Can the Na'vi breathe an Earthlike atmosphere?

[Clarke]

Originally Posted by Human No More

Wrong.
Pressure is not necessarily higher, you're thinking of density. Vehicles and buildings are pressurised above ambient so in case of minor damage, atmosphere leaks out and not in. Pandora's atmospheric pressure is actually 0.9atm, so presumably the vehicles and buildings are ~1.0 or more.

H2S is not toxic on skin contact except as a liquid. He would have had some eye irritation, as well as to mucous membranes of nose/throat, but it is perfectly possible not to actually inhale it.

I was under the impression that H2S was a vapour, when it is actually a gas, just, at STP. That does save Quaritch some pain, but the fact that vehicles might be pressurized doesn't, since he walks straight out of the building into the open air at one point.

However, I think you are downplaying how much damage it'll do; the WW1 era British Army commented that H2S was "not an ideal war gas," at 1/10th the concentration it's apparently has in the atmosphere. Even small amounts of it in his eyes or windpipe would cause severe damage.