A recent small study in people has shown that cell phones may also have some other effects on the brain, although it’s not clear if they’re harmful. The study found that when people had an active cell phone held up to their ear for 50 minutes, brain tissues on the same side of the head as the phone used more glucose than did tissues on the other side of the brain. Glucose is a sugar that normally serves as the brain’s fuel. Glucose use goes up in certain parts of the brain when it is in use, such as when we are thinking, speaking, or moving. The possible health effect, if any, from the increase in glucose use from cell phone energy is unknown.
However, there have been some studies that have shown that rats can develop a specific type of brain tumor, called a schwannoma, if they're subjected to prolonged radiofrequency radiation. These studies examined thousands of rats and mice, and exposed them to a variety of radiations — everything from "near-field" (which is what you get holding a phone to your ear) to "far-field" (which is what you get walking through everyone's Wi-Fi signals at Starbucks).
Still, despite the odds, these fears could be around for a while — because it’s hard to prove that cellphone radiation doesn’t cause harm. There are just too many combinations of genes, environmental exposures, patterns of cellphone use, plus a healthy helping of random chance to consider. It’s why we’re still having the conversation about whether coffee, for example, is good or bad for us. So while the bulk of evidence points to no health effects from cellphone radiation, the scientific literature is still somewhat mixed, Foster says. “Someone who wants to worry can pick and choose and find a lot of evidence that would support their theories.”

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Scientists have reported adverse health effects of using mobile phones including changes in brain activity, reaction times, and sleep patterns. More studies are underway to try to confirm these findings. When mobile phones are used very close to some medical devices (including pacemakers, implantable defibrillators, and certain hearing aids) there is the possibility of causing interference with their operation. There is also the potential of interference between mobile phones signals and aircraft electronics. Some countries have licensed mobile phone use on aircraft during flight using systems that control the phone output power.

Just why Schwann and glial cells appear to be targets of cell phone radiation is not clear. David Carpenter, a physician who directs the Institute for Health and the Environment at the University at Albany, S.U.N.Y., explained the purpose of these cells is to insulate nerve fibers throughout the body. These are electrical systems, so that may be some sort of factor, he wrote in an e-mail. “But this is only speculation.”

In one type of study, called a case–control study, cell phone use is compared between people with these types of tumors and people without them. In another type of study, called a cohort study, a large group of people who do not have cancer at study entry is followed over time and the rate of these tumors in people who did and didn’t use cell phones is compared. Cancer incidence data can also be analyzed over time to see if the rates of brain tumors changed in large populations during the time that cell phone use increased dramatically. These studies have not shown clear evidence of a relationship between cell phone use and cancer. However, researchers have reported some statistically significant associations for certain subgroups of people.