Ground-level ozone or smog irritates the lungs and can make it difficult for people to breathe. Ozone is formed when chemicals emitted by motor vehicles, power plants, certain industries and processes, and even vegetation react chemically in the presence of sunlight. Both ozone and some of the pollutants which cause it are carried by the wind, so pollution generated in one state can affect ozone levels in downwind areas.
The U.S. Environmental Protection Agency has established standards requiring that the maximum daily 8-hour average ozone level at any location be no greater than .08 parts per million. The EPA is considering lowering the standard to .07 or .075 parts per billion.