Tuesday was another code red day for smog.

For the second consecutive day, the state's air quality forecasters issued a red alert warning for expected high levels of ground-level ozone, a main ingredient in smog that can trigger asthma attacks and burn lungs.

John Spink / AJC

Westbound traffic crests the hill near Thornton Road on I-20 Tuesday where the hazy skyline of Atlanta was under a code red for smog conditions.

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On code red days, public health officials advise people to exercise indoors or limit outdoor activities to morning and late evening hours when ozone is lower.

Ozone levels tend to peak from about 2 p.m. to 7 p.m.

Tuesday's red alert was materializing as of 5 p.m. Data from the state Environmental Protection Division's Web site showed ground-level ozone levels shot up at the ten air quality monitors across metro Atlanta. The highest level was registered in McDonough, southeast of Atlanta, at 99 parts per billion measured over one hour.

The U.S. Environmental Protection Agency considers the air unsafe at 75 parts per billion of ozone measured over eight hours. At 96 parts per billion measured over eight hours, air quality enters the red zone, when even healthy people can find it hard to breathe.

Metro Atlanta has had a string of bad-air days this month, largely due to hot, stagnant air that cooks emissions from vehicles, construction equipment and industrial plants. Not counting Tuesday, the region has exceeded the federal limit for ozone 12 days this year, including 10 days in June and six in a row last week.