We breathe air in, absorb some of the oxygen, and then breathe out carbon dioxide.
Plants breathe in carbon dioxide, absorb it to build their tissue, and breathe out oxygen.
When contaminants enter the picture, however, nature's cycle is disrupted. Today's air, particularly over cities, is a foul brew of natural air plus smoke from automobiles, factories, leaf blowers, and hundreds of other sources. There are noxious gases like sulfur dioxide and oxides of nitrogen. There are specks of coal dust and rubber. Our bodies can tolerate only so much.