Logan has the worst air quality in the nation, according to airnow.gov.
Brigham CIty, Ogden, Provo and Salt Lake City also made the list for poor air quality.
The Utah Department of Environmental Quality (DEQ) forecasts another red air day for many counties in northern Utah. A storm moving into the area is expected to help clear out the pollution. Moderate air quality days are forecast for much of northern Utah beginning Friday.
In Logan, everyone experience health effects due to the air quality. DEQ suggests people limit prolonged outdoor exertion.
Also, many northern Utahns experienced a foggy commute Thursday morning.
The National Weather Service tweeted a graphic showing areas of dense fog surrounding the Great Salt Lake from Brigham City to the bottom of the Salt Lake Valley.
Cpl. Todd Johnson, Utah Highway Patrol, said there is heavy fog in Salt Lake, Davis, Weber and Box Elder counties and it may be causing some slick roads.
"The moisture can turn to ice very quickly, so everybody just needs to assume that there is going to be icy spots and reduce their speed and stay to the right and be looking down the road for any possible problems," Cpl. Johnson said.
A semi-trailer rolled over on southbound I-15 near mile post 369, which is north of Brigham City near Honeyville. Cpl. Johnson said there were no injuries and traffic is able to get around the crash.
The South Davis Metro Fire Agency said via Twitter suggested drivers allowing for greater following distances while driving due to the fog.