(CNN) The incidence rate of aggressive malignant brain tumors in England has more than doubled in recent decades, and a new study questions what could be driving that rise.

The rate of glioblastoma climbed from 2.4 to 5.0 per 100,000 people in England between 1995 and 2015, according to the study, published Wednesday in the Journal of Environmental and Public Health

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 paper itself is not about cell phones; it's just about this change in the tumors ... but cell phones seem like really they're the most likely cause," said Alasdair Philips, lead author of the study and a trustee of Children with Cancer UK

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.

Around the world, research into cell phone radiation and a potential link to cancer risks has left more questions than answers.

Some studies have found no relationship between cell phone radio frequency and certain health problems, such as increased risks of tumors , while others suggest the opposite.

There are three main reasons why people are concerned that cell phones might have a link with cancer or other health problems, according to the National Cancer Institute in the United States.

Cell phones emit radiofrequency energy , a form of non-ionizing radiation, and the tissues in our bodies closest to the phone can absorb this energy. Second, our use of cell phones has increased rapidly. Lastly, the number of phone calls we make and the length of those calls also have increased.

However, the US Food and Drug Administration notes that since cell phones emit low levels of radiofrequency energy that are non-ionizing, they're not considered strong enough to permanently damage our biological tissues.

There's a rise, but should cell phones take the fall?

The new study included cancer registration data for malignant brain tumors diagnosed in England from 1995 to 2015. Within those data, which came from the UK Office of National Statistics , there were 81,135 diagnosed cases.

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.

The researchers also mentioned atomic bomb testing fallout , particularly ingesting or inhaling radioactive substances, as a possible factor, and they mentioned traffic-related air pollution in addition to cell phone use.

"This paper provides evidence for a rise in specific malignant brain tumors in England, showing that incidence has more than doubled over the last two decades. What the analysis does not show is that this rise is caused by mobile phones. This is for a number of reasons," Lion Shahab, senior lecturer in epidemiology and public health at University College London, said in a written statement released by the Science Media Centre

"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.

The number of mobile phone subscribers worldwide is estimated to be 5 billion , according to GSMA, a trade association that represents the interests of mobile operators.

In the UK, about 93% of adults own or use a cell phone, according to the National Health Service . Similarly, the vast majority of Americans, 95%, own a cellphone of some kind, according to the Pew Research Center

One report from that series, released in February, found a link between high levels of cell phone radiation and some evidence of carcinogenic activity in male rats, including a rare type of tumor called a schwannoma in their hearts.

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

"One of the things that we found most interesting about our findings was that the malignant schwannomas -- even though they occurred in the heart and not in the head of these animals -- were in fact schwannomas," John Bucher, a senior scientist at the National Toxicology Program and one of the authors of the reports, said when they were published in February

"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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In the meantime, those worried about radiofrequency radiation from cell phones can take steps to limit exposure, according to the American Cancer Society

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 texting instead of talking, and try to simply limit your cell phone use, as well as the cell phone use of any children in your household.