Also explains the open door - Gene trying to get air, went back to save others and died. And the pills - alls you know is you're light headed and confused so you panic and look for medicine, pass out opening it. And the dogs - one near them died, the air cleared up from the open door before dogs in other areas were affected.

Regardless, get a detector if you don't have one because by the time you feel the effects you are likely too confused to even make the right decisions. You may go back to rescue your toothbrush or turn the water off or something nonsensical like that.