Atlanta is one of more than a dozen metropolitan areas around the country that gets too smoggy in the summer, according to the U.S. Environmental Protection Agency. Others include Los Angeles, New York City, Charlotte and Houston. Here's what you need to know about smog:

THE SMOG EQUATION

NOx + VOCs + Sunlight = SMOG

NOx: Nitrogen oxides

Cars, trucks and power plants contribute this pollutant.

VOCs: Volatile organic compounds

Most of metro Atlanta's VOCs come from trees and other vegetation.

Sunlight

The heat cooks the pollutants together to form ground-level ozone, a main ingredient in smog.

SMOG

The word comes from combining "smoke" and "fog" and is a mixture of pollutants.

SMOGGY DAYS

In the past decade, the smoggiest summer was 1999. Here's a look at the number of days metro Atlanta exceeded the federal standard* for ground-level ozone, and the peak level it reached.

1998: 62, 141 parts per billion

1999: 69, 139 ppb

2000: 46, 139 ppb

2001: 20, 115 ppb

2002: 37, 130 ppb

2003: 13, 109 ppb

2004: 11, 114 ppb

2005: 17, 114 ppb

2006: 30, 121 ppb

2007: 29, 124 ppb

2008: 15, 109 ppb

*Until this year, the federal standard for ozone was 84 parts per billion measured over 8 hours. Now it is 75 parts per billion.

OZONE MONITORING STATIONS

Here are the metro Atlanta ozone monitors where the air violated the federal standard from May 1 through July 2.

COBB

Kennesaw

Days in orange violation: 2

COWETA

Newnan

Days in orange violation: 2

DEKALB

South DeKalb

Days in orange violation: 7

Days in red violation: 1

DOUGLAS

Douglasville

Days in orange violation: 1

FAYETTE

Fayetteville

Days in orange violation: 3

FULTON

Confederate Ave.

Days in orange violation: 7

GWINNETT

Gwinnett Tech

Days in orange violation: 4

HENRY

McDonough

Days in orange violation: 6

ROCKDALE

Conyers

Days in orange violation: 8

Days in red violation: 2

Source: Georgia Environmental Protection Division and U.S. Environmental Protection Agency

SMOG ALERTS

Green: No action needed.

Yellow: Sensitive groups should consider limiting outdoor activity.

Orange: Children and adults who play, work out or work outside, and people with respiratory disease such as asthma should limit outdoor exertion. Outdoor activity should occur in the morning or late evening.

Red: Active children and adults and people with respiratory disease should avoid prolonged outdoor activity. Everyone else should limit outdoor activity.

Source: Clean Air Campaign

DALE E. DODSON / Staff

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Air quality is improving

Thanks to cleaner-burning fuels, more efficient cars and trucks, pollution controls on coal-fired power plants and outdoor burning bans, metro Atlanta's air quality has steadily improved over the past three decades. But the region still violates federal standards, which have gotten more stringent.

Twenty metro Atlanta counties do not meet the federal standards for ground-level ozone or particle matter.

The counties are Barrow, Bartow, Carroll, Cherokee, Clayton, Cobb, Coweta, DeKalb, Douglas, Fayette, Forsyth, Fulton, Gwinnett, Hall, Henry, Newton, Paulding, Rockdale, Spalding and Walton.

Smog is a mix of pollutants

> Ground-level ozone is an invisible gas that forms when pollutants emitted by vehicles, power plants and other industrial sources react chemically to the sun's heat. Gases emitted from trees and other vegetation also contribute to ozone. It is most likely to form during the warmest months, from May 1 through Sept. 30.

> Particle matter, or soot, is microscopic material emitted from vehicles, power plants, factories, fires and some industrial processes.

Smog is unhealthy

> Though it's worse for children and people with heart and lung ailments, ground-level ozone can make it hard for even healthy people to breathe, and it can trigger asthma attacks. It can cause permanent lung damage.

> The tiny particles in soot or dust can lodge deep in the lungs and work their way into the bloodstream. Particle exposure can lead to chronic bronchitis and even heart attacks.

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FORECASTING SMOG

The state's air quality forecast team gets an "A" for correctly predicting smog alerts 94 percent of the time this year. Here's how they're doing on forecasts in each category since smog season started May 1:

........Number of days correct ..Number of days forecasted

Green ............2........................15

Yellow ..........26........................29

Orange ..........10........................17

Red......No correct forecasts ..............2

Source: Georgia Environmental Protection Division

DALE E. DODSON / Staff

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Limit your exposure to bad air

> Sign up for the Clean Air Campaign's e-mail smog alerts at www.cleanaircampaign.com and watch for the air quality forecast on the front page of this newspaper and on weather reports.

> When a smog alert is issued for the next day, plan to limit outdoor activities. Learn more about what you can do at the Clean Air Campaign's Web site, and at the U.S. Environmental Protection Agency's Web site, http://airnow.gov/.

> Know the difference between an alert for high ozone, high particle matter, or both, and check with a doctor if you have concerns about your risk.

Meet the forecasters

About 1:30 p.m. every day, including weekends, the seven members of the state's air quality forecast team huddle in cyberspace to determine whether to issue a code orange or code red alert for the next day. The team includes meteorologists, atmospheric scientists, a geochemist and an expert in statistics from the state Environmental Protection Division and Georgia Tech.

Each team member independently reviews various weather forecasts and air quality computer models to predict the next day's ozone and particulate matter levels.

Bill Murphey, EPD's chief meteorologist. His mother, who was also his sixth-grade teacher in Coweta County, sparked his interest in science.

Carlos Cardelino, senior research scientist in Georgia Tech's School of Earth & Atmospheric Sciences. His research helped regulators understand how ozone forms in metro Atlanta.

Michael Chang, senior research scientist at Georgia Tech. He's an expert in air quality modeling that can be used to determine how to reduce ozone and other pollutants.

Susan Zimmer-Dauphinee, EPD's manager in charge of air quality monitoring. She has degrees in geochemistry and natural science, and makes the final call on all forecasts.

Nyasha Dunkley, EPD meteorologist. She is an adjunct professor of physical sciences at Troy State University's Atlanta campus.

Scott Sutherland, EPD meteorologist. He joined the team in April 2007 after three years at The Weather Network, Canada's version of The Weather Channel.

Jim St. John, research scientist at Georgia Tech. He has consulted with CNN and studied the role of power plant emissions on metro Atlanta's ozone levels.

 DALE E. DODSON / Staff