As altitude increases in Earth's atmosphere, air pressure decreases. This is because there is less air and therefore less molecules pressing down on an area. As air pressure decreases, the amount of oxygen also decreases. This is because oxygen molecules make up about 21% of Earth's atmosphere, so as the overall air pressure decreases, the concentration of oxygen also decreases. This is why at higher altitudes, such as in mountains or when flying in an airplane, it can be more difficult to breathe and people may experience symptoms of altitude sickness due to the lower oxygen levels.
