The available 'air' to us will not run out in the near future, it is the quality that matters.
Carbon dioxide is produced by all life, even fungi.
Forests play an important part in absorbing the carbon dioxide and replenish the oxygen content.
The oceans also play an important part in air quality; it contains a lot more carbon dioxide then the air and slowly sucks it out of the air. However, where the oceans are warmer the carbon dioxide is released to the atmosphere instead so without forests in the tropical regions the breathable air will be getting worse, making the 'beings' that breath this more liable to all sorts of diseases.
The greenhouse gasses such as methane burbed by cows makes the hole in the protective ozone layer larger so warming up the oceans even more.
I know the global population increase is alarming. When I was at school it was 3 billion. I believe the misery experienced by African population now will slowly spread to even the more effluent societies as the food production shrinks. The previously 'silent' Asian nations are now major global players and the gigantic China has become the most resource hungry nation in the world. Ironically, we scorn at China's 'one child' policy but they have seen the world population crisis coming years ago.