The Wisconsin Department of Natural Resources issued the advisory because of "persistent elevated levels of fine particles in the air" that have brought -- or is expected to soon bring -- the Air Quality Index to the orange level, which "is considered unhealthy for people in sensitive groups," according to the advisory. Those groups are advised to reschedule or cut back on strenuous activities.

Sensitive groups include older adults, children and people with heart or lung disease or asthma. Those with lung diseases, including asthma and bronchitis, or heart disease should pay attention to cardiac symptoms like chest pain and shortness of breath or respiratory symptoms like coughing, wheezing and discomfort when taking a breath. If they experience symptoms, they should consult with a physician.

Fine particle pollution deposits itself deep into the lungs and cannot easily be exhaled. At-risk individuals are particularly vulnerable after several days of high particle pollution exposure.

The fine particles come primarily from combustion sources, such as power plants, factories and other industrial sources, vehicle exhaust and wood burning.