Radio waves are not ionizing radiation, so they aren't directly cancerous in the same way as say, x-rays or UV light (UV light is about the threshold for this), but they are still absorbed by water in tissue, which heats it up slightly. There is a limit to how much this is safe, and so radio safety standards impose limits on the average power you can use to transmit in most consumer devices. The standards are stricter for something intended to be carried in a pocket or worn on the body.