Several wildfires burning across the northwestern Canadian provinces are again sending vast amounts of smoke into the upper Midwest and northeastern US, swamping the area with unhealthy air quality – some of the worst in the world, according to some metrics.

Air quality will deteriorate across 12 states through Saturday, mainly in the Upper Midwest, as wildfire smoke pours into the US from northwestern Canada brought by the cold front that is ushering in a brief summer cooldown.