There is an industrial chlorine plant next door to the Reserve Center I work at. We have all kinds of alarms and detectors, and they've had two spills in the 20+ years I've worked here. One spill was mostly alarms, and a bleachy smell when we got back from the evacuation. The second closed the entire block for 48 hours, sent three of their employees to the hospital, and had all of us coughing as we evaced. They had spilled about 2 cups of liquid chlorine that had become gas.
It is NASTY stuff.