An air quality alert for east-central and southeastern Minnesota issued by the Minnesota Pollution Control Agency will remain in effect till 8 p.m. Friday.
Affected areas include the Twin Cities metro area, St. Cloud, Rochester, Winona, Red Wing and the tribal nation of Prairie Island, the MPCA said.
Air quality index values have reached the orange or "unhealthy for sensitive groups" category, the agency said, adding that sunshine, heat and light winds combined to cause an increase in ground level ozone.
Light winds over the past two days have caused the pollutants nitrogen dioxide (NOx) and volatile organic compounds (VOCs) to accumulate over the region, the MPCA said. Sunshine and heat cause those pollutants to react in the air and form ozone.
Air quality will improve Friday night as the sun sets and temperatures drop.
