In addition to seated or unseated and the peak applied pressure, it is also necessary to define the blast duration (i.e. how long does the peak pressure take to decay back to ambient pressure) measured in milliseconds (ms) or the blast impulse (i.e. the measure of blast energy represented by the area under the peak pressure versus duration triangle of the positive phase of the blast) measured in pounds per square inch per millisecond (psi/ms). Additionally, the mitigation or performance level required after the blast must be defined. In both the UFC-4-010-01 and the ASTM F2927 documents there are four levels of protection offered.