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Smog/Regional Transport of Ozone

Environmental Issues

Smog is the brownish haze that pollutes our air, particularly over cities in the summer. Smog can make it difficult for some people to breathe and it greatly reduces how far we can see through the air.

The primary component of smog is ozone, a gas that is created when nitrogen oxides (NOx) react with other chemicals in the atmosphere, especially in strong sunlight. NOx is produced whenever we burn something, such as coal in a power plant or gasoline in a car's engine.

Like sulfur dioxide (SO2), NOx can travel large distances before reacting to form ozone. For this reason, it creates regional pollution problems, rather than simply affecting the local area where it is emitted. EPA has taken several steps to reduce NOx emissions across large regions, including the national Acid Rain Program and several regional NOx allowance trading programs such as the NOx Budget Trading Program and the new Clean Air Interstate Rule.

For more information about ozone and its effects, including maps of ozone concentrations, visit EPA's ozone Web site.


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