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

Canadian environmental officials said smoke from forest fires that was causing reduced visibility and poor quality 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) is expected to reach the red or unhealthy category in a large swath of Minnesota and will likely remain through Saturday. AQI is a system used to communicate how much air pollution is in the air. It breaks pollution down into six categories and colors, and advice on what isn’t safe to do. They range from “good” (the color green) to “hazardous” (maroon).

The smoke could start to decline beginning Saturday before spreading as far south as Tennessee and Missouri. The air could remain unhealthy for sensitive groups through Monday, state health officials said.

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 keep windows and doors closed to keep the smoke from getting inside.

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