The static pressure test determines the maximum pressure inside the facepiece without any air flow.  The NIOSH requirement is 1.5 inches of water pressure maximum.  The exhalation resistance test determines the force necessary to exhale at a flow rate of 85 lpm.  This requirement is that the exhalation pressure must not exceed the static pressure by over 2 inches of water.  The maximum allowable facepiece pressure must not exceed 3.5 inches of water.