The question of whether cellphone use causes brain tumours remains unsettled after the results of a major study on almost 13,000 cellphone users over 10 years.

The World Health Organization's International Agency for Research on Cancer study called INTERPHONE found most cellphone use didn't increase the risk of developing meningioma, a common and frequently benign tumour, or glioma, a rarer but deadlier form of cancer.

"I think the results are tantalizing in the sense that overall we're not seeing evidence of an increased risk," said Prof. Daniel Krewski of the University of Ottawa, a Canadian member of the study group. "But … in this one exposure group, the 1,640-plus cumulative call hours, a possible increased risk in glioma."

"We're not sure about the significance of those results because some of the people in that category reported usage levels which were somewhat improbable."

Handsets and texting have changed how people use cellphones, which are now less-often held against the head. (CBC)

The overall findings are "comforting" from a public health point of view, but the possible increased risk of glioma among heavy users talking for more than 30 minutes per day on the same side of the head warrants a closer look, Krewski said.

The authors noted that "biases and error prevent a causal interpretation" that would directly blame radiation for the tumour.

The paper, published in the International Journal of Epidemiology, was compiled by researchers in 13 countries including Britain, Canada, France, Germany and Japan. The study looked at almost 13,000 participants, including 2,409 meningioma cases and 2,662 matched controls of the same age, gender and location, and 2,708 glioma cases with 2,972 matched controls.

It takes 10 to 20 years for many tumours in the head and neck areas to develop, which is why the study was designed to follow so many users.

Difficult to interpret

The potential effects of using hands-free devices during calls or the risk of having cellphones nearby, such as in a pocket, were not examined. Researchers mainly relied on people to recall their cellphone use. For some groups, cellphone use appeared to lessen the risk of developing cancers, which the researchers called "implausible."

A separate study will look into the effects of cellphone use on children. (David Kohl/Associated Press)

One explanation is that cellphone users may be healthier, but a more likely reason is people showing manifestations of neurological disease use cellphones less, Rodolfo Saracci of Italy's National Research Council and Jonathan Samet of the University of Southern California's department of preventive medicine said in a journal commentary accompanying the study.

"For the time being, INTERPHONE’s findings, interpreted in the context of prior studies, tells us that the question as to whether mobile phone use increases risk for brain cancers remains open," Saracci and Samet wrote.

"The tired refrain 'more research is needed' fully applies in this instance: without more research the public's question about the acceptability of cancer risk from mobile phones will remain unanswered."

Texting changes cellphone usage

Last month, European scientists launched what will be the largest study into the long-term effects of cellphone use. The study aims to track at least a quarter of a million people in Europe for up to 30 years in real time. A separate study will look into the effects of cellphone use on children, who are believed to be more susceptible to the effects of radiation.

"It's kind of embarrassing as a scientist to say we've just spent 12 years and 20 million euros and we think there's not much of a risk there, but we're not entirely sure," Krewski acknowledged of the research so far, which included almost $2 million in Canadian funding.

At this point, researchers don't have a plausible biological mechanism by which radio frequency fields could increase cancer risk in humans, said epidemiologist Dr. Mary McBride of the B.C. Cancer Agency, another Canadian member of the study.

"The way in which people use cellphones today and the number of users has certainly changed since the study data were collected," said McBride. "In particular the exposure levels by using handsets and by texting are less than by holding the phone up to one's ear."

Precaution options

Both the study's authors and the International EMF Collaborative, which wrote a counterview to the study, pointed to flaws in the research.

"When randomly selected controls are asked if they would like to participate in a 'mobile phone' study, they are far more likely to agree if they use mobile phones than if they don’t, and if they do not use a mobile phone, they are likely to refuse participation," the counterview said.

The counterview authors also noted the study did not consider other sources of radio frequency radiation such as cordless phones, calling the five-year results "woefully inadequate as a gauge of risk today."

“Four billion people own mobile phones worldwide; many of those users are children," said Eileen O’Connor, director of the Radiation Research Trust and member of the International EMF Collaborative.

"Responsible governments must advocate for public transparency of risks so that an informed public may have more options to exercise precaution," O'Connor added in a statement.

The Mobile Manufacturers Forum welcomed the study.

"The INTERPHONE project is the biggest study of its kind ever undertaken in this field and provides significant further reassurance about the safety of mobile phones," said Michael Milligan, secretary general of the Mobile Manufacturers Forum, which provided partial funding for the study along with the GSM Association, European Commission and national research funding bodies.

With files from The Associated Press