Re: Call the hospitals just before.
Call all the hospitals, prepare the doctors, inform them about the gas used, suggest the antidotes, remind the first ones on the spot about the first things to do (and not to do) with the victims.
There regularly are dress rehearsals, here, faking major nuclear contaminations, airplane accidents, fire in major constructions, etc...It involves many professions and it is shown on TV.
Raoul