A helmet contains impact attenuating fluid (e.g., CO2, Air or Water) containing cells, sensors and electrically actuated exhaust valves for cushioning impact and decelerating a wearer's head after an impact. Accelerometers and a pressure sensor supply signals to a microcontroller, which opens an exhaust valve if accelerations (or pressure) exceed a threshold. Expelling fluid provides a cushioning and damping effect, decelerating the wearer's head. Acceleration data, fluid pressure data and GPS position data may be wirelessly communicated to a monitor computer system.