What's with this ozone stuff? First, everyone is worried that earth's ozone layer is disappearing so you shouldn't use aerosols. Then you hear that we have "ozone exceedance days" in Houston and other large cities. Do we have too much ozone or too little ozone? And why should we care? What is ozone, anyway? The following is a brief explanation of the basic facts about ozone and what the controversy is about. 

The air we breathe is composed mostly of nitrogen and oxygen. About 75 percent is nitrogen and about 20 percent is oxygen. Argon gas makes up slightly less than one percent of the air while water vapor can vary from almost zero to about three percent on really hot, humid days. Air pollution is only a small part of our air. For example, when ozone levels reach the federal one-hour average standard of 0.12 parts per million, the air only contains 0.000012 percent ozone. 

Even though the percentage of air that is ozone is small, ozone in the air can be a problem. The normal, life-giving oxygen in our air exists as molecules made up of a stable configuration of two atoms of oxygen. The paired forms of either gas create a stable, balanced molecule, not easily divided.

easily gives up the "extra" atom of oxygen and therein lies the problem. 

The problem is oxidation. Fire is a form of rapid oxidation with the release of energy which causes heat. We are also familiar with another, slower kind of oxidation: the oxidation of iron we call rusting. Given enough time and oxygen, a large collection of iron such as an automobile will become a pile of iron oxide or rust. This is why we paint metal - to prevent oxygen from attacking it. Paint serves as a barrier between oxygen and metal.

Ozone, an energetic form of oxygen, can attack respiratory tissues as oxygen attacks iron. That is why, during an ozone warning, citizens are urged to stay indoors and not exercise outdoors. Outdoor exercise causes panting which increases exposure of lung tissue to high concentrations of the gas.