Abstract: Employee #1, a water treatment worker, was exposed to chlorine gas, which is used to treat the water, when he opened one gas cylinder but did not close the valve on the yoke to prevent the gas from coming out of the disconnected line. Employee #1 was working by himself at a remote location in a small shed-like cabin. Employee #1 works in this facility for three days for about five hours per day. The facility takes sewage water from Camp Miller, a County juvenile detention center and purifies it. Employee #1 was in the process of changing a chlorine gas cylinder. There are two tanks mounted on a weight scale. Employee #1 knows what a full tank weighs and what an empty tank weighs. One of the tanks was almost empty by weight, so Employee #1 shut off the valve of the nearly empty cylinder and opened the valve of the full tank. Each tank has a very small 0.25-inch diameter pvc gas line that goes into a small manifold where both lines meet. There is no bypass valve on the manifold to prevent backflow. Employee #1 then started unscrewing a spring tension valve that holds the yoke to the gas cylinder. The yoke has two spring tension valves; the other side has a valve that would shut an orifice that would prevent the backflow of gas once the yoke is completely disconnected from the gas cylinder. Employee #1 did not screw in the valve which would close the orifice. There were no immediate supervisors around at the time of the accident. The injuries sustained were minimal, a slight irritation of the throat. The factor that caused the accident was a lack of proper training in the changeover process. While Employee #1 was trained in the changeover, no one told him to shut of the valve to the orifice before removing the yoke. Employee #1 was taken to Los Robles Hospital in Thousand Oaks, California, where he was hospitalized and kept overnight for observation.