Ozone is a form of oxygen with three atoms, while most gaseous oxygen just has two. It is a photochemical oxidant and, at ground level, is the main component of smog. Ozone is not emitted directly into the air by any one pollution source, but is formed through chemical reactions between natural and man-made emissions of volatile organic compounds (VOCs) and nitrogen oxides in the presence of sunlight. These gaseous compounds mix in the air, and when they interact with sunlight, ozone is formed. Sources of these pollutants include automobiles, gas-powered motors, refineries, chemical manufacturing plants, solvents used in dry cleaners and paint shops, and wherever natural gas, gasoline, diesel fuel, kerosene, and oil are used as fuel.
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The ozone layer in the upper atmosphere is considered "good" ozone because it filters harmful ultraviolet radiation from the sun and reduces the amount that reaches the earth's surface. On the other hand, high collections of ozone in the air we breathe can be harmful to people, animals, crops, and many common materials. This ground-level ozone is the air pollutant of most concern in Texas and the nation.
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The Texas Commission on Environmental Quality forecasts days when ozone formation is likely to be high. The program asks people to take voluntary action on those days to prevent exceeding the National Ambient Air Quality Standards for ozone.

Since automobiles are one of the main culprits in ozone formation in the San Antonio region, we should try to limit our driving on Air Quality Health Alert days. Actions like carpooling, riding the bus, riding a bicycle, and trip chaining add up to less pollution.
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