As altitude increases, what happens to the relative percentage of oxygen in the atmosphere?

As altitude increases, the relative percentage of oxygen in the atmosphere remains constant at approximately 21% of the total air composition, even though the absolute partial pressure of oxygen decreases due to the overall reduction in atmospheric pressure. This means that while the density of air decreases with altitude, the proportion of oxygen compared to other gases does not change.

At sea level, the atmosphere is composed of about 78% nitrogen and 21% oxygen, with trace amounts of other gases. As altitude increases, the total atmospheric pressure drops, leading to a decrease in the number of oxygen molecules available per breath. However, the ratio of oxygen to other gases stays the same. This distinction is crucial for understanding oxygen availability at high altitudes and is why acclimatization or supplemental oxygen may be necessary for individuals flying or climbing at high elevations.