The air thing is odd, since there also appears to be wind blowing from the human air to the moon air, and fairly strongly, too. The Na'avi also look like creatures evolved in a low-pressure environment (large, sensitive ears, room for more than their fair share of lungs, etc.) and a small moon like Pandora wouldn't be able to hold onto much in the way of atmosphere........ yet tree goop burns (or are the trees somehow self-oxidising?!), and the choppers fly on relatively small rotors, and people's voices don't sound funny. CO2 concentrations were one of the things I hadn't considered, though...... I guess it might work....
I bothered to do a bit of research... and Cameron's 200+ page book goes rediculously indepth into this shizz... i only did the amazon look-in-side thing... but this is what i found about about the air from that book and alot of other credible sources.
1. All the gunships, AMPs (the mechs), buildings, and so forth are pressurized to .3 psi above that of the outside. This is to prevent, the outside air from leaking in (instead the inside air would leak out)... and in the case of a large breach... you would get your wind blowing from the human air to the moon air.
2. the gravity of pandora is .8 of earths... and the atmosphere is 1.2 that of earths. dont ask me to do the math on that to see if thats possible... cause its not happening.
3. All the military equipment is retooled versions from 21st century earth military equipment.... where due to the constant emp attacks everything had to be shielded from EMPs... (and UAVs had to be phased out) Pandora is so rediculously magnetic... that this EMP shielding is REQUIRED... by a lot. Also... it means that all these things worked at 1g and 1atm... which means they work even better at .8g and 1.2 atm.
4. The atmosphere has a higher level of oxygen than earths 21%... but not by much... but the C02 concentration is 18%... (some chemistry nerds could probably do the math and figure out how much more oxygen is needed to allow earth-like fire thanks to the increase in the products (CO2) concentration in those lovely balancing equations and such) and most the rest is Nitrogen, but it has some amount of Xenon, Ammonia, Methane, and Hydrogen Cyanide... but the page cuts off but i am sure most of these are trace only.
But seriously... for a sci-fi film they did a SHIZZTON of work on this... enough to stand up to the average forum nit-picker. (geezz... the Venture Star (the big starship)... they seriously did the math... and they designed that thing so completely, you plug in a 100 years of extrapolation in robotics and nano-technology... a room temperature superconductor (unobtanium... which... appearently the original script says that it was a "joke/slang" name that just kinda caught on), and those Antimatter/fusion engines (which dont look too far fetched if you read up on em) and your there.