Abstract: At approximately 11:00 a.m. on December 23, 1991, at the Sierra Conservation Center, a medium security prison, Employee #1, an experienced water/sewage plant supervisor, preparing to change a 1 ton chlorine cylinder, disconnected a vacuum line from the nearly empty cylinder. The cylinder valve was in the OFF position and the vacuum had been allowed to purge the regulator for several minutes. Because an electric regulator heater had never been installed, according to an in-house investigation, entrained water vapor apparently froze in the drip trap, isolating recondensed chlorine. When the regulator was removed from the tank, the chlorine flashed to vapor and caused a chemical burn to Employee #1's right wrist. Employee #1 also suffered a delayed reaction to chlorine inhalation. He was hospitalized for 24 hours for observation and treatment. A regulator heater was installed immediately. Note that December 23 was an extremely cold day. No chlorine release had ever been experienced at this location during cylinder changes before this accident. An approved SCBA had been moved up to within 50 ft of the cylinder before the cylinder change.