For the third day in a row, the haze blanketing metro Atlanta is expected to be hazardous for children and people with heart and lung diseases.

Forecasters issued an orange alert for Monday. If ground-level ozone exceeds the federal limit of 75 parts per billion, it will be the region's sixth violation since May 1.

Typically, ozone begins to build during the morning rush-hour, when cars, construction equipment and other motorized vehicles begin to emit nitrogen oxide and volatile organic compounds. 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 p.m and 8 p.m., during the evening commute.

With the region is sweating through a string of 90-plus degree days, ozone levels have built up. Possible rain this afternoon and tonight could break the heat, and clean the air.

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.

In March, the U.S. Environmental Protection Agency tightened the federal ozone standard after scientists determined the old standard was not protective enough of public health. The old limit was 84 parts per billion of ground-level ozone. Industries and environmental groups opposed the new limit. Industries said it is too strict and will cost too much money to comply; environmental groups and public health advocates said it is too lenient.

Metro Atlanta has exceeded the old standard on only one day, which was Sunday.