We present a theory of pneumatic protection based onthe laws of thermodynamics, elasticity and fluid mechanics.A general pneumatic protection system is made up of severalcommunicating compartments, the differences in pressure ofthe compartments generating a transfer of mass andenergy between them. The transfer offers interesting possibilities toimprove the performance of the system. An example of this typeof protection in aerial sport is the airbag for para-gliders, it is usedin this paper to illustrate the theory. As the pressure in theairbag depends uniquely on its volume, the geometric modelin the theory can be simplified. Experiments carried out withcrash-test dummies equipped with sensors have confirmed thetheoretical predictions.