The U.S. Environmental Protection Agency's standard for ground-level ozone -- smog's main ingredient -- sets a limit of 75 parts per billion over an eight-hour average.
Wednesday's reading of 77 parts per billion was the second violation this year. On July 9, an eight-hour average of 81 parts per billion was recorded.
Although there were no violations in 2009, there were four in 2008, three in 2007, four in 2006 and one in 2005, according to Georgia's Environmental Protection Division.
Typically, the region's most unhealthy air accompanies the stifling heat and humidity of late July and August.
The current standard of 75 parts per billion is stricter than an earlier standard of 85 parts per billion. A proposal is under way to tighten the levels even more -- to a range of between 60 and 70 parts per billion to better protect public health.
According to the American Lung Association, ozone is formed by chemical reactions of nitrogen oxides and hydrocarbons, also called volatile organic compounds. They are produced primarily when fossil fuels such as gasoline, oil or coal are burned or when some chemicals, such as solvents, evaporate. They combine with heat and sunlight to form ozone smog.
People vary widely in their susceptibility to ozone. Healthy people, in addition to those with respiratory difficulty, can experience breathing problems when exposed to ozone. Exercise during exposure to ozone causes a greater amount of ozone to be inhaled and increases the risk of harmful respiratory effects.
Other Georgia cities and regions have also had ozone violations this year, including 10 in metro Atlanta, four in north Georgia, two each in Macon and Athens and one in Columbus.