In the composition of the air is oxygen. Without it man cannot live. When we breathe, we inhale air with oxygen and exhaled it already with carbon dioxide. The more carbon dioxide, the harder it is to breathe, but running factories, power plants, cars produce whole flow of carbon dioxide.
But come to the aid of the plants that process the carbon dioxide back into the air oxygen. That is why it is so important that we were surrounded by forests, fields, parks and gardens, giving the Earth oxygen. Save the forest, guys!
Cm. also: What is the atmosphere?