Why were the humans unable to breathe the air in Pandora?

Quoting from Pandora wiki:

Pandora's Atmosphere components:

Pandora's atmosphere is a mixture of nitrogen, oxygen, carbon dioxide (>18%), xenon (>5.5%), methane, and hydrogen sulfide (>1%) and is about 20% denser than the atmosphere on Earth primarily due to the high percentage of Xenon; a heavy, colorless, odorless, and generally unreactive noble gas.

Why can't human breathe in Pandora:

The high concentration of carbon dioxide in the Pandoran atmosphere makes it extremely poisonous to humans, rendering them unconscious in about 20 seconds and causing death in about 4 minutes. The hydrogen sulfide present is also quite poisonous; concentrations over 1000 ppm (0.1 %) can cause immediate collapse with subsequent loss of breathing, even after inhalation of a single breath.

So Oxygen is present there and that's why combustion is possible.

I concur... In high Carbon Dioxide situations, a human will hyperventilate as the brain forces the body to expel the excess and rebalance it's oxygen. If the imbalance persists, loss of consciousness and death may follow. In the film, this was portrayed very accurately when the hero was exposed to the Pandoran atmosphere. There is Oxygen present, (as was mentioned- combustion is possible) and thus death was not immediate, and recovery was rapid once Carbon dioxide was reduced.

Both answers are correct. There is oxygen so combustion is possible, but because there are so many other elements in the air on top of oxygen, it would make it impossible for human to breathe that air and survive. The human body would hyperventilate as it tries to reduce the amount of Carbon Dioxide and get more oxygen in but this put the body under stress because it is also trying to keep you conscious. The human body is designed to try to keep you alive with out stressing the body out to much, keeping you conscious and trying to expel Carbon and increase the oxygen consumption would just over stress the body, therefore, it would try to relieve some of the stress by stopping certain unnecessary or currently less vital functions so it would make the human lose consciousness for a period of time until it has the oxygen levels back under control. Having this stress on the body would make you lose consciousness within 20 seconds but you would continue to live for the next few minutes because your body is still trying to make a feeble attempt to regain control, which of course will not happen with insubstantial amounts of oxygen in the air, so, after a few minutes your body slowly starts to shut down until you die.

In conclusion, there is oxygen in the Pandora atmosphere so combustion and limited breathing is possible but there isn't enough oxygen for the human body to survive for any longer than a few minutes, in which you would be unconscious. But the Na'Vi people have bodies that are designed to breathe the air with all of those different chemicals in it. Mixing the Human DNA with the Na'Vi DNA would mean that the body would be primarily Na'Vi design but the mind would be by human design. It would be a happy medium between the two species so the Avatar is able to survive just as perfectly as the Na'Vi.