertainty. This is always the point where critics start. However, “probably carcinogenic“ does mean that a substance causes cancer in animals and experts do not question that the mode of action is similar in the human body. The remaining question is, how many cancer cases are caused by the substance. This can only be estimated and is not IARC’s task.

Dr. Sabine Bonneck