If you use some kind of vented bung (provided you keep your water in a drum like me) and the water has chlorine in it, there would be no buildup of chlorine gas. Unless you put chlorination in your water yourself, municipalities use very little when chlorinating water for use by residents of that city. Therefore, you should have no fear of chlorine gas buildup. I work with it everyday, not to mention other chemicals. Chlorine is absolutely hazardous if mixed with say Hydrochloric acid(muriatic)which will definitely emit chlorine gas. In water I would say that you have little to worry about really.