The air in the Twin Cities generally meets state and national air quality standards -- except for the occasional "air inversion" day when pollution can shoot way up.
But standards change, and in the next few days the U.S. Environmental Protection Agency is expected to announce new rules on fine particles, or soot -- the kind of pollution produced by cars, trucks and wood stoves.
Depending on how stringent the new standards are, the metro area may not be able to meet them, and failure to meet them could lead to restrictions on new buildings, roads and other projects.
A team of air quality experts from the Minnesota Pollution Control Agency recently visited the roof of Harding High School on the east side of St. Paul.
Six metal air monitors, each the size of a small refrigerator, are attached to the roof. They constantly record and give real-time readings of the number of pollutants, including fine particles, in the air.
Rick Strassman, the supervisor of the Air Monitoring Unit at the MPCA, says the particles they're capturing and measuring are about one-thirtieth the width of a human hair.
"It's a soot-size particle," Strassman says. "Wood smoke is a classic example of a fine particle. You can see it in the form of haze and wood smoke. It will obstruct visibility, and when we have a hazy day it's an indicator of a fair amount of fine particulates in the air."
Monitoring stations like this are scattered around Minnesota, with eight in the metro area.
Most of the time the stations report that the Twin Cities are meeting current federal air standards, both as a yearly average and during 24-hour measurement periods.
There have been times when the metro area has exceeded the 24-hour standard -- and it's usually because of the weather.
Catherine Neuschler, a staff member of the MPCA, says: "Because we've had some kind of weather event where we have a temperature inversion and the air just doesn't clean out, we don't get clean air moving through."
New research shows that in amounts smaller than previously thought, these fine particles can aggravate asthma, cause heart attacks and even bring vulnerable people to an early death.
As a result the Environmental Protection Agency is expected to lower the standard for the amount of fine particles in the air.
If the metro area can't meet the new standards, the state will have to respond with a plan to cut pollution. Neuschler of the MPCA will write the state's plan if the federal government comes up with a new standard that the metro area fails to meet.
Paul Aasen, the former head of the Minnesota Pollution Control Agency, says the federal government could put a cap on pollution. Until the area gets under the limit, it couldn't add anything that would make the problem worse.
"So you couldn't add new buildings, you couldn't add new roadways," Aasen says. "You'd have to do something if you're planning to do an addition to then somehow create a subtraction to balance it out in the big picture. So there's a real big economic potential here."
But it's not going to be easy.
The problem is not necessarily with the big sources of pollution like power plants or factories. There are a lot of smaller, spread-out sources, like cars and trucks, diesel generators and wood burners.
And unlike power plants and industrial facilities, there are no regulations in place to force these smaller sources to cut their emissions. So either the Legislature has to pass some new laws, or someone has to come up with voluntary and creative ideas.
One ideas is Project Green Fleet. Nine years ago, Minnesota faced a similar problem with a different pollutant: ground-level ozone. A non-profit group, Environmental Initiative, created a voluntary program to retrofit diesel trucks and school buses with pollution controls.
Executive Director Mike Harley says it accomplished a lot.
"At this point we've reduced emissions that would be the same as removing 350,000 cars from the road, so you can have a big impact with some of these collaborative, voluntary, proactive strategies."
Money for the program comes from industry and government.
Now, Environmental Initiative has brought together a diverse group of business leaders, environmental and health organzations, and government officials to try to come up with other ideas to meet stricter fine particle standards.
They will report to the state a year from now, and then they can start working on the next challenge -- another revision of ground-level ozone standards.
The EPA is scheduled to tackle ozone in 2013, and the Twin Cities will likely be unable to meet a more stringent standard.