Ozone is a toxic gas.
It occurs naturally in the stratosphere from solar-induced chemical reactions, and at that altitude it protects us and our environment from harmful ultraviolet rays. But when ozone forms at ground level, it becomes the unnatural and predominant ingredient of smog. Ozone molecules are highly reactive oxidizing agents that tend to stick to large molecules lining our respiratory tracts. They react with these large molecules, breaking their chemical bonds and adding oxygen atoms to their structures. In fact, they can be as different as, well, night and day.