Where does our breath go?

All gases, when exposed to other gases, tend to get mixed with them, until, if nothing interferes the various gases are equally distributed or diffused.

The breath we breathe out has much more carbon dioxide and also much more water vapour than ordinary air. They begin to diffuse into the air around them. In this the expired air is aided by the fact that it is almost warmer than the air it meets and is therefore lighter and must rise.