The data analyzed in the study only reflect trends in brain cancer cases and do not shed light on why these trends could have occurred, but the researchers pointed to examples of lifestyle factors that they think could have played a role.
The tumors "primarily are in the frontal and temporal lobe areas, by your ear and forehead," which raises the cell phone suspicion, he said.
Yet, he added, since brain tumors are very rare, his suspicion should not raise alarm, because even if cell phone use could raise your brain tumor risk, "it's still a very low risk of you individually getting a brain tumor," he said. "My advice would be if you're going to have a long call, make sure it's hands-free, but I wouldn't panic about it, either."
Several experts in the UK have cautioned that although the study found evidence of an increase in brain tumors, the suggestion that cell phone use could be responsible is not proven.
There's a rise, but should cell phones take the fall?
While comparing new case numbers in 2015 with those in 1995, the researchers found an additional 1,548 aggressive glioblastoma multiforme tumor cases annually.
"I was very surprised by how big the rise is and how consistent it is over the years," Philips said. "The statistics are phenomenally tight."
The researchers wrote in the study that ionizing radiation, especially from X-rays used in CT scans, has the most "supportive evidence" as a possible factor behind the rise in glioblastoma diagnoses.
"Firstly, this paper does not attempt to link the rise in mobile phone use with a rise in brain tumor incidence directly," said Shahab, who was not involved in the study.
"There are statistical techniques available to do this, for instance using time-series analysis, which attempt to link changes in a putative risk factor with changes in the outcome of interest over time. This was not done here," he said. "Second, even if such a link were found, correlation does not imply causation."
In other words, just because a rise in brain tumor incidence appeared to occur at the same time as a rise in cell phone use, that does not mean one caused the other.
"While mobile phone usage in the population increased from less than 15% to 95% over the time period studied, we do not see the same increase in malignant brain tumors. This suggests that the strength of any effect, if present, would have to be small," Shahab said.
"For now, linking a rise in malignant brain tumors to mobile phone usage remains speculative and should not detract from encouraging lifestyle changes which are known to reduce cancer risk, such as adopting a healthy diet, reducing alcohol consumption and stopping smoking," he said.
In response to criticism that the study has received, Philips said that he would ask, what else could be contributing to the increase in brain tumor incidence?
"It has to be a fairly universal thing or change in lifestyle that would cause such a trend," he said.
No such findings were seen in female rats, and another report from the series found no such findings in mice being studied.
What to do if you're worried
"These experimental animal studies are but one approach to understanding whether exposures to radiofrequency radiation pose a risk to human health," Bucher said, adding that studies are continuing at the National Toxicology Program to examine changes on the molecular level in tissue samples from the rodents.
He added, "I have not changed the way I use a cell phone."
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They include using the speaker mode or hands-free devices while talking on the phone, in order to keep the phone away from your head. Try