Abstract: An employee was working in a water supply penstock, using a gasoline-driven water pump. The pipe was 1.8 meters in diameter, with an access opening that was 0.6 meters in diameter. Equipped with a self-contained breathing apparatus tank and regulator for use as a respirator, he was working about 15 meters from the access opening. He was doing heavy work, breathing normally (through his nose). After he had been in the penstock for about 10 minutes, he became dizzy and started toward the access opening. He passed out just as he made it to the opening. The attendant outside the confined space pulled him out, and he revived in the fresh air. He was taken to the hospital, where he was checked for carbon monoxide in the blood. His blood carbon monoxide level was found to be very high.