A study of people who used cell phones for an average of less than three years found no evidence the devices cause brain cancer. But the research didn’t say if longer-term use of cell phones is dangerous. The study, funded by the industry group Wireless Technology Research and the National Cancer Institute, appears in the Dec. 20 issue of Journal of the American Medical Association. The study of 891 people found a slight increase for a rare type of brain cancer, but the researchers said it wasn’t statistically significant. While researchers acknowledged the need for longer-term studies, they said that overall results should reassure the more than 86 million cell phone users nationwide. Unlike regular phones, hand-held cell phones contain an antenna inside the receiver, which puts the user’s brain close to the electromagnetic radio waves emitted by the antenna. Since cell phones were introduced in 1984, conflicting data have emerged from safety studies on animals and humans.