Oura is a BT Smart class 2 device which has adjustable output power; 2.5 mW at maximum, but we use lower power levels with the current design, typically 1 mW. For comparison, cell phones output power is typically 1000-2000 mW peak, approximately 120-250 mW during call.
BT Smart is not continuously transmitting, but only for short periods according to selected advertising intervals (well below 1% of the time) and more continuously during data syncing to app or firmware update for a few minutes altogether/day. During periods with no movements, especially when sleeping, also advertising is turned off. Additional flight mode to turn BT completely off is also being planned.