There are more than 5 billion cell phones in the world, and many of us spend our days glued to our devices, bathed in their fields of electromagnetic radiation. So you’d think it would be important to have absolute scientific certainty about whether cell phone use can contribute to the risk of brain cancer. Despite years of studies, however — including a World Health Organization review in May — the answer is still unclear. The bulk of the research has shown no clear method for cell phones’ weak radiation to lead to tumors, but there is enough outlying, epidemiological evidence that some experts recommend using a wired headset, which can greatly reduce radiation exposure. For 5 billion people, safe is better than sorry.