The Na'vi and our oxygen

[Human No More]

That is actually an interesting point. The difference would be manageable, though may be a problem if a Na'vi was in a human atmosphere for the long term (although then, they would likely just need to be exposed to a higher level of CO2 for maybe a few hours per day to maintain levels)

Woodsprite, RE pressures: Pandora's air pressure is 0.9 atmospheres. If we assume that RDA vehicles and buildings are therefore pressurised to 1 (makes sense in terms of comfort for humans while being above the ambient air pressure) then that is within the range for a Na'vi to survive with no discomfort or ill effects, humans can survive and even live in significantly further from average air pressures.

I agree on pollution though... a Na'vi may find it difficult to breathe while actually on Earth (as opposed to just an Earthlike gas mixture) without some kind of filter system because it would be their first encounter with pollution. While earth today would be likely tolerable (if unpleasant), 22nd century Earth is so badly polluted that exopacks are starting to be used there, so Na'vi would certainly need some kind of filter system for the air there.