Parts of Salt Lake and Utah counties are smothering in dangerous smog, air pollution that carries a serious health risk, warns the state's air-quality chief.

Burnell Cordner, director of the Utah Bureau of Air Quality, said Friday a temperature inversion has caused stagnation so severe that Salt Lake City violates the federal standard for PM10, the tiniest particulates. In Utah County, the PM10 standard is being violated, and carbon monoxide concentrations reached the standard twice in the past two days but did not violate it.Air in these places is unhealthy. People especially vulnerable to pollution - the very young, the old and anyone with heart disease or respiratory illness - should reduce exertion and stay indoors.

The standard for PM10 is 150 micrograms per cubic meter, based on a 24-hour average.

Friday morning the readings in downtown Salt Lake City were 229 micrograms on the 24-hour standard, a little higher than on Thursday. Hourly readings Thursday peaked around 330.

Utah County's PM10 level reached 250 on Friday.

Cordner said the cause is "the real cold air and the really tight, intense temperature inversion that's lasted now for several days, creating a foggy condition. In that fog there's a lot of air pollution."

The bureau requested that residents curtail use of wood-burning stoves and reduce driving if possible.

"There's an emergency episode plan that we can implement if it gets much more serious," he said. That plan kicks in if the 24-hour average reaches 340 micrograms of PM10.

Under the plan, industries that contribute to PM10 pollution must start to shut down in stages. The amount of the cutback depends on the air's foulness. The companies already know what they must do at each stage.

Not all industries are affected.

"It's pollutant-specific," Cordner said. "If it were a sulfur dioxide problem, then you're looking at the source of sulfur dioxide."

In Salt Lake Valley's case, this time the emergency probably would concern PM10. The plan could require production cutbacks by Kennecott, the North Salt Lake oil refineries and "any smokestack industries," he said.

"We're not at that level yet," he emphasized. But Salt Lake City could reach it, if the inversion tightens its grip on the capital.

"It's the worst we've had for several years," in northern Utah, he said. In Utah County, in recent years some PM10 concentrations have been even worse.

Readings at a monitoring station in the Cottonwood area near Salt Lake City weren't as high. "So far, the only ones (monitors) that are showing serious problems are downtown Salt Lake and Orem," Cordner said.

"We need something to blow this out."

A brisk frontal system sweeping through would clear the air. But that isn't likely for several days at least.

William J. Alder, head of the U.S. Weather Service office at the Salt Lake International Airport, said Friday that while the inversion may relax a bit over the weekend, it will remain.

Snow that fell in Salt Lake and Utah counties "tends to cleanse the air some," he said. Also, Alder said, people drive less on weekends, reducing their autos' considerable contribution to the problem.

The inversion has forced the Environmental Protection Agency to delay the detonation of small supplies of potentially explosive chemicals.

The "shock-sensitive" chemicals were found at the Midvale slag site, part of the Sharon Steel environmental cleanup. They could explode if left too long, so the EPA officials planned to blow them up this week.

They wanted to detonate them on Thursday but held off because of the air pollution - the smoke from the blasts would only make things worse.

They thought the inversion had thinned enough Friday to go ahead. But later in the day, Mary Hagen, EPA public affairs specialist in Denver, told the Deseret News, "Apparently because of the air . . . they changed their mind and it's all off."

EPA officials will track the air quality day-to-day and blow up the chemicals whenever the atmosphere allows.

On Thursday, a malfunctioning air monitor in Lindon, Utah County, registered alarming readings: three times the allowable level of fine particulates. The false reading reached 497 micrograms.

Before the readings were discounted, the Bureau of Air Quality issued a level "C" health advisory and warned Utah County residents to stay indoors, avoid traffic, and curtail outdoor activity. They advised school officials to keep children indoors, and were prepared to ask Geneva Steel to close down some operations.

While pollutant levels aren't as extreme as officials feared on Thursday, they are still a concern.

Residents of Salt Lake County can get the latest information on air quality by calling the air pollution hotline after 9 a.m., at 533-7239. Residents of Utah County can reach hotline information at 373-9560.