What do people breathe into the air?

People ( Humans) exhale nitrogen, carbon dioxide, and trace amounts of other gases.

The nitrogen and other gases remain unreacted. from the atmosphere.

Carbon dioxide is the result of breathing in (inhaling) oxygen travelling around the body in the blood stream, being used at the muscles etc., thereby being converted to carbon dioxide, which is carried in the blood stream to be exhaled.

The carbon dioxide, being a heavy gas, falls to earth, is breathed in by green plant life, undergoes photosynthesis, and oxygen is released. The carbon compoent remains in the plant as biomass.

The two gases , oxygen and carbon dioxide, are part of the oxygen/carbon cycle.