The largest study to date on the safety of mobile phones has found no clear link to brain cancer, but researchers say further research is needed given the increasingly intensive use of the technology.
The study, by the World Health Organization's International Agency for Research on Cancer (IARC), appears this week in International Journal of Epidemiology.
"The results really don't allow us to conclude that there is any risk associated with mobile phone use, but... it is also premature to say that there is no risk associated with it," says IARC's director Dr Christopher Wild.
The study looked at almost 13,000 mobile phone users, including 2,708 people with glioma tumours and 2,409 people with meningioma tumours in 13 countries.
It found no increased risk of glioma or meningioma tumours after 10 years of using a mobile phone, although it found "suggestions of higher risk" for the heaviest users.
The heaviest users who reported using their phones on the same side of their heads had a 40% higher risk for gliomas and 15% for meningiomas, but the researchers said "biases and errors" prevent making a causal link.
Given that the heaviest users in the study talked an average of half an hour per day on their mobile phones, a figure which is not heavy by today's standards, the researchers recommend further research.
The research, involved 21 scientists from the Interphone International Study Group, which received 19.2 million euros (A$27 million) in funding, around 5.5 million euros (A$7.7) of which came from industry sources.
Young people not included in study
The researchers also cited the need for the study of the impact of mobile phone use among young people, who were not included in the Interphone study.
The say it is not unusual for young people to use mobile phones for an hour or more a day.
"Observations at the highest level of cumulative call time and the changing patterns of mobile phone use ... particularly in young people, mean that further investigation of mobile phone use and brain cancer is merited," says Wild.
The researchers noted, however, that the latest mobile phones have lower emissions, and the popularity of hands-free devices and texting reduce exposure to the head.
Scientific Advisor to Cancer Council Australia, Professor Bernard Stewart agrees the findings merit further research.
"This study involves phone usage for 12 years at most and therefore tells us little about any risk associated with mobile phone use over decades," he says.
"In particular, insufficient time has passed since mobile phones were introduced to determine whether or not there is a risk in children."
He says until there is research into this area, the Cancer Council recommends caution in relation to children.
"They should not use, or minimise their use, of mobile phones. Anyone concerned about the harmful effects of electromagnetic energy should reduce their use of mobile phones, or employ hands-free technology."
Puzzle points to study methods
In some analyses of the Interphone study data, mobile telephone users appeared to have a lower risk of brain cancer than people who had never used one.
But researchers say this paradoxical finding suggests problems with the method, or inaccurate information from those who took part.
The study relied on people searching their memories to estimate how much time they spent on their cell phones, a method that can throw up inaccuracies.
European scientists last month launched what will now become the biggest ever study into the effects of mobile phone use on long-term health.
It aims to track at least a quarter of a million of people in five European countries for up to 30 years.
This kind of study, called a prospective study, is considered more accurate because it does not require people to remember their cell phone use later but tracks it in real time.