Medical researchers pointed out long time ago that mobile phones might cause cancer. But because of the lack of strong evidence, such warnings often ended up with nothing. Now Dr. RonaldB.Herberman, the director of Cancer Institute of University of Pittsburgh in the United States issued this warning again. He warned people not to let children use mobile phones unless in an emergency, because they are most likely to be affected by electromagnetic field. He also urged adults to keep mobile phones away from their heads and use external speakers or wireless earphones. In addition, his suggestions also included not to carry a mobile phone around all the time,