Hall County was included in Atlanta’s Code Orange air quality alert Thursday. But that doesn’t mean the county was smoggier than usual.

Susan Zimmer-Dauphinee, director of the Georgia Environmental Protection Division’s ambient air monitoring program, said Hall always has been part of metro Atlanta’s 20-county nonattainment area for ozone.

"But the National Weather Service only recently began posting the alerts (on its Web site)," she said.

Zimmer-Dauphinee said ground-level ozone is invisible, and outlying counties may have more of it than people think.

"DeKalb and Fulton (near downtown Atlanta) are sources of the precursors to ozone, but suburban sites have higher concentrations of actual ozone because it is carried on the wind," she said.

She said Hall’s inclusion in the nonattainment area is based on computer modeling, because the EPD doesn’t have an ozone monitor in Hall.

Ground-level ozone forms when pollutants interact in the presence of sunlight.

A Code Orange alert means ozone concentrations are high enough to cause problems in sensitive people, such as children and people who have heart and lung problems.

Susceptible people should avoid outdoor exercise during Code Orange days, especially during the afternoon and early evening.

Ozone can irritate and inflame the airways when inhaled, even causing shortness of breath and tightness in the chest at higher levels, according to the Georgia Clean Air Campaign’s Web site. Particulate pollution, which also is measured by the air quality index, can aggravate the existing conditions of heart and lung disease.

Zimmer-Dauphinee said Atlanta already has had 27 ozone alerts this year, but that’s partly because federal standards have changed and cities have to meet stricter ozone limits.

The air quality index is reported for the following day based on observations of weather patterns and other factors by a team of forecasters. The Georgia Clean Air Campaign issues smog alerts when the index reaches a level of Code Orange or above.