Bluetooth devices transmit signals via a short-range radio frequency. That frequency operates at a much lower power level than your phone, so the threat from radiation is insignificant. Also, like cell phones, Bluetooth headsets disperse their signals in random directions, rather than in a straight beam. If you're really worried about radiation, keep in mind that when using a headset, your phone will probably be resting away from your body. Since your handset will have a much higher SAR rating than the headset, you'll probably feel better with this arrangement.