This made me laugh. Although, a biological toxin does seem interesting to me.
This makes sense to me. It sounded to me like they were both coughing at first but that
Saul ended up growing accustomed to the air quality. If that's the case, I don't see why he couldn't go back in. However, it might end up being bad for him over long periods of time so this would be a gamble.
Personally, I expected when they pulled the camera up out from the crack for there to be something insane / creepy / gross down there.