By 1 p.m. Wednesday, smog levels had started creeping toward the red zone, when the air quality is considered unhealthy for most people.

The highest eight-hour measurement of ground-level ozone, an ingredient in smog, was 56 parts per billion in Douglasville. The U.S. Environmental Protection Agency considers ozone unsafe at levels above 75 parts per billions. Smog enters code red at 96 parts per billion.

At that level, public health officials advise people to confine their workouts to indoors, or do their outdoor exercise in the morning or late evening. Ground-level ozone, a colorless gas formed when manmade emissions mix with the heat, can be particularly bad for people with asthma and other lung and heart diseases.

The state's air quality forecasters had expected Tuesday's air quality to enter the red zone. While smog levels did violate the EPA's limit in Conyers, south Dekalb County, central Gwinnett County and McDonough, they did not reach the red zone.

Forecasters issued an orange alert for Thursday's air quality, which would be another violation of the EPA standard. As of Wednesday afternoon, the region had violated the EPA standard on 13 days, starting May 6.