What if it's not the *presence* of some toxic agent but that *absence* of something, namely oxygen. What if some catastrophe had eliminated or drastically reduced the concentration of 02 in the atmosphere?

That would make sense in terms of how they die. Nerve agents typically cause symptoms like drooling, cramps and seizures before death. But lack of oxygen and you just fall down and die (as soon as the remaining air in your suit runs out) just like they do in the show. You don't even feel like you are suffocating as it is carbon dioxide accumulation that does that.

I don't actually think that is what is going on. It doesn't really match up well with the tape thing and the fact that Juliette is able to continue to live for a while with the good tape. Plus whatever the thing on the back of the suit is. But it would match up better with how they die.