It's called a weak synoptic environment.
That's what we call it when there are no big pressure systems tearing at Minnesota. The result can be some nice weather in much of the year. But when we're in winter, the light winds and stagnant air can make for some air quality issues.
That's when MPCA issues air quality alerts like they did today. The AQI (Air Quality Index) considers four pollutant types. Ground level ozone, sulfur dioxide, carbon monoxide and fine particulates. Several MPCA monitors around the state measure these pollutants.
Today the culprits are fine particulates. They come from things like wood smoke and other sources, so it may be best to light the fireplace after the winds kick up again on Friday night.