Chapter 16 Cabin Environmental  Control Systems Physiology of Flight Composition of the Atmosphere The mixture of gases that make up the earth’s atmosphere is  commonly called air. It is composed principally of 78 percent  nitrogen and 21 percent oxygen. The remaining 1 percent  is made up of various gases in smaller quantities. Some of  these are important to human life, such as carbon dioxide,  water vapor, and ozone.  Figure 16-1  indicates the respective  percentage of the quantity of each gas in its relation to the  total mixture. As altitude increases, the total quantity of all the atmospheric  gases reduces rapidly. However, the relative proportions of  nitrogen and oxygen remain unchanged up to about 50 miles  above the surface of the earth. The percentage of carbon  dioxide is also fairly stable. The amounts of water vapor  and ozone vary.