THE’International Agency for Research on Cancer (IARC) of the WHO has drawn up some lists, the IARC monographswhich group chemicals, foods and pathogens based on the level of carcinogenicity demonstrated in scientific studies. The classification is not made based on the probability of the appearance of cancer but on the solidity of the scientific data present in the literature. There are 4 classification groups: group 1 – carcinogenic to humans (135 agents), group 2A – probably carcinogenic to humans (97 agents), group 2B – possibly carcinogenic to humans (324 agents) e group 3 – not classifiable due to its carcinogenicity for humans (499 agents). It is important to note that the IARC lists do not specify which is which dose dangerous nor for how long time you have to expose yourself to a substance to get sick. Also, they don’t do potency rankings, putting two substances in the same group doesn’t mean they are equally dangerous. This is why, when we read in the newspapers that a new substance has been classified as carcinogenic, as happened with cooked ham which has come back into fashion in recent days, we must not stop at the title but delve deeper into the issue.
The IARC groups and the classification method
Group 1 – carcinogenic to humans
This category applies whenever there is sufficient evidence of carcinogenicity in humans. It can be applied when there is simultaneously fort evidence in exposed humans that the agent displays key characteristics of carcinogens and sofficial evidence of carcinogenicity in experimental animals.
They are part of this category:
Group 2A – probably carcinogenic to humans
This category generally applies when the work group has performed at least two of the assessments explained above, of which at least one must involve exposed humans or human cells/tissues (limited evidence in humans, sufficient evidence in animals, strong evidence on key characteristics).
In this group we find:
- there red meat
- The androgenic steroids (anabolic)
- Hair salon or barber (professional exposure)
- emissions from high temperatures derived from frying
- shift work which involvesdisruption of circadian rhythms
- the very hot drinks above 65 °C (to drink)
- The talc
- The DDT
- internal emissions resulting from domestic combustion of biomass fuel (mainly wood)
Group 2B – possibly carcinogenic to humans
This category generally applies when it was made just one evaluations valid for previous groups. May be based exclusively on animal studies, it is not required that the strong evidence comes from exposed humans or human cells.
Examples include: whole leaf extract of aloe verait engine exhaust (petrol), i contraceptives based on progestin only, theaspartame and the chloroform.
Group 3 – not classifiable due to its carcinogenicity for humans
Agents who do not fit into any other group are typically placed in this category. This includes the case where there is strong evidence of the carcinogenic mechanism in animals it does not work in humans or the other categories are not supported by human or mechanistic data.
This includes the coffeeThe teathe acrylic fibresthechlorinated drinking wateri hair coloring productsthe leather processingand the silicone breast implants.
At this link you can consult the complete monographs
IARC classification of substances: how it happens
IARC monographs evaluate the intrinsic ability of an agent to cause cancer, they do not assess the likelihood of cancer occurring based on actual levels of exposure. For example, the IARC tells you whether cooked ham can cause cancer, not how likely you are to get sick if you eat a slice.
To reach these conclusions, numerous interdisciplinary experts are involved working groups (working groups) who evaluate and review scientific data, specialists (Invited Specialists), agency representatives (Representatives), observers (Observers) e IARC secretaries who coordinate the entire process. Specifically, studies are evaluated on three levels: i carcinogenic mechanisms (data on how a substance acts at the molecular level), studies on animal models And evidence in humans through epidemiological studies derived from observations in populations exposed to the elements.
Agents are considered potentially carcinogenic if they exhibit one or more of these properties:
- AND electrophilic or it can be metabolically activated to become so (reacts chemically with DNA/proteins).
- AND genotoxic (damages DNA).
- Alter the DNA repair or causes genomic instability.
- It induces epigenetic alterations (changes the expression of genes without changing the DNA).
- It induces oxidative stress.
- It induces chronic inflammation.
- AND immunosuppressant (weakens the immune system).
- Modulates mediated effects receptors (e.g. acts like a hormone).
- Cause immortalization (prevents cells from dying when they should).
- Alter the cell proliferationcell death or nutrient supply.
At the end of the process, the agent – which can be a fooda pathogen like bacteria or viruses, one generic chemical substance but also professional exposure in jobs such as the fireman or the hairdresser – is placed in one of 4 groups, based on the strength of the evidence collected (not on the potency of the substance). They are evaluatedevidence of carcinogenicity in humanstheevidence of carcinogenicity in experimental animals and the presence of le key characteristics of carcinogens. The lists used for this article are updated to 21/11/2025.
