How important is your mobile phone, better known to Americans as a cellphone? Jack said, "I use my phone all the time. I'm always on my phone." Ann said, "It's important to me because it's like my life saver." Jenny said, "I need my cellphone. I cannot live without it." So far, no studies have proven beyond question that the radio signals from cellphones cause brain cancer or other health problems. But a new study by government scientists in the United States has some people wondering what to think. The scientists have found that holding a cellphone to your ear for more than fifty minutes increases brain cell activity. Even the scientists themselves are not sure about the meaning of their findings. Dr Nora Volkow led the study. She heads the National Institute on Drug Abuse, part of the National Institutes of Health. Dr Volkow says she is not concerned that fifty minutes of cellphone exposure   would harm anyone. But she says the research does show the need to study whether there are long-lasting effects of repeated exposure over several years. Her team studied forty-seven healthy volunteers between January and December of 2009. The volunteers had cellphones placed against both ears while the scientists made images of their brain activity. As part of the test, one phone was activated  for fifty minutes. The other phone was off. After that, the volunteers were tested with both phones turned off. Dr Volkow says the brain scans  showed increased activity in brain cells closest to the activated phone. The scans showed how the brain cells used sugar to produce energy, a normal activity. The activity was seven percent higher in areas of the brain closest to the cellphone antenna . Dr Giuseppe Esposito is a nuclear medicine expert at Georgetown University Medical Center in Washington. He says after many years of studies, better kinds of research are still needed to settle questions about cellphone safety. These studies would take years. The results of the study tell us that  _  .
people use cellphones for too long every day using cellphones has an effect on brain cells cellphones that are powered off can also affect the brain increased activity in brain cells has many causes