(AP) — Omaha and other areas under Douglas County have been issued an air quality alert due to trapped particles close to the ground.

The Douglas County Health Department issued the alert Thursday after fog, lack of winds and a temperature inversion led to high levels of particulates trapped close to the ground. An inversion occurs when cool air near the ground is trapped by a layer of warmer air. Particulates can include dust and soot.

Officials say the air quality index is unhealthy for “sensitive groups” such as people with heart and lung disease, older adults and children. They advise that people with breathing problems avoid outside exercise.

Officials expect the alert will be over as winds pick up and the fog lifts.