Why is it difficult to breathe at higher altitudes?

Solution

Low air pressure is usually the most significant limiting factor in high mountain regions. The percentage of oxygen in the air at 3.2 km is essentially the same as at sea level (21%). However, the air pressure is 30% lower at the higher altitude due to the fact that the atmosphere is less dense—that is the air molecules are farther apart. At high altitudes, the lower air pressure makes it more difficult for oxygen to enter our vascular systems.