[alexander_busek]

Carbon monoxide would work as well, for the same reason. And a lot less toxic to the outside.

Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air.

Regards,

[steve86]

CO is so close in density to air that it is considered neutral with respect to buoyancy in most situations. Little differences in temperature are more significant. And with respect to cyanide, propane, etc., I think using any toxic gas is anathema to the Jewish people for obvious historical reasons.