The Minnesota Pollution Control Agency issued an air pollution health advisory that applies to the Twins Cities area and Rochester.
Hot, humid weather with light winds produced Sunday's alert, which continues through Monday. The MPCA says pollution concentrations are unhealthy for sensitive groups such as children and the elderly.
The MPCA tracks the current air quality index, with readings for particles and ozone at ten locations around the state.
The Minnesota Department of Health has lots more on particulate matter (which can range in size from a few atoms in diameter to the thickness of a human hair) -- what it is, where it comes from, and how it can affect your health.
The heat index reached the mid-90's across much of the state on Sunday, with Monday's forecast similar. The National Weather Service says southern Minnesota may see some severe storms on Tuesday before cooler temperatures move in by mid-week.