The Canadian Military uses the same reasoning, or so I've been told. Do you guys actually get equipped with a gas mask? I can't imagine, given the world's capabilities, that a gas mask is enough anyway, seeing as many chemicals available for warfare can be just as damaging by having contact to skin, not even having to bother with inhalation. I'm thinking its just an outdated policy, but that's just an assumption.
The military explanation is chemical warfare.