Kevin Perry had just begun his morning routine, stepping outside to get the newspaper, when he noticed something was wrong with the sky.

“Within 30 seconds, I was coughing and my throat hurt,” Dr. Perry, an atmospheric scientist at the University of Utah, said of that morning in August. “It was the absolute worst air quality I’ve ever experienced in my life.”

Shrouded in smoke drifting from California’s colossal wildfires 500 miles away, Salt Lake City had on that morning edged past smog-choked megacities like New Delhi and Jakarta to register the most polluted air of any major city in the world.