For the fourth day in a row, the haze blanketing metro Atlanta is expected to be hazardous for children and people with heart and lung diseases.
On Monday, forecasters issued another orange alert for today. If final readings show ground-level ozone exceeded the federal limit of 75 parts per billion on Monday and today, it would be the region's seventh violation since May 1.
"Hot, stagnant air is going to lead to poor air quality events, and that's what we're seeing," said Michael Chang, an air quality forecaster and senior research scientist in Georgia Tech's School of Earth & Atmospheric Sciences.
The region's string of seven 90-plus-degree days is expected to end today in the wake of possible thunderstorms.
Ground-level ozone is a hot weather problem, typically occurring between May 1 and Sept. 30.
Ozone, a major component of smog, begins to build during morning rush-hour when cars, construction equipment and other motorized vehicles emit nitrogen oxides and volatile organic compounds. Coal-fired power plants are also a major source.
The pollutants cook in the sun's heat to form ozone, a colorless gas that can trigger asthma attacks and exacerbate other diseases. Ozone levels generally peak between 4 and 8 p.m., during the evening commute. On days when temperatures are at least 90 degrees, ozone is often at its worst.
On orange-alert days, public health experts advise parents to keep children inside during the hot afternoon and early evening hours. They also advise adults who are sensitive to ozone --- those who have a hard time breathing --- to limit outdoor activities during the peak ozone times.
Marlin Gottschalk, an air quality expert with the state Environmental Protection Division, said, given the heat, metro Atlanta's air is showing improvement over recent years. He credits cleaner cars and gasoline, and pollution controls on power plants.
"A lot of the programs that have been put in place to reduce emissions are in fact working," Gottschalk said.
In March, the U.S. Environmental Protection Agency tightened the federal ozone standard after scientists determined the old standard was not stringent enough to protect public health. The old limit was 84 parts per billion of ground-level ozone.
Metro Atlanta has exceeded the old standard on only one day, which was Sunday.
Copyright 2008 The Atlanta Journal-Constitution