Yeah the room they are in seems rather large, and the amount of poisonous gas put into one of the balloons would not definitely not be concentrated enough to kill everyone in the total space of the room. Example: a fart is twice as bad in a closet vs. your bedroom. Also, this is not considering the fact that the poisonous gas is more dense than air, and would therefore have to be mixed with quite a bit of helium to make the balloon float like the others. Hence, the total volume of gas would be even less. Proof: Gas laws, law of entropy, and gay-Lussac's law.

Unless it's chlorine gas. All it takes is a little.

dude chlorine gas Cl2 weighs approximately 71g/mole. Oxygen gas, or O2 weighs approximately 32g/mole. however, air is actually 70% nitrogen which in gas form is N2 and weighs 28g/mole. since air is a combination of both and a couple other gasses, a mole of "air" will end up significantly weighing less than Cl2. Now go sit in a corner and think about what you just said.

Ok, how about Zyklon B? Is that okay with you Dave? Tell me Dave, is that okay with you?

I would look into it but I've taught quite enough chemistry for tonight.

wait... what the fuck was i replying to? anyway... the point is that any toxic gas would become too diluted to have any serious effects on the people in the room.