Skip to main content


Ozone is one of the criteria air pollutants and is a form of oxygen, the molecule contains three oxygen atoms and has the same chemical structure whether it is found high in the atmosphere or at ground-level. Ozone is unstable, particularly at ground level, and will form when other air contaminates are present in sunlight. The process can reverse at night, meaning ozone levels near the surface often vary greatly during a single day. In the upper atmosphere the stratospheric ozone layer, which contains about 90% of the ozone in the atmosphere, makes the Earth habitable by absorbing harmful solar ultraviolet (UV) radiation before it reaches the planet’s surface. In the lower atmosphere ozone can be formed by sunlight reacting with precursor pollutants such as nitrogen oxides and volatile organic compounds. There is evidence from controlled human and animal exposure studies of the potential for ozone to cause adverse health effects at high concentrations.

Because ozone originates with sunlight, the ozone near the surface displays strong seasonal and daily patterns, with higher concentrations typically, but not always, in summer and in the afternoon. Besides being formed locally, ozone is subject to long-range atmospheric movement.

Precursor pollutants (the molecules that react to create ozone) are the result of human activity and natural processes. The term “background ozone” relates to ozone not controllable by a local region, meaning it is sourced from both natural processes such as wild fires and lightning, as well as ozone created by human activity far away from local control including outside the U.S.

Production of cleaner burning fuels, reduction of gasoline vapors at fueling stations, emission controls at refineries and production facilities are among the contributions the petroleum industry is making to reduce hydrocarbons and nitrogen oxides for ground-level ozone.

What Can I Do?

Using gasoline wisely reduces the pollutants that form ozone, please review the Fuel-Saving Tips for Drivers to see what you can do help. Emission controls at gasoline stations keep the fuel loading emissions from escaping into the atmosphere. When the pump stops, don't top off your tank if you want to reduce the hydrocarbon vapors from going into the air.