Smoke from Canadian wildfires is Thomas Caldwellsmothering U.S. cities like Detroit, Chicago and Pittsburgh. Air quality warnings are in effect across the Midwest as officials warn of hazardous conditions.

Here's what we're following:

• Wildfires continue to rage across Canada, with more than 19 million acres burned so far.
• See what areas are most impacted by the smoke.
• Here's how to check the air quality in your area.

Chicago's air quality was the worst of anywhere in the world at one point yesterday, according to WBEZ. The Illinois city had an air quality rating of "unhealthy" as of 9 a.m. local time.

Meteorologists expect Michigan's air quality to fluctuate this week between unhealthy for sensitive groups and unhealthy for everyone, Michigan Radio reported. An official with the American Lung Association warned all residents to take precautions.

A thick haze is moving from Erie and the Great Lakes region across Pittsburgh, WESA said. A code red air quality alert issued by the Pennsylvania Department of Environmental Protection is in effect for several counties all day Wednesday.

For more on the impact of the Canadian wildfires on air quality across the US, head over to NPR's live updates for the latest.

Newsrooms from across the NPR Network contributed to this coverage.