Here's something else to blame on the weather: bad air.

State pollution control officials have issued 18 air quality alerts for the Twin Cities since Nov. 1.

Last year's total for Nov. 1-March 31: seven.

Lighter winds and a healthy snowpack are trapping contaminants that usually rise and blow away.

"It certainly seems like one of the worst years we've had," said Cassie McMahon, an air quality specialist with the Minnesota Pollution Control Agency.

An alert day means that air quality for one or more pollutants is unhealthy for tens of thousands of sensitive individuals, including the elderly, children and those with respiratory and heart conditions.

They are advised to avoid strenuous activities, which means no recess for kids at many schools, and no outings for many others.

Beyond that, even healthy adults should consider reducing or postponing exercise.

"I try to avoid going outside on those days as much as I can," said Dana Lansky of Minneapolis, who watches for the alerts. "Breathing feels more difficult, and I can become particularly tired and unwell."

The troublesome pollutants in winter are fine particles such as soot, dust and smoke that are 30 times smaller in diameter than a human hair.

Emitted from smokestacks, fireplaces and tailpipes, the particles hang in the air. They become deeply embedded in the lungs, which aggravates asthma, emphysema, bronchitis and other ailments.

"The particles are so tiny that they can also get into the bloodstream and trigger heart attacks," said Chuck Stroebel, a research scientist with the Minnesota Department of Health.

Sitting under high pressure

A statewide network has been monitoring fine-particle pollution since 2002.

Power plants and vehicles are not producing more particles this winter, but weather patterns are trapping them, usually for two or three consecutive days.

"We've had a few things conspiring as far as the weather," said Tom Hultquist, science and operations officer at the National Weather Service in Chanhassen.

One factor is the main storm track, which has been well to the south of Minnesota much of the past few months, said Hultquist. "We've been sitting under high pressure most of the time, with lighter winds," he said. "That keeps us from having the pollutants mixed up."

An icy snowpack has also contributed.

Sunny, longer days have been heating the air, but snow and ice on the ground reflect much of the sunlight. The result is an inversion, said Hultquist, in which the air near the ground is colder and remains trapped beneath warmer air hundreds of feet above.

"It's like the weather is creating an ever-smaller box for the same amount of pollution that we keep creating every day, so concentrations go up," said McMahon.

"I guess you could say that boring weather makes for unhealthy air," said Hultquist.

The stagnant air usually lasts for two or three days before winds pick up and blow it away, or snow or rain cleanse the air.

It's difficult to tell how many people have suffered, or how severely.

"It's very hard to look at just one day and say hospitalizations up or down on that day are necessarily due to air pollution," said Jean Johnson, an environmental epidemiologist for the Minnesota Department of Health. She and others are working on a new project to compare air pollution data with hospitalizations, but she said it will take several years to determine trends.

What you can do

When an air alert day is declared, authorities recommend that people do their part by driving less, conserving electricity, skipping wood fires and minimizing the use of small engines.

Bob Moffitt, communications director for the American Lung Association in Minnesota, said that the state generally receives good grades for its air quality, but that's partly because the Twin Cities area is lucky. It doesn't receive much pollution drift from other large metro regions, he said, and relatively flat terrain usually allows prevailing winds to disperse pollutants quickly and carry them away. But Moffitt warns that the Twin Cities metro area was recently ranked as the nation's 12th most traffic-congested area, and vehicles are a major source of air pollutants.

"People say that the air here is fine," said Moffitt. "But we're very close to it not being fine, and we're learning that this year."

Tom Meersman • 612-673-7388