After reports from Germany and Sweden last winter concluded regular cell-phone use over many years increases the risk of certain types of brain cancers, the first reaction anyone might have was to wait – surely, the next peer-reviewed, highly respected study would conclude just the opposite. But subsequent reports seem merely to strengthen the idea that cell-phone users should take precautions. Clearly, cell-phone users are not prepared to judge the relative safety of their phones and their phoning habits, leaving the federal government with an obligation and an opportunity to step in and offer guidance to the more than 100 million cell-phone users nationally.

The debate sounds like one of those e-mail blasts you wish your spam filter had picked up: What you should know about cell phones and brain tumors. But for more than 15 years, researchers have dissected the question of whether and under what conditions radiation from cell phones increases the risk of cancer. Most recently, the German Federal Office of Radiation Protection and the Swedish Radiation Protection Authority warned that prolonged cell-phone use could lead to higher rates of brain cancer.

According to the Swedish study, spending at least an hour a day using a cell phone over many years raises the chance of suffering from a brain tumor by 240 percent, compared with someone who never uses a cell phone. The tumors were more likely to appear on the side of the head where users placed their phones.

The simple answer, conclude researchers, is to use a hands-free headset when making cell-phone calls, though it isn’t clear on what part of the body a caller should wear a phone and not be affected by radiation. The studies were noteworthy because they provided very different conclusions from earlier work that found no link between phone use and cancer. For instance, a four-year British study concluded in 2005 that cell-phone use didn’t lead to glioma, a common type of brain tumor.

The issue is further confounded by questions over cell-phone radiation frequencies, age of the phone and distance of users from cell-phone towers, among other factors. Despite these limitations, the ubiquity of cell-phone users – try to walk through a mall or an airport and not see someone using a cell phone – adds urgency to this question. The public shouldn’t expect a definitive answer on the risk of cell phones, but it deserves a thorough federal review.