When it comes to its chronically poor winter air, Salt Lake City already has topography working against it. Steep mountain ranges on either side of the city create a giant bowl, where dreaded winter temperature inversions trap cold air that stagnates with dust, factory smoke, and diesel exhaust from the city’s expanding web of freeways.

At least a million people have moved to the valley since 2000, and the air on some days looks — and actually can be — dirtier than Mexico City’s.

That makes Dr. Tom Nelson cringe. Nelson, who grew up here, manages the emergency room at one of Utah’s largest hospitals. And on bad air days, they see admissions go up for respiratory illnesses and even heart attacks.

“You’ll feel it in your throat and in your lungs, you’ll feel it stinging your eyes,” he says. “You can’t avoid it.”

Lately, Nelson and others worry about an additional threat: dust storms stirring up toxins off the shrinking Great Salt Lake blowing east into the city.