The Utah Department of Environmental Quality is issuing the yellow ozone advisory for the Wasatch Front, from Box Elder and Cache counties as far south as Utah County.
The advisory is in place throughout Wednesday.
A yellow advisory is not as severe as a red alert, but officials are urging people to limit driving and tailpipe emissions.
Wildfire smoke is causing part of the problem this summer. Emissions of all kinds mix with sunlight and heat to form ozone - a lung irritant that can make breathing difficult.
Other counties on the ozone watch list - Tooele, Uintah and Washington - were expected to have good air Wednesday.
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