The weather phenomenon known as an inversion, when a warm high pressure system seals cold air and pollution into Utah's valleys and creates a dirty haze that can settle for days, is likely to blame for the index's high ratings.
The EPA measures five pollutants to judge air quality, including ground-level ozone, particle pollution (also known as particulate matter), carbon monoxide, sulfur dioxide, and nitrogen dioxide. Air quality is measured on a ratings value scale index of 0 to 500. Any measurement over 151 is considered unhealthy for most people.
Monday afternoon, Provo's air quality score was 172. Logan was a close second at 155.
The Utah Division of Air Quality continues to forecast unhealthy air into Wednesday for many areas of the state, issuing "red" air alert warnings for Box Elder, Cache, Davis, Salt Lake, Tooele, Utah and Weber counties. The air will be harmful in those areas for the very young, very old and people with respiratory illnesses. People are encouraged to cut back on driving and the division calls for no burning in those areas.
Duchesne and Uintah counties have a "yellow" air quality warning in effect for Tuesday and Wednesday.
The National Weather Service is forecasting a storm late Wednesday into Thursday that could break up the inversion.
Until then, temperatures are expected to stay about the same as they have been. Overnight lows along the Wasatch Front will reach the single-digit neighborhood, while Logan is expected to get as low as 10 below zero before Tuesday morning. During the day, the temperatures in most of northern Utah won't break 21 degrees. Park City and Heber could reach as high as 43 and 34 degrees, respectively.
The National Weather Service expects daytime highs of 20 degrees in Salt Lake City, 14 in Ogden, 21 in Provo and 7 degrees in Logan.
Temperatures in the south will be a little warmer, with St. George forecast at a daytime high of 60 degrees, Kanab at 57 and Cedar City at 36. In the eastern part of the state, Manila will see a daytime high of 41, Blanding will get to 42 and Vernal will remain at a chilly 15 degree high Tuesday.
Also for Tuesday, the Utah Avalanche Center predicts the Salt Lake, Provo, Ogden and Logan mountain areas will be a low risk for dangerous snow slides; the Uintas will be at a moderate risk.
Jennifer Dobner contributed to this report.