How much, oxygen is in the air?

The air, of course, is a mixture of several gases, plus fine, floating particles of dust and soot, pollen grains, bacteria and other assorted fragments. Most of the air around us, however, is nitrogen and oxygen. These two gases make up 99% of the atmosphere. Nitrogen is a lazy gas. It rarely combines with other gases and refuses to aid the burning process. Oxygen is an active, energetic gas. It is always ready to combine with other elements and act as a fuel for the burning process.

Nitrogen, fortunately, makes up about 78% of the air. Oxygen makes up about 210. This neat balance provides us with enough oxygen to keep our fires going and carry on our breathing processes, The bulk of lazy nitrogen acts as a brake on the busy oxygen and prevents these vital processes from going too fast.