Brain activity surges in people who hold cell phones to their ears for long conversations, raising new questions about the devices, according to a new study.

The research measured the effects from phone radiation and is the first to document that mobile phones have an impact on brain function.

While the health implications remain unknown, the discovery, published in Wednesday's Journal of the American Medical Association, is certain to fuel the global debate over potential risks, particularly for children.

The current generation of children and teens is the first to use cell phones almost exclusively, and there's no body of medical evidence to determine if there are long-lasting effects, experts said Tuesday.

The study by the National Institutes of Health and Long Island's Brookhaven National Laboratory in Upton, found an increase in the amount of glucose metabolized in the brain closest to the antenna for people using their phone for 50 minutes. Glucose is the sugar that fuels brain cells.

"We found the brain is indeed sensitive to this type of radiation," said neuroscientist Nora Volkow, director of the National Institute of Drug Abuse and lead author of the study.

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"My recommendation for individuals who want to play it safe . . . and don't want their brain stimulated by anything that is not physiological, I suggest putting the phone on speaker, or using an earpiece. In these instances, the radiation has no impact on the brain," Volkow said.

Volkow said she was inspired to conduct the study after noticing changes in brain activity among patients who underwent medical imaging studies. Cell phones were a natural follow-up, she said, because they produce a form of electromagnetic radiation to which an estimated 5 billion people worldwide are exposed.

Mobile phone safety has been rigorously debated for at least 15 years and manufacturers now add warnings in the insert materials accompanying new phones, cautioning users not to hold the device against the head. BlackBerry's manufacturer, Research in Motion, cautions users to maintain a distance of at least 1 inch.

Samsung and BlackBerry did not return calls Tuesday seeking comment on the study.

Dr. Dardo Tomasi, a visiting Brookhaven National Laboratory scientist, said the study was conducted between Jan. 1 and Dec. 31, 2009, involving 47 Long Island volunteers.

The participants were outfitted with a Samsung cell phone on each ear and not told which one was activated.

After a 50-minute muted message, Tomasi conducted positron emission tomography - PET - scans of the brain. Glucose metabolism increased an average of 7 percent, the scans revealed.

Volkow said the surge in brain activity was seen in the area of the brain directly behind the eyes.

The study should motivate parents to stop cell phone use among children and teenagers, said Dr. Devra Lee Davis, founder of the Environmental Health Trust, a nonprofit that has pushed for greater cell phone safety.

"By the time this generation of children is middle-aged, they will have been using cell phones for more than 40 years," she said.

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Brianna Olfano, 15, of Long Beach spends about an hour a night on her purple cell phone talking to a friend who recently moved to New Jersey. She often cradles the phone between her shoulder and ear.

"Sometimes I'll put her on speaker phone if I'm in the middle of doing something, but, yeah, usually it's up to my ear," she said.

Olfano wasn't alarmed by the new study, but her mother was.

Penny Olfano said she wants to know more about whether the phones pose any health risks to her daughter and her three other children.

"Fifty minutes isn't all that much during the course of a day. That's a couple of minutes per phone call," said Olfano, 39, a project manager for a construction company where she's constantly on a cell phone.

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With Candice Ferrette