## 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. 

