New Delhi, July 16 -- a classification that health agencies describe as unsafe for everyone, with no exceptions for healthy adults or outdoor workers. The smoke came from Canada, carried south by wind from more than 800 fires burning simultaneously across the country's forests.

The Canadian Interagency Forest Fire Centre reported 836 active wildfires as of Thursday, with 194 classified as completely out of control. That volume has sent smoke across 20 American states, covering a band from the upper Midwest through the Great Lakes region into New England and parts of the Mid-Atlantic. Roughly 100 million Americans were estimated to be living under some degree of unhealthy air, according to CBS News. The scale placed the event among the la...