In May 2011, a working group of 31 scientists from 14 countries met at IARC in Lyon, France, and reviewed roughly 900 published studies on radiofrequency electromagnetic fields and cancer. They evaluated evidence from human epidemiology, animal experiments, and mechanistic studies.

The group found "limited evidence of carcinogenicity" in humans based on positive associations between cell phone use and glioma (a type of malignant brain tumor) and acoustic neuroma (a non-cancerous tumor of the nerve connecting the ear to the brain). Based on this, they classified radiofrequency electromagnetic fields as Group 2B - possibly carcinogenic to humans.

Group 2B means that a link to cancer has been detected but chance, bias, and confounding factors cannot be ruled out with enough certainty. Other substances in this category include pickled vegetables and talcum powder. It is one step below Group 2A ("probably carcinogenic") and two steps below Group 1 ("carcinogenic to humans").

The US National Toxicology Program (NTP) conducted a $30 million, decade-long study that exposed rats and mice to whole-body radiofrequency radiation at 900 MHz, using the same GSM and CDMA signals used by cell phones. Rats were exposed from before birth through their entire lives, about 2 years.

Male rats showed a clear dose-related increase in malignant schwannomas of the heart - a rare cancer of the cells that wrap around nerves. About 2% of rats at the lower exposure level and 5-6% at the higher level developed these tumors, compared to 0% in the unexposed control group. The NTP classified this as "clear evidence of carcinogenicity." Male rats also developed gliomas (malignant brain tumors) at elevated rates, which the NTP called "some evidence of carcinogenicity."

The heart schwannomas are biologically significant because schwannomas are the same cell type involved in acoustic neuromas - the tumor type linked to heavy cell phone use in some human studies. Female rats and mice of both sexes showed weaker or no effects. The results were published as NTP Technical Report 595 in November 2018.

The Ramazzini Institute in Bologna, Italy, published its final results in 2018 in Environmental Research. Researchers exposed 2,448 Sprague Dawley rats to 1.8 GHz GSM radiofrequency radiation - the same frequency used by cell phone base stations - for 19 hours per day from prenatal life until natural death. The exposure levels (whole-body absorption rates of 0.001, 0.03, and 0.1 watts per kilogram) were designed to mimic what people receive from cell towers, which is much lower than what a phone held to the head delivers.

At the highest dose (0.1 watts per kilogram), male rats showed a statistically significant increase in malignant schwannomas of the heart. This is the same rare tumor type found in the NTP study, despite the Ramazzini study using much lower radiation levels and a different frequency.

The fact that two independent, large-scale studies in the same rat strain found the same rare tumor type strengthens the case that the finding is real rather than a statistical fluke. Critics note that the exposure levels were still higher than what most people receive from cell towers in daily life.

Swedish researchers led by Lennart Hardell published a pooled analysis of their case-control studies in Pathophysiology in 2014. The analysis combined data from studies conducted in Sweden during 1997-2003 and 2007-2009, comparing brain tumor patients to matched controls drawn from the general population.

For people who had used mobile phones for more than 25 years, the odds ratio for glioma was 3.0 - meaning they were 3 times as likely to develop a glioma compared to non-users. The risk was highest on the same side of the head where the phone was typically held (called ipsilateral use). For all mobile phone users combined, the odds ratio was 1.3.

These findings stand out because they show a dose-response pattern - longer use meant higher risk - and because the effect was strongest on the side of the head receiving the most radiation. However, other research groups have not consistently replicated these results, and critics argue the Hardell studies may be affected by recall bias, where people with brain tumors overestimate their past phone use.

The INTERPHONE study, coordinated by IARC and published in the International Journal of Epidemiology in 2010, was the largest case-control study of cell phone use and brain tumors at the time. It enrolled 2,708 glioma cases and 2,972 controls across 13 countries, interviewing participants about their phone use history.

Overall, regular mobile phone use was not associated with increased glioma risk. However, when researchers looked at the top 10% of users by cumulative call time - those reporting 1,640 or more hours of total calls - the odds ratio was 1.40, meaning a 40% higher risk of glioma. The risk was concentrated on the same side of the head where the phone was used.

The study authors cautioned that biases and errors in the data prevented a firm causal interpretation of the finding in heavy users. Some of the reported usage amounts were implausibly high, suggesting recall errors. Nevertheless, the elevated risk in the heaviest-use group was one of the key findings that contributed to the IARC Group 2B classification.

Yang and colleagues published a systematic review and meta-analysis in the International Journal of Environmental Research and Public Health in 2017. They focused specifically on glioma risk and analyzed data from multiple case-control studies that assessed mobile phone use duration.

For long-term use of 10 years or more, the pooled odds ratio for glioma was significantly elevated. When looking specifically at tumors in the temporal lobe - the part of the brain directly adjacent to the ear and closest to where a phone is held - the odds ratio reached 1.71 in the group with 10 or more years of use.

The temporal lobe specificity is considered biologically plausible because this brain region absorbs the highest dose of radiofrequency energy during phone calls. If cell phone radiation truly caused brain tumors, one would expect the tumors to cluster in the area receiving the most exposure, which is what these data suggest.

Choi and colleagues published a systematic review and meta-analysis in the International Journal of Environmental Research and Public Health in 2020, pooling data from 46 case-control studies on cell phone use and tumor risk.

The overall random-effects meta-analysis found no statistically significant association between regular cell phone use and tumor risk. However, the subgroup analysis revealed a striking split: the Hardell group studies consistently showed a statistically significant positive association, while the INTERPHONE-related studies showed a statistically significant negative association (suggesting a protective effect, which is biologically implausible and likely reflects participation bias). Other research groups showed no significant association.

This pattern highlights the core challenge in this research area. Whether cell phones are linked to cancer depends heavily on which studies you prioritize. The Hardell studies, conducted in Sweden with detailed exposure assessment, consistently find elevated risks. The larger multinational studies tend to find no overall risk but acknowledge possible elevated risk in the heaviest users.

Lennart Hardell and Michael Carlberg published a review in the International Journal of Oncology in 2017 examining the accumulated evidence from case-control studies on cell phone use and brain tumor risk, with particular attention to laterality - whether tumors developed on the same side as the phone was typically used.

Their analysis found that for 10 or more years of mobile phone use, the odds ratio for ipsilateral glioma (on the same side the phone was held) was approximately 2.0 - meaning roughly double the risk. Contralateral glioma (on the opposite side) showed no significant increase. A similar pattern was found for acoustic neuroma.

This laterality pattern is considered important evidence because it matches what would be expected if the radiation from the phone were actually causing the tumors. The side of the head closest to the phone receives substantially more radiofrequency energy than the opposite side, and that is where the excess tumors appear.

In 2024, multiple researchers published formal responses to the WHO-commissioned systematic review by Karipidis and colleagues, which had concluded that radiofrequency exposure likely does not increase cancer risk. The criticism, published in Environment International, argued the review had serious methodological flaws.

The main criticism was that most studies included in the Karipidis review had follow-up periods too short to detect tumors that develop slowly. Brain tumors like glioma can take 15 to 20 years or more to grow large enough to be diagnosed. Since widespread cell phone use only began in the late 1990s and early 2000s, many studies may simply not have waited long enough to capture the full cancer risk.

Critics also noted that the review dismissed the NTP and Ramazzini animal studies and downgraded the Hardell case-control studies, which had the longest exposure assessment periods and consistently found elevated risk. The debate highlights that the scientific community remains divided, with disagreements about which studies deserve the most weight.