Herberman and 22 other prominent doctors, medical researchers and health officials warned in a statement, "The most recent studies, which include subjects with a history of cell phone usage during the last 10 years, show a possible association between certain benign tumors ... and some brain cancers on the side the device is used."

Cell phones operate on radio frequencies and release low levels of electromagnetic radiation. There's a theory that long-term exposure to this radiation could increase the risk of tumors, possibly by altering DNA or heating brain cells. However, the American Cancer Society says, "Cell phones wouldn't be expected to cause cancer because they don't emit ionizing radiation," which is "the type that damages DNA and is known to have the ability to cause cancer."

Studies are mixed. Some research studies have found an association between cell phone use and risk of brain tumors, but other studies have found no connection. At the University of Utah, a 2008 review of studies "found no overall increased risk of brain tumors among cellular phone users." However, long-term studies have not been completed. The Utah review includes some of the studies cited by Herberman and his colleagues, but they also referred to new findings, not yet published, from a 13-nation project called Interphone.

The U.S. Food and Drug Administration (FDA) has this to say: "The available scientific evidence does not show that any health problems are associated with using wireless phones. There is no proof, however, that wireless phones are absolutely safe."