Things you notice on repeat viewings...

[Patrice Maire]

I noticed Neytiri pausing the first time. Understandable in her experience. That's the same one that nearly ate them!
She paused because instinct says, 'This is a bad idea!' but when Jake hops on and takes hold, she's all in.

[Patrice Maire]

Okay... something else that just struck me. In another thread someone asked if Na'vi can breath human air and someone replied 'probably' (likely as a response to seeing Neytiri in the ruined link-mod)
But... when Jake enters his Na'vi body for the first time, all the med-techs are in full scrub gear and rebreathers... which means that it is pressurized and filled with Pandoran air.
Now yes, they have perfectly brand spankin' new Na'vi bodies right there but if the Na'vi could breath human air, they wouldn't need the exo-packs.

So, given that little details, Na'vi cannot breath human air due to the lack of gases as well as the high content of oxygen. Or whatever the reason.

[Human No More]

The oxygen level is essentially the same. There is no biological use for hydrogen sulphide or xenon.

The room the avatars were in has a Pandoran atmosphere because the airlock was open - after all, the avatars were going to be going outside, probably still with the doctors making sure they were OK at first.

Also, I never said that the reason Na'vi could breathe an Earth atmosphere was because of that scene, it is simply based on the composition of Pandora's atmosphere and the fact that only oxygen is actually absorbed for respiration.