Our Atmosphere

• Our atmosphere is a blend of nitrogen, oxygen, carbon dioxide and other gases and substances including water vapor. Nitrogen comprises 80 percent with oxygen filling out most of the rest at almost 20 percent. The remaining fractions of a percent are where carbon dioxide and other gases come in. There are five layers of the atmosphere They are, in order starting at the ground, the troposphere, the stratosphere, the mesosphere, the thermosphere and the exosphere. The atmosphere thins out the farther up you go. Most of the time, we stay in the troposphere which extends up to 7 miles from the surface.
  
  

Oxygen in Air

• The 20 percent of our atmosphere that is oxygen is the most important for our lives. Our bodies have evolved in an oxygen-rich environment and metabolize oxygen quickly as we breathe it in. This oxygen is then sent via our bloodstream to every corner of our body where it is used as part of our body's many life-sustaining processes.
  
  Oxygen is also the gas given to patients in hospitals or people who have trouble breathing. This pure oxygen is acceptable to breathe in short bursts and for medical reasons but since we evolved in an atmosphere where our oxygen is diluted with other gases, it can be harmful to breathe pure oxygen for extended periods of time.
  
  

Oxygen in Water

• Water, the plentiful substance that is necessary for life, has oxygen as one of its key ingredients. The other one is hydrogen. Hence the chemical formula for water, H20. There are two hydrogen molecules for every one oxygen. Water also has three main forms: solid (ice), liquid and gas (water vapor). In every form, the chemical composition remains the same and water still consists of hydrogen and oxygen.
  
  

Air vs. Water Vapor

• The oxygen that occurs naturally as part of the makeup of our atmosphere is not bonded with any other elements. Water vapor enters the atmosphere due to the heating and evaporation of water. There is naturally oxygen in this water vapor but it is not counted as part of the oxygen that comprises 20 percent of the atmosphere because our bodies do not metabolize the oxygen in the vapor the same way regular oxygen does.