Abstract: Employee #1 was working below a condenser in a chlorination plant. He was wearing a canister respirator because of the possibility of chlorine or carbon dioxide in the area. As he tried to retrieve a light from inside the condenser, Employee #1 passed out. He was removed from the platform and administered oxygen. He revived, but continued to receive oxygen until he arrived at the hospital. Blood samples showed that Employee #1 had elevated levels of carbon dioxide, but the levels of other gases were normal. Sampling done in the work area did not reveal any concentration of gases that would cause loss of consciousness. The most probable cause for Employee #1 to lose consciousness was oxygen deficiency. This suspicion could not be verified with the blood gas sample, as he had been on 100 percent oxygen for at least 40 minutes before his blood was taken. The unit on which Employee #1 had been working had been out of service for a few days and had been under a nitrogen purge.