On Friday more than 100 million Americans were under air quality alerts as smoke from hundreds of Canadian wildfires drifted across the Midwest toward the East Coast.
The air quality alerts extended from Wisconsin and northern Illinois through Michigan and Ohio and eastward.
On Friday morning, ranking site IQ Air listed three U.S. cities among the top six major cities in the world with the worst air quality: Detroit, Washington DC and New York City, though Toronto and Montreal in Canada ranked as the very worst.
An area spanning nearly 20 million acres—bigger than West Virginia—has already burned. About half the fires were considered out of control as of Wednesday, spreading from the Pacific to the Atlantic coasts.
Storms on Thursday brought some relief from the smoke in parts of the Midwest, and more rain there Friday would hopefully help further. But smoke was expected to dissipate less quickly in the Ohio Valley and the Mid-Atlantic, where storms on Friday were forecast to be more isolated.