Though the health risks of cell phone radiation exposure are still "up in the air," scientists have finally figured out that it is doing something. According to a new study, the radio waves emitted by mobile devices cause a light flurry of activity in nearby brain tissue, causing it to burn 7 percent more energy than it would normally.

"We have no idea what this means yet or how it works, said Nora Volkow, a neuroscientist at the National Institutes of Health and lead author of the study. "But this is the first reliable study showing the brain is activated by exposure to cellphone radio frequencies."

To obtain their data, Volkow and her colleagues did extensive brain scans on 47 study subjects. They taped a phone to both of each subject's earsone phone off and the other on a call, but mutedand compared neural activity on the two sides of the brain for 50 minutes. Their results have been published in the Feb. 23 issue of the Journal of the American Medical Association.

All past studies of the effects of cell phone radiation, according to Volkow, used 14 subjects at most, and looked at brain activity for only minute-long intervals. "A cellphone's effect on the brain is very weak, so you lose statistical power with small sample sizes and durations," she told the press. The thoroughness of Volkow's study has led experts to describe it as "the best to date" on the effects of cellphone radiation exposure, according to Wired Science.

A kilogram of brain tissue absorbs just under 1 watt of radiation from a Samsung Knack phone, the model used in the new study. For comparison, you absorb 5 watts per kilogram of solar radiation while sunbathing . It has been speculated that cell phone radiation, like UV radiation , may cause cancer. Scientists are trying to find out.

In the meantime, Nora Volkow uses an ear phone rather than placing her cell phone against her ear.

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