Wildfire smoke was helping raise ozone to hazardous levels. No alerts have been issued, but the air quality may approach “code orange” levels again late Thursday afternoon in parts of the region, as it appears that the ozone factory will still be operating while the Canadian wildfire smoke persists in the skies over Philly.

Smoke from wildfires in Canada has spread more than 5,000 miles, reaching parts of Russia and Europe during the past week.