I am not sure how you figure this, considering we cannot breath on the summit of Mt. Everest, which has a higher atmospheric pressure than anywhere on Mar's surface, including the lowest observable impact craters. Mars average - 0.6 kilo-pascals (0.087 psi) Hellas Planitia bottom - 1.16 kilo-pascals (0.168 psi) Lowest altitude on Mars. Armstrong limit - 6.25 kilo-pascals (0.906 psi) Point at which water boils at the normal temperature of the human body: 37 °C (98.6 °F) Mount Everest summit - 33.7 kilo-pascals (4.89 psi) Also to put this in even more perspective CO2 is a heavier gas than O2. So the only way we can ever live on Mars will be within closed environments (Biospheres), Unless we find a way to generate an Magnetosphere around Mars (Or some other method) to protect the atmosphere from being torn away by the Sun's radiation.