Blasts are complex physical events. An initial shock wave is followed by a supersonic blast wind and a reverse and prolonged front of underpressure, all of which fade in rough proportion to the inverse square of distance. Tissues are damaged when the shock energy is dissipated at the interface between air and liquid that presents a change in acoustic impedance. The blast wind is the source of separate injury, throwing people against fixed objects and dispersing projectiles that penetrate the body.