Ozone (O3) is a highly reactive gas composed of three oxygen atoms.
Depending on where it is in the atmosphere, ozone affects life on Earth in either good or bad ways.
Stratospheric ozone is formed naturally through the interaction of solar ultraviolet (UV) radiation with molecular oxygen (O2).
The stratospheric "ozone layer" extends from approximately six to thirty miles above the Earth's surface and reduces the amount of harmful UV radiation reaching the Earth's surface.
Tropospheric, or ground-level, ozone forms primarily from reactions between two major classes of air pollutants, the volatile organic compounds (VOCs) and nitrogen oxides (NOx).
These reactions depend on the presence of heat and sunlight, meaning more ozone forms in the summer months.
NOx is emitted by cars, power plants, industrial plants, and other sources.
Significant sources of VOC emissions include gasoline pumps, chemical plants, oil-based paints, auto body shops, print shops, consumer products and some trees.