Smoke from Canadian wildfires hovered over several Midwestern states Saturday, bringing warnings of unhealthy air for at least the third day.

Air quality alerts were in effect in Iowa, Minnesota, Wisconsin and Michigan, as well as eastern Nebraska and parts of Indiana and Illinois. Forecasters said the smoky skies would remain for much of the day.

Canadian environmental officials said smoke from the forest fires would persist into Sunday for some areas.

The Switzerland-based air quality monitoring database IQAir, which assesses air quality in real time, listed the city of Minneapolis as having some of the worst air pollution in the world since Friday. The Air Quality Index (AQI) was expected to reach the red or unhealthy category in a large swath of Minnesota.

AQI is a system used to communicate how much air pollution is in the air. It breaks pollution down into six categories and colors, along with advice on what is and is not safe to do. They range from "good" (the color green) to "hazardous" (maroon).

People with lung disease, heart disease, children, older adults and pregnant women are most susceptible to the poor breathing conditions.

"What's been unique in this go-around is that we've had this prolonged stretch of smoke particulates towards the surface, so that's where we've really had the air quality in the red here for the past few days," said Joe Strus, meteorologist with the National Weather Service in the Minneapolis–Saint Paul area in Minnesota.

"We've sort of been dealing with this, day in and day out, where you walk outside and you can taste the smoke, you can smell it," Strus said. "Sometimes we've been in higher concentrations than others. Other times it's just looked a little hazy out there."

The air was improving Saturday, he said, specifically across the Twin Cities and southwestern Minnesota, but state health officials warned that conditions could remain unhealthy for sensitive groups through Monday.

Officials said the smoke could spread as far south as Tennessee and Missouri.

The EPA's Air Quality Index converts all pollutant levels into a single number. The lower the number, the better. Anything below 50 is classified as "healthy." Fifty to 100 is "moderate" while 100-150 is unhealthy for "sensitive groups." Anything above 150 is bad for everyone. Parts of Minnesota exceeded that number on Saturday.

Health officials advise people with asthma and other lung disease, heart disease, children and older adults to avoid prolonged exposure to smoke and limit strenuous activities. They said to avoid burning things that could make the air pollution worse and to keep windows and doors closed.

"This is something that's become part of our summer here the last few years," Strus said, "and I think a lot of us are just looking forward to seeing a little more movement in the atmospheric winds and we'll be able to hopefully disperse some of the smoke out of here in the next few days."