A bit late for me to come into this discussion, but I always thought that the alien that he sees in the field, and the one they chase around the house, is just a scout, so would not attack. They check out the area and report back on things like numbers, resistance, layout, possible attack points or defensive positions, that kind of thing. They don't attack as they need to get their butts back to base so they can pass on that information.

And yeah, with the gas question I always thought it was poison and that his asthma saved his life as he did not manage to breath any of it in (have a listen to the dialog of Graham when they are outside hoping he will wake up).