The average human adult at rest inhales and exhales about 7 to 8 liters (about one-fourth of a cubic foot) of air per minute. That amounts to around 11,000 liters of air (388 cubic feet) in a day.
The air that we inhale is about 20-percent oxygen rich. The air that we exhale contains about 15-percent oxygen. So, we consume about 5-percent oxygen contained in the air in each breath. Therefore, we humans use about 550 liters of pure oxygen (19 cubic feet) per day.
A person who is exercising obviously uses a lot more oxygen than that.
Have you ever given thought to what produces this 550 liters of oxygen that each one of us need daily?
The oxygen we consume is exhaled as carbon dioxide.
Now, have you ever given thought to what assimilates that carbon dioxide and converts it back to the vitally needed oxygen?
The plants and trees of course.