Relief is in sight from Canadian wildfire smoke that has been drifting into Minnesota and degrading air quality across the state this week and pushing pollution to record levels.

The Minnesota Pollution Control Agency (MPCA) said Thursday that air quality should be improving across northern Minnesota, where an alert is on the verge of expiring.

However, heavy smoke will continue to linger Thursday over east-central and southern Minnesota, but "air quality should steadily improve for the Twin Cities today," the latest notice from the MPCA read.

Much of southern Minnesota, however, will still be experiencing unhealthy breathing conditions for sensitive groups of the population throughout Thursday, the agency said.

The MPCA's air-quality alert is set to expire at 6 a.m. Friday, with conditions nearing something closer to normal later Friday as traces of smoke linger into Saturday.

Wednesday afternoon, air quality levels in parts of the state reached the red level, which is unhealthy for everyone and particularly perilous for people in sensitive groups — those with asthma, chronic obstructive pulmonary disease, diabetes or heart disease, as well as children and the elderly.

Nick Witcraft, an air quality meteorologist with the MPCA, said Wednesday's smoke was the worst on record for the Twin Cities. At 6 p.m. Wednesday, St. Paul had the worst air quality in the United States, with a score of 256, according to AirNow.gov, the official U.S. air-quality index site. While the index hits 500, readings above 200 are considered sufficiently hazardous that outdoor activities are discouraged.

Fine particle pollution from wildfire smoke can irritate the eyes, nose and throat, and cause coughing, chest tightness, shortness of breath, dizziness or fatigue. Ozone is produced on hot, sunny days by a chemical reaction between volatile organic compounds and oxides of nitrogen. High levels can aggravate lung diseases such as asthma, emphysema and COPD, according to the MPCA.

People in sensitive groups are encouraged to reduce outdoor physical activity and avoid prolonged time outdoors until air quality improves, the MPCA said.

Among cancellations Wednesday because of the air quality were the Minneapolis Park and Recreation Board's outdoor programming, including Music and Movies in the Park events, athletic field rentals and events, and outdoor activities for youth and adults. St. Paul Parks and Recreation also canceled all youth and adult athletics Wednesday.

The Minnesota Aurora women's professional soccer team moved its outdoor match at TCO Stadium against the Green Bay Glory to a private indoor setting with no fans allowed. Ticketholders will get a full refund within the next three to five business days.

Canterbury Park also called off Wednesday's horse races.