Phosgene is very dangerous. It reacts with the moisture in the airways, turns to (amongst other things) hydrochloric acid, which then disolves the linings of the airways and the lungs.
It takes a lot longer to act, however, than the show suggests.
Also, because it's so dangerous, it's almost always made and then consumed within the *same* chemical plant, almost *never* transported any distance. (But if it were to be transported, the NFPA warning diamond on the truck would be exactly as shown!)