WHO and IARC: how substances get labeled cancer-causing

This page is for patients and caregivers who have seen a news story saying something "causes cancer." It explains what IARC is, what its labels mean, and what they cannot tell you.

The International Agency for Research on Cancer is the specialized cancer agency of the World Health Organization. It sits in Lyon, France. It has been working since 1965.

Its own short summary of its mission is: cancer research for cancer prevention.

IARC does several kinds of work. It runs research programmes. It publishes, including the WHO Classification of Tumours, the Monographs, and handbooks of cancer prevention. It also offers training and postdoctoral fellowships.

For most patients, though, one part of IARC is the part they have heard of.

The full title is worth reading slowly: IARC Monographs on the Identification of Carcinogenic Hazards to Humans.

Every important limitation is already in that title. It says identification. It says hazards. It does not say risk. It does not say danger.

The programme reviews published evidence on an agent and reaches a classification. Agents can be chemicals, mixtures, occupations, or particular exposures. The results are published as monographs. Volumes have been appearing for decades.

Across volumes 1 to 142, agents are sorted into four groups.

• Group 1, carcinogenic to humans: 135 agents
• Group 2A, probably carcinogenic to humans: 98 agents
• Group 2B, possibly carcinogenic to humans: 329 agents
• Group 3, not classifiable as to its carcinogenicity to humans: 498 agents

The formal definitions of these groups are set out in the Monographs Preamble.

Here is the part news coverage usually loses. The group tells you how confident the working group was that the agent can cause cancer. It does not tell you how much cancer any real-world exposure produces. Two Group 1 agents can differ enormously in how much they matter in ordinary life.

IARC is also careful about the edges of its own list. It strongly recommends consulting the complete monograph, its publication date, and the list of studies considered. That is because significant new information might support a different classification. And it says directly that for agents that have not been classified, no determination of non-carcinogenicity or overall safety should be inferred.

Counts are for volumes 1 to 142 and change as new volumes are published.

It can give you a stable reference point. Say a headline claims a substance causes cancer. You can look up whether IARC has evaluated it and which group it landed in.

It can show you the reasoning, not just the label. A monograph records the studies considered and when they were considered. So you can see the basis of a classification rather than taking a summary on faith.

It can put your reading in context. IARC classifies hazards worldwide. Its lists include occupational exposures, infections, and everyday agents side by side. That makes it easier to see that "on the list" covers a very wide range of real-world importance.

It can also help you feel less alone in the confusion. Maybe you have read one article saying something is safe and another saying it is a carcinogen. The classification system explains how both writers could be looking at the same evidence.

It cannot tell you your own risk. A classification is not a personal risk estimate. It does not account for how much you were exposed to, when, or for how long.

It cannot explain your diagnosis. No classification can trace one person's cancer to one cause.

It cannot regulate. IARC produces evidence for policy. Setting exposure limits, warning labels, and bans is the job of governments and regulatory agencies. It cannot certify anything as safe. The absence of a classification means the question has not been settled there. It does not mean the answer is no.

And it cannot help with your treatment. It is a research agency, and nothing in a monograph speaks to your treatment plan. For questions about your care, call your care team.

Maybe you are here after seeing a warning about food, a product, or a job you once held. Try separating two questions.

The first is what the evidence says an agent can do. That is what IARC answers.

The second is what it means for you, given your actual life and exposure. People who know your history answer that question, not a group label.

Both questions are reasonable. Only the second one belongs in an exam room, and it is a fair thing to raise there.