I haven’t seen Avatar yet, so I don’t know if it’s spelled out why the atmosphere is unbreathable to humans. The simplest and easiest reason I can think of is if the atmosphere had too much CO2. 10% CO2 would leave humans hyperventilating until they died.
However I don’t know if this would be plausible or not. A high CO2 atmosphere would strongly effect the geochemistry of the planet. In particular, it would mean the very rain would be soda water. Not a strong acid but an acid to be sure. Maybe life there could have evolved to tolerate an acidic environment, but it’s a stretch. More importantly, in a geologically short period of time either most of the CO2 would be scrubbed from the atmosphere or else the crust of the planet would consist almost entirely of carbonate minerals like limestane. So does the high-CO2 idea fail?
The presence of Carbon monoxide in much smaller concentrations would make the atmosphere toxic for humans. I’m not sure how stable carbon monoxide is though in the presence of oxygen. CO is about 0.1 ppm in the atmosphere. Car exhaust is about 7000 ppm (or 0.7%) and that is certainly lethal.
According to Avatar: A Confidential Report on the Biological and Social History of Pandora, which I have just been reading, Panadora’s atmosphere includes nitrogen, oxygen at a similar partial pressure to Earth, 18% CO2, 5.5% argon, and a small amount of hydrogen sulphide. The atmosphere is denser than Earth’s (due partly to the argon) but air pressure at sea level is 10% less than Earth’s due to lower gravity. The high CO2 and the hydrogen sulphide make the air unbreathable without a breathing mask.
There are anaerobic life forms on Earth that prefer high levels of CO2 and low levels of O2, so there’s no hurdle to life itself.
As for geochemistry… you’d certainly have some differences, but I don’t see any reason why it would prevent life. Many of the rocky planets and moons in our solar system have high levels of CO2 in their atmospheres without it being scrubbed out. We also haven’t give up the search for life on many of them (including Mars, of course).
Many plants prefer acidic soil, and would probably thrive in a high CO2 atmosphere with slightly acidic rain. Note that many (all?) plants require O2 for seed germination, so a pure CO2 atmosphere wouldn’t work.
Humans and other animals that have adapted to our current atmosphere wouldn’t do well with 10% CO2, but presumably animals better suited to that atmosphere would evolve.
I haven’t seen the movie, but my understanding is that the planet is covered with giant-ass trees and other plants. Plants, of course, absorb CO2 and release oxygen, which is why our atmosphere is the way it is. So you’d think the CO2 levels would be dropping at some rate. Perhaps. I just ran out of sufficient interest to continue typing.
Pandora has a high level of volcanic activity, which could maintain the high levels of CO2. The volcanoes are also the source of the hydrogen sulphide.
Early in Earth’s history, CO2 levels may have been as high as 10%.