I know that Chlorine Gas is deadly. After we put in a watermain system, and before the copper lines got ran to the houses, we would fill them with water, close the valves and have someone come out and put chlorine gas in the mains. Then two days later, blow it out the hydrants and whips, and take chemical tests for bacteria and such. But anyhow, the truck had the skull and crossbones placard on it. And it was important that the feed valves did not get opened up and release the gas into the existing water main.
With that said, is it possible that enough of a concentration of chlorine could effect someone the same as the gas? I am sure that the gas is probably 100% though so it is a guess.