Chemicals and particles too small to be seen from wildfire smoke can cause problems ranging from minor to serious

Over 800 wildfires are burning through forests in Canada, prompting heavy smoke warnings for millions of Americans.

Wildfires in Canada are pouring thick clouds of smoke into the US and will usher in unhealthy air quality for millions of people in Northeast and Midwest.

A massive plume of smoke from wildfires burning across Canada has swept into the United States, degrading air quality across the Midwest, Northeast and Mid-Atlantic.

Residents have been warned to stay indoors to avoid both the smoke and the extreme heat