The Twin Cities will be under an air pollution health alert on Friday between 11 a.m. and 9 p.m., with intense heat and bright sunlight expected to produce dangerous levels of ozone.
The Minnesota Pollution Control Agency said people with respiratory or heart conditions, as well as the elderly and young children, should limit strenuous outdoor activities, especially in the afternoon and evening.
High airborne ozone levels can cause shortness of breath, coughing and throat pain and can aggravate preexisting lung and heart ailments.
Air quality is expected to improve late Friday as temperatures cool, and again as storms move through the region on Saturday or Sunday.
The alert covers Anoka, Carver, Dakota, Hennepin, Ramsey, Scott, and Washington Counties and follows an air quality advisory issued for Thursday.
Ozone is produced on hot, sunny days by a chemical reaction between pollutants known as volatile organic compounds and oxides of nitrogen, according to a statement by the agency.
State officials said driving, use of power garden equipment and other small engines contribute to ozone pollution. They encouraged Twin Cities residents to take public transit and reduce the use of lawn equipment for the next few days.