India, July 17 -- Wildfires are burning through the boreal forests of central Canada, blanketing Toronto and sending smoke across the border to choke Detroit, Chicago, Minneapolis and New York. Flash floods have submerged parts of the Texas Hill Country, one year after a deluge that killed more than 130 people in the same region. California's Sierra County is burning. Washington DC has been sweating through a heat index close to 40degC. And roughly 64 million Americans on Thursday were dealing with both high temperatures and dangerously polluted air, according to the US Weather Prediction Center, which is part of the National Weather Service.

Officials from Minnesota to New Jersey have urged residents to stay indoors, distributed KN95 fa...