why is there 80% nitrogen in earth's atmosphere? jw
I don't claim to be an expert on the subject, but we can use some basic science to determine what is probable.I suspect they could breathe our air just fine. Probably the biggest issue they would face is our atmosphere is less dense than Pandora's atmosphere. It would be like someone from sea level going up several thousand feet or thereabouts.
The Na'vi have evolved to withstand 18% carbon dioxide in Pandora's atmosphere. If for humans having too much carbon dioxide in the air they breathe causes Hypercapnia (also called CO2 retention) which interferes with the blood's ability to absorb oxygen, it would be a logical conclusion to make that without that much CO2 in the air for which they have evolved, a Na'vi breathing Earth's atmosphere would develop Hyperoxia (breathing too much oxygen). This would lead to Oxygen toxicity that leads to cell damage, etc. If exertion is limited, and breathing is controlled and purposefully minimized I think Na'vi and Avatars could breathe Earth's atmosphere for a longer time than humans can breathe Pandora's atmosphere.
Of course, there is also the Hydrogen Sulfide to take into account that is toxic to humans and prevents cellular respiration, thus stopping the metabolism of cells. Since Na'vi have resistance to at least a small amount of Hydrogen Sulfide, not breathing it could cause increased cellular respiration and increased metabolism. If taken to the logical extreme, lack of Hydrogen Sulfide could cause them to age faster.