See My Post 10.
Chlorine is indeed a very nasty gas. It forms hydrochloric acid when it comes into contact with water, such as inside your lungs. There was a railroad accident in the Northwest around 1980 which released a carload of chlorine. A young brakewoman died from chlorine inhalation, though not for months after the event. Terrible way to go.
Chlorine and mustard gas are not the same thing, however, even though both were used as weapons in WWI.