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Logan stood out Tuesday for having the dirtiest air in the nation. And the Salt Lake Valley followed closely behind.

Bob Dalley, Utah director of air monitoring, pointed out March is usually a time when most of America doesn't have air-quality problems.

But Utah does, thanks to a rare combination of snow, geography, a high-pressure system, windless skies and a buildup of fine-particle pollution coughed out mainly by vehicles and industry.

"I expect this to continue into [today] - the winds might pick up in the evening," Dalley said. "We should get relief of some type Thursday. But until then, we're here."

Pollution jumped in northern Utah to levels shown to harm health, making communities ordinarily known for healthy lifestyles to register one-day pollution worse than such congested cities as Los Angeles, Houston, New York, Boston and Chicago.

The health warnings applied, in particular, to people considered ''sensitive'': the young, the old and those with heart and lung problems.

In Cache County, pollution levels were declared unhealthy for everyone for outdoor exertion. Outside recess was canceled at schools.

For Carmen Sorenson, it is a matter of trying not to let the pollution affect daily life more than it must in a household where three of five people have asthma.

A resident of Hyde Park and lifelong resident of Cache County, the Greenville Elementary PTA president backs the breathing-impact tests being done since January on 100 of the school's 500 students, including her daughter. She suspects test results will validate the health problems she sees before pollution warnings are issued - her daughter's fatigue, pale skin and dark circles under the eyes.

"It doesn't just affect the kids with asthma," she said. "It affects everybody."

This week's pollution warnings were unusual, not because northern Utah normally doesn't have high pollution but because the wintertime pollution season normally ends in mid-February.

Particularly bad pollution spells in January and December, plus the lowering of the level of pollution at which alerts are declared, made Salt Lake City rack up a record 28 "red" air-quality days when health warnings were issued. During a few of those days, northern Utah pollution was among the highest in the nation, if not the highest.

But Logan had a markedly mild year until the late February snowstorms helped create perfect conditions for a rare March pollution buildup.

"We've got more fog now than we've had all winter," said Grant Koford of the Bear River Health Department in Logan.

Koford noted there are practical things people can do to help protect the air.

The most important: "Reduce driving. Now's the time to do it."

"And, if you're sensitive to this stuff," he added, "you should remain indoors and limit your outdoor activities."

People in "sensitive" groups - the very young, the very old and people with heart and lung problems - are advised to avoid exertion outdoors in Salt Lake, Davis and Weber counties.

In Cache County, the health warning applies to everyone.

Microscopic particles of soot, the byproduct of combustion engines, are responsible for these wintertime warnings. "Particulate matter" 2.5 microns and smaller, commonly called PM2.5, builds up in the air, and when people breathe it, it can cause a wide variety of health problems.

Studies have shown that the particles, about one-fortieth the width of a human hair, burrow deep into the lung tissue.

High concentrations in the air can worsen symptoms of asthma, for instance. High levels also are blamed for an increase in heart attacks and deaths from heart and lung disease.

Pollution by the numbers

Air-quality officials measure fine-particle pollution as a concentration, micrograms pollution per cubic meter of air. The higher the number, the worse the pollution. Here is what the Utah Division of Air Quality's monitors recorded through Tuesday afternoon:

* 25.1 Ogden monitor high reading, Monday 10 p.m.

* 34.1 Lindon monitor high reading, Tuesday noon

* 35.5 triggers "unhealthy" warning for everyone

* 47.1 Salt Lake City high reading, Tuesday noon

* 56.8 Logan monitor high reading, Tuesday 5 a.m.