The rig was drilling with foam and compressed air was being injected at the standpipe manifold at approximately 800psi. After drilling the Kelly down, the driller contacted the third party air drilling vendor to initiate the connection process. The third party service hand diverted the air to the blow-down line but did not close the supply valve to the standpipe manifold. The driller pumped the required fluid volume to displace the pipe with fluid to the BHA check valve. The lower Kelly valve (LKV) was closed and the connection broken off in the rotary table. The Kelly was made-up to the mouse hole joint and the rig crewmember opened the LKV. Compressed air trapped inside the Kelly blew down through the mouse hole joint and came out through the top of the mouse hole. The crewmember was startled by the incident and slipped while moving away from the mouse hole, injuring his shoulder and arm.