Smog pollution in King and Pierce counties could reach unhealthy levels this week as temperatures rise throughout the region, according to forecasters at the Puget Sound Clean Air Agency.

Exposure to smog can trigger asthma attacks, make breathing difficult, exacerbate lung and heart problems and weaken the immune system. The Washington State Department of Health recommends that people who are sensitive to air pollution limit time spent outdoors. Air pollution is especially harmful to children, people with heart and lung problems, and adults over age 65.

Smog builds when summertime sunlight “cooks” everyday emissions from motor vehicles, industry, paints, solvents and gasoline fumes. When the pollutants react with the summertime sunlight, they form ground-level ozone, the main component in smog.

Smoggy conditions will mostly impact communities in the Cascade foothills of King and Pierce counties. Pollution generated by vehicle traffic and activities in the urban areas of Everett, Seattle and Tacoma accumulates as it moves to outlying suburban and areas.

Smog in the Cascade foothills may remain elevated through Saturday.