When Mathieus' air hose is burnt through, they don't consider trying to fix it (considering that they are in a science lab) or even breathing from the hose. They also don't think to take the bottle so they have a spare. And, they don't consider if the suits in the lab (several of which are available) might fit them, particularly Anna, who is not much larger than Sarah.
The goof item below may give away important plot points.
basically everybody who inhales the mist dies, including birds and dogs. However, we see at least two dogs that are alive and kicking, and no explanation is provided at all. We can surmise that they, like the kids, are immune, but it's a stretch, especially given that one of the two dogs is not a puppy.