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Smog pollution in King and Pierce counties could reach unhealthy levels this week as temperatures rise throughout the area.

The Puget Sound Clean Air Agency says the smog could trigger asthma attacks, make breathing difficult, exacerbate lung and heart problems and weaken one’s immune system.

If you’re sensitive to air pollution, the Washington State Department of Health recommends limiting the time you spend outdoors. Air pollution is especially harmful to children, people with heart and lung problems, and adults over age 65.

Emissions from vehicles, industry, paints, solvents and gasoline fumes get “cooked” by sunlight, according to the Clean Air Agency. The cooking forms ground-level ozone, the main component in smog.

Most of the smog will been seen in communities in the Cascade foothills of King and Pierce counties.

Smog in the Cascade foothills may remain high through Saturday.