The air pollution that smothered mountain valleys in northern Utah for more than a week made a dramatic improvement Sunday.

Regulators called for at least two days of clean air and lifted a ban on wood-burning starting Monday for the urban corridor anchored by Salt Lake City. The relief came courtesy of a storm that brought heavy snow to northern Utah.

Winds finally loosened an icy fog that was trapping tailpipe and other emissions in mountain valleys.

Doctors had declared a health emergency, but Gov. Gary Herbert refused to follow suit with a decree of his own. The doctors called for lower highway speed limits, curbs on industrial activity and free mass transit.

Soot along the greater Salt Lake region topped out at up to 130 micrograms per cubic meter last week. That was more than three times the clean-air limit set by the U.S. Environmental Protection Agency. But on Sunday, the count of extremely fine particulates was down to 10 micrograms per cubic meter in Salt Lake and Davis counties.

Later Sunday, regulators issued an all-clear for the greater Salt Lake region as the winter storm scoured out mountain valleys. Only two counties in eastern Utah — Duchesne and Uintah — remained under an advisory for people sensitive to air pollution.

Weather conditions could set the stage this week for a return of bad air. Regulators blame temperature inversions, when winds calm and cold temperatures sink into valleys, trapping pollutants.

The U.S. Environmental Protection Agency had singled out the greater Salt Lake region for much of January as having the nation's worst air.

More than 100 Utah doctors signed a petition Wednesday calling for immediate relief. They also criticized expansion plans by Salt Lake-area oil refineries that they said were almost certain to be approved by the Utah Division of Air Quality, the same agency that puts out pollution alerts.

The refineries say they are cutting some emissions even as they expand, but doctors say other emissions will rise and that nobody measures errant emissions from thousands of valves at a refinery that can wear out and leak over time.