Hot summer days are getting stuffier for more than 100 million people in the U.S. as wildfire smoke drifts south and imposes dangerous risks to their health.

After sitting together with Prime Minister Mark Carney during Sunday’s World Cup final in New Jersey, U.S. President Donald Trump isn’t letting up on his threat to tariff Canada over wildfire smoke.

"We are holding Canada responsible for the fact that they are not properly maintaining their Forests, and Brush therein, and the United States is being unnecessarily invaded by filthy, polluted, and u...