The air over the Salt Lake Valley was difficult to see through Friday, and nearly as hard to breathe — and that’s not likely to change over the weekend.

Thick fog descended over much of the valley Friday, limiting visibility for travelers. With the fog came unhealthy levels of particulates, known as PM 2.5, in Salt Lake County, according to the Utah Division of Air Quality.

Solid-fuel burning devices, such as wood stoves, cannot be used when air quality is in the red zone. No open burning is allowed, including fire pits, fire rings and campfires. People are encouraged to consolidate trips to cut down on vehicle use.

(Francisco Kjolseth | The Salt Lake Tribune) A window washer cleans the glass of the Huntsman Cancer Foundation (HCF) as the Salt Lake valley skyline is obscured by heavy fog and haze and staff from the library is reflected from the inside on Friday, Jan. 11, 2019.

The same restrictions apply when the air is in the “orange” zone, unhealthy for sensitive groups — which the division forecasts through Sunday in Salt Lake, Box Elder, Cache and Weber counties. Moderate, or “yellow,” air is predicted this weekend in Davis, Duchesne, Tooele, Uintah and Utah counties.

The National Weather Service reported Friday that “fog and deteriorating air quality will persist through the weekend.”