The area had up to 130 micrograms of soot per cubic meter on Wednesday.

SALT LAKE CITY — Michelle Francis keeps one eye on Utah's air quality index and the other on her 9-year-old daughter's chronic asthma these days. The air pollution is so awful in her Salt Lake City suburb that Francis keeps her daughter indoors on many days to prevent her cough from being aggravated.
"When you add all the gunk in the air, it's too much," Francis said.
The U.S. Environmental Protection Agency has singled out the greater Salt Lake region as having the nation's worst air for much of January, when an icy fog smothers mountain valleys for days or weeks at a time and traps lung-busting soot.
The pollution has turned so bad that more than 100 Utah doctors called Wednesday on authorities to immediately lower highway speed limits, curb industrial activity and make mass transit free for the rest of winter. Doctors say the microscopic soot — a shower of combustion particles from tailpipe and other emissions — can tax the lungs of even healthy people.
"We're in a public-health emergency for much of the winter," said Brian Moench, a 62-year-old anesthesiologist and president of Utah Physicians for a Healthy Environment, which delivered the petition demanding action at the Utah Capitol.
The greater Salt Lake region had up to 130 micrograms of soot per cubic meter on Wednesday, or more than three times the federal clean-air limit, according to the U.S. Environmental Protection Agency.
That's equivalent to a bad day in the Los Angeles area.
For 2 million Utah residents, there is no escape except to the snow-capped mountains that gleam in the sunshine thousands of feet higher, or to resort towns like Park City, where the Sundance Film Festival is under way.
"I wish there was something we could do about it," Francis, a school teacher 10 miles north of Salt Lake City, said.
Authorities have prohibited wood burning and urged people to limit driving. Vehicle emissions account for more than half of the trapped pollutants.
Utah regulators are working on a set of plans to limit everyday emissions, including a measure to ban the sale of aerosol deodorants and hair spray that contain hydrocarbon propellants. Those plans, however, will take years to show results.
Doctors say people — especially pregnant women and children — should stay indoors, or at least avoid active outdoor exercise under the sickening yellowish haze. Elderly people with heart disease are most at risk, Moench said.
"If you can see it, you don't want to breathe it. Think about what's going into your body," Salt Lake City pediatrician Ellie Brownstein said. "It's essentially like smoking. Instead of breathing clean air, you're breathing particles that make it harder for your lungs to function and get oxygen."
Snow cover amplifies the phenomena called a temperature inversion — Salt Lake City was a foggy freezer box Wednesday at 18 degrees, while Park City basked in sunny 43-degree weather. The warmer air aloft acted like a lid on the frigid valley air, leaving it with no place to go.
For weeks, industrialized cities in northern China have been dealing with bouts of sickening smog several times more toxic than Utah's. But by U.S. standards, Utah's pollution index is off the charts with readings routinely exceeding a scale that tops out at 70 micrograms a cubic meter. The EPA sets a standard for clean air at no more than 35 micrograms.
"People think the health implications are limited to asthma — that's only a drop in the bucket," Moench said. "For every pregnant woman breathing this stuff, this is a threat to her fetus through chromosome damage. It sets people up for a lifelong propensity for all sorts of diseases."
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