Ozone is a colorless gas that can be found in the air we breathe. The extra oxygen atom makes ozone extremely reactive. While it occurs naturally in the Earth's upper atmosphere, where it shields the Earth from the sun's ultraviolet rays, it is also found close to the surface. This ground level ozone is a harmful air pollutant.
Ozone is the primary ingredient in urban smog.
Although it acts as a protective layer when high above the earth, ozone near the ground can cause or contribute to serious health problems. Remember it by this rhyme. If you have to drive on an Ozone Action Day, avoid having to refuel until late in the day. On the day before an Ozone Action Day, if your car is low on fuel, remember to stop at the gas station on the way home from work Be careful to avoid spills and overfills at the gas station, since gasoline fumes supply a major ingredient for ozone formation. Apply the same precautions against sloppy refueling when filling lawnmowers, outboard motors, etc.