text: Actually, over a long term, we don't know.  Over a short term, the EM waves obviously don't mess with our current mental processes.  But I have seen studies that showed people who use cell phones next to their heads are slightly more likely to get brain tumors, and men who carry cell phones in their pockets are more likely to get testicular cancer, but I've also seen studies that show there are no long term effects.  Since the widespread use of this technology is relatively new, no one can say definitively "yes" or "no" on whether they are harmful long-term.