Do mobile phones cause cancer? What the 2011 “possibly carcinogenic” label meant, and what came after

May 2011: what was decided

A Working Group of 31 scientists from 14 countries met at IARC in Lyon from 24 to 31 May 2011 and, on the last day, classified radiofrequency electromagnetic fields as possibly carcinogenic to humans (Group 2B). The full evaluation was published in 2013 as Volume 102 of the Monographs.

The evaluation did not concern only phones. The Working Group looked at three kinds of exposure: occupational exposure to radar and microwaves, environmental exposure to signals from radio, television and wireless telecommunications, and personal exposure from using wireless telephones. The classification itself covers radiofrequency fields as a physical agent.

The context explains the attention. IARC noted at the time that the number of mobile phone subscriptions was estimated at 5 billion worldwide.

What “limited evidence” meant here

The decision rested on studies in people. For glioma, a malignant brain tumour, and for acoustic neuroma, a tumour of the hearing nerve, the evidence from wireless phone users was judged “limited”. For all other cancers it was “inadequate”.

In the vocabulary of the Monographs, limited evidence means that a causal interpretation of the observed association is credible, but chance, bias or confounding cannot be ruled out with reasonable confidence. That is the typical profile of a 2B classification: a signal worth taking seriously, not a demonstrated cause. How the levels of evidence combine into groups is described on Groups 1, 2A, 2B and 3.

The chair of the Working Group, Jonathan Samet, said that the evidence, while still accumulating, was strong enough to support a conclusion and the 2B classification.

The “40%” figure

One number from 2011 has had a long life in the news. IARC’s press release mentioned that one study of mobile phone use up to 2004 found a 40% increased risk of glioma in the highest category of heavy users, who reported on average about 30 minutes of calls a day over a 10-year period.

That figure came from a single analysis of the heaviest users, not from an estimate made by IARC. The press release said explicitly that the Working Group did not quantitate the risk. A Group 2B label, in IARC’s own terms, says how convincing the evidence of a hazard is; it carries no information about how many people, if any, might be affected. The general point is explained on Can it, or will it?.

The IARC Director of the time, Christopher Wild, also stressed that additional research into long-term, heavy use of mobile phones was important.

2024: a systematic review for the WHO

That research followed. In September 2024 the journal Environment International published a systematic review of human observational studies on radiofrequency fields and cancer, led by Ken Karipidis of the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA). It was commissioned by the WHO, with co-financing from New Zealand, Italy and Australia.

The authors screened more than 5000 studies and included 63 articles published between 1994 and 2022, from 22 countries, covering 119 combinations of exposure and outcome. Ever or regular use of a mobile phone was not associated with an increased risk of glioma, meningioma, acoustic neuroma, pituitary or salivary gland tumours, or of brain tumours in children. For glioma, the pooled relative risk was 1.01, with a 95% confidence interval of 0.89 to 1.13 – in other words, no detectable difference.

Crucially for the 2011 debate, the review also found no rise in risk of glioma, meningioma or acoustic neuroma with longer time since first use, more cumulative call time or more calls. Using the GRADE system, the authors rated their conclusion for mobile phones held to the head as of moderate certainty. Their conclusions for cordless phones and for occupational exposure were of low certainty, because there were fewer studies.

Do the two findings contradict each other?

Not exactly. They answer different questions with different evidence. The 2011 evaluation was a hazard identification made on the studies available then, using all three of IARC’s evidence streams. The 2024 review covers only observational studies in people, but many more of them, published over a longer period, with a formal assessment of certainty. It does not replace an IARC evaluation; it is part of the evidence a future one would weigh.

The same pattern appeared with coffee, which IARC moved out of Group 2B in 2016 once better studies were available. That case is told in Is hot tea or coffee carcinogenic?.

What happens next

In March 2024 an IARC Advisory Group of 28 scientists from 22 countries recommended priorities for the Monographs in 2025–2029. It listed radiofrequency non-ionising radiation among the high-priority agents for re-evaluation, citing new evidence on cancer in humans and in experimental animals.

A recommendation is not yet a meeting. The July 2026 newsletter of the Monographs announced the next two meetings – cannabis smoking (3–10 November 2026) and lead and cadmium (19–26 October 2027) – and neither concerns radiofrequency fields (status as of 3 November 2026). Until a new Working Group meets, the 2011 classification remains the one on IARC’s list. Announced meetings are tracked on Conferences & Events.

This article explains how evaluations work; it is not medical advice.