Abstract: On September 11, 1990, Employee #1, an ironworker for Wickman Steel Company, was using a portable Emglo compressor pneumatic hand tool, model KU, serial #A315799. He was chalking and securing seismic floor bolts at the northeast corner of the second floor of a four-story building under construction. Employee #1 had filled 4 or 5 bolt holes when the rear air chamber suddenly blew off the end of the tool, striking him on the right side of his chest, his clavicle, neck, and face. The pneumatic hand tool, which was rated to a maximum air pressure of 80 psi, was new to the job site and the employees were unfamiliar with its operation. Prior to the accident, the foreman attempted to set the air pressure on the new tool to the 80 psi maximum, but the compressor did not have an air regulator valve. He attempted to regulate the pressure by adjusting a motor control valve and a tank pressure gauge. The foreman stated that when he checked the compressor after the accident, the pressure gauge was reading 160 psi. He released the air, but the gauge still read 120 to 140 psi. When a coworker tapped the gauge it went to zero. The pneumatic hand tool was getting direct tank pressure. The foreman's attempt to set it to 80 psi without a regulator valve connection, and a possibly faulty pressure gauge on the motor control valve, were the main causal factors in this accident.