[gjhan]

As you point out, the body can tolerate significant variation in absolute pressure, corresponding to the temperature, composition, and density of the gas you breathe.

The pressure that must be narrowly controlled in a ventilator application is the relative pressure: the difference between the absolute pressure outside the lung (in the room) and the absolute pressure inside the lung. This relative pressure is what does all the work --- mechanically opening airways and alveoli --- and could cause all the injury.