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Fears concerning the possible dangers of prolonged cell phone use won't go away, despite numerous studies showing no conclusive link between cell phones and brain tumors or cancer.

Now new data from the National Institutes of Health suggesting that cell phone radiation boosts brain activity is poised to stir the debate even further.

Researchers used PET scans to measure brain activity in 47 participants when they had cell phones held to their ears in both off and on but muted positions and found that exposure to an in-use cell phone for more than 50 minutes increased brain activity by about 7 percent in the regions closes to the antenna.

This suggests that while no link has been proven between adverse health effects and cell phone radiation, the human brain is sensitive in some way to the electromagnetic waves coming off of a cell phone.

Whereas past studies have looked at cerebral blood flow to measure changes in brain activity, this study measured the brain's consumption of glucose -- the fuel of the brain -- in order to measure localized activity near the antenna.

"There have been several studies since the late 1990s trying to address whether the human brain is affected by the electromagnetic radiation from cell phones because it's very, very weak," said the lead author on the study, Dr. Nora Volkow of the National Institute on Drug Abuse. "The studies were very inconsistent, but we designed this study so it would be powered to detect small activity.

"This shows that the human brain is sensitive to these weak magnetic impulses."

But sensitivity does not equal harm or an increased risk of cancer, Volkow says, and it may even turn out that the ability of this radiation to boost brain activity could have therapeutic effects. Further study is needed, however, to explore the potential detrimental or beneficial effects of such an increase in activity.

Although the research on this topic has been mixed and inconclusive, Volkow says, their findings reopen the debate about cell phone concerns and make it impossible to ignore that prolonged use over many years might have some kind of unknown effect on the brain.

Justified Caution or Cancer Conspiracy?

This study, published today in the Journal of the American Medical Association, does not speak to the possible cancer risks supposedly associated with cell phone use, but its findings may stir up past debates concerning the health hazards of America's cell phone habit.

The cell phone-cancer debate began in the late '90s as increasing use of cell phones both for work and play stirred interest in the potential effects of the small amounts of radiation emitted from any phone in use.

"Our bodies are exposed to electromagnetic radiation all the time, and it is relatively harmless in the low doses we receive," Dr. Jennifer Smullen of the Department of Otology and Laryngology at Harvard Medical School said.

"But cell phone antennas are associated with strong electromagnetic fields and are placed close to our body."

The largest-ever study of cell phones, known as the Interphone study, released its long-term results in May of 2010 and found no increased risk for benign or cancerous brain tumors, except at its highest level of use, a level deemed "implausible" as a comparison to real-life by the investigators. Even this link was only seen with benign tumors, not with cancer.

So why does the fear of cell phones' frying our brains still persist? The inconsistency in the data fuels the concern, Volkow says.

"Some studies have shown a significant association with cancer, others show the opposite," she said. "When you have data that is not consistent there is uncertain and uncertainty of course generates fear."

Especially considering the integral, frequent, increasing role that cell phones play in both our work and home lives. The number of mobile phone users has gone from 7.6 million in 1991 to 223 million in those older than 13 as of 2010, according to Nielsen industry statistics.

What's more, more than half of Americans age 25 to 29 use only cell phones as a means of communication and do not live in a household that even has a landline.

It also doesn't quell fears when doctors publicly advocate for cell phone precautions. In 2008, CNN Chief Medical Correspondent Sanjay Gupta, along with Cedars-Sinai Medical Center brain surgeon Dr. Keith Black said that they use earpieces instead of holding the phone against their heads out of fear of brain tumors.

Also in 2008, the director of Cancer Research at the University of Pittsburgh bolstered concerns about cell phone dangers when he released a controversial memo to his staff concerning modified cell phone practices to ward off potential harm:

"Although the evidence is still controversial, I am convinced that there are sufficient data to warrant issuing an advisory to share some precautionary advice on cell phone use," Dr. Ronald Herberman wrote in his memo.

He went on to warn that children shouldn't use cell phones except in emergency situations because "the developing organs of a fetus or child are the most likely to be sensitive to any possible effects of exposure to electromagnetic fields."

And many other doctors take this better safe than sorry approach. Dr. Andrew Sloan, director of the Brain Tumor and Neuro-Oncology Center at University Hospitals Case Medical Center in Cleveland, agrees that there's no "definitive proof" that cell phones are bad. But he still advises patients to use speaker phones or ear buds when talking for prolonged periods of time because, "why take the chance?"

With doctors advising patients to "play it safe," it's hard to shake the feeling that cell phones may be unsafe.

The Significance of Brain Sensitivity

That researchers were able to show a consistent effect on the brain through prolonged cell-phone use is "landmark," said Dr. Maciej Lesniak, director of neurosurgical oncology and neuro-oncology research at the University of Chicago.

"Previous articles have tried to link cell phones to brain tumors based on large population studies, but those studies have many inherent flaws and too many variables to control," he said.

This experimental kind of research lays the groundwork for further research into exactly what's going on in the brain, whether harmful or not, when people use cell phones over long periods of time, he adds.

That said, there is little more than can be gleaned from this study beyond the fact that cell phones are doing something to the brain and increasing activity.

"Clearly there is an acute effect, and the important question is whether this acute effect is associated with events that may be damaging to the brain or predispose to the development of future problems such as cancer as suggested by recent epidemiological studies," Smullen said.

"Cell phones have become an essential part of our culture both for personal reasons and for work ... they are here to stay, but we need to evaluate whether cell phone design and usage needs to be adjusted to avoid or minimize potential risks," she adds.