South Dakota State health officials are warning that smoke from Canadian forest fires may be a threat to people susceptible to breathing problems.

The Department of Environment and Natural Resources issued an air-quality alert Thursday for areas of western South Dakota.

The smoke is reducing visibility and carries enough fine soot - called "particulate matter" - to raise concern for public health, officials said. The pollution levels Thursday exceeded the National Ambient Air Quality Standard.

The DENR recommends that elderly people, young children and those with breathing problems reduce outdoor activities. Residents also are encouraged to close windows and doors to keep indoor air clean.

The state monitors air quality with equipment in Rapid City, Black Hawk, Wind Cave National Park and Badlands National Park. The automated stations report readings hourly, and the DENR posts results on its Web site.

Individuals may check on the readings at www.state.sd.us/denr/DES/AirQuality/aarealtime.htm