CC/Flickr/Paul Carroll Overall, the region stretching from the Twin Cities to St. Cloud went from 36th most polluted urban area in the country to 42nd place among the 235 places that were tabulated.

On Wednesday the American Lung Association issued its annual State of the Air report, and in general Minnesota’s cities ranked just a bit better than they had in the 2012 statistics for levels of ozone and soot.

Overall, the region stretching from the Twin Cities to St. Cloud went from 36th most polluted urban area in the country to 42nd place among the 235 places that were tabulated.

A separate analysis of small-particle pollution — soot, that is — showed the Twin Cities improving from 104 to 116 out of 220 metro areas. Rochester’s ranking moved slightly in the other direction, from 142 to 140.

Both Duluth and Rochester ranked among the cleanest U.S. cities for ground-level ozone, and Duluth also did well on soot.

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Last year’s headlines were about Ramsey County getting a failing grade on particulate pollution because it posted 10 days of “orange alerts” — the least severe among three alert levels, indicating soot levels that could be unhealthy for people with asthma and other sensitivity.

This year’s report shows just 7 of those alerts, which moved Ramsey County from an F to a D in the gradebook. (A look at the association’s national findings is here; an interactive presentation of the data is here; and the full report in PDF format is here.)

Curious to know more about how the grades are calculated and how they should be interpreted, I called up Robert Moffitt, the association’s longtime spokesman in Minnesota. Following are excerpts from our conversation:

MinnPost: Do these grades mean something as literal as they would in, say, a school setting — does an F mean failing by EPA standards, a D almost failing, and so on?

Robert Moffitt: They don’t necessarily equalize to federal standards per se. They’re meant to give you a general snapshot of where were going, what the trends are: Is the air getting better? Is getting worse? Are we having more air alert days for both ozone and high particle pollution?

It’s not necessarily there to show that we’re about to go out of attainment status — although, if you continue a pattern of bad grades, you’re not on the right path.

MP: Do the grades line up with percentiles, then, or some other standardized measure?

RM: There’s a detailed explanation of the methodology in the full report, but the simplest way to say it is they do a count for the number of alerts for 24-hour periods for particulates and ground-level ozone, and they also have a weighted average that they use to get to that grade for each of the two pollutants.

MP: If we extend the letter-grade metaphor a bit, if you’re holding a teacher-parent conference with Air Quality’s concerned parents, what would you tell them about how we got to this report card and what needs to happen to make it better?

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RM: I would tell them that in some areas we seem to be doing very well, particularly in ozone, where we consistently get good grades. But particulate pollution is more of a concern.

I would also say that perhaps were not getting enough tests to really know what’s going on. There are roughly 18 air monitors in the state of Minnesota. I would love to see one in every county, personally.

Particle pollution is the area we need to focus some work on. One thing we’ve learned in recent years is that the small particles — much smaller than a human hair — are much more harmful to human health than we had first realized.

They tend to be breathed deeply into the lungs and the respiratory system, so even healthy adults can suffer ill effects. But it’s particularly bad for people who have existing respiratory diseases like asthma, chronic obstructive pulmonary disease, heart issues.

We don’t want to have to go through another great smog to get our wake-up call, like London did in the 1950s — and we’ve had that happen in the U. S. as well. There was a small factory town in Pennsylvania that had a very similar situation.

The good news is, since the Clean Air Act was passed, our air actually is getting better. Sometimes the grade does not always reflect that, but of course our standards have gotten tighter, too, which is the way the Clean Air Act was written. They knew that they didn’t know everything they needed to know, in 1970, about the science of air pollution and human health.

MP: I think your Pennsylvania town was Donora, and if I remember correctly that event involved a smelter or other big smokestack source and an inversion. But that kind of heavy pollution from big smokestacks isn’t really such a problem anymore, is it?

RM: Yes, but the situation today is much more complex to deal with, and not as visible as a single tall smokestack.

Now you have millions of cars driving around, and trucks, with longer waiting times in traffic congestion. You’ve got backup diesel generators, volatile organic compound sources, dry cleaners, paint shops, you name it.

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So with the mobile sources, you’ve dispersed the pollution somewhat, but they are still somewhat concentrated in a fairly small area.

MP: Ramsey County’s grade for high-particulate days has gone from a B in 2009 steadily downward to an F last year, then back to D this year. What explains that — more pollution or stricter standards?

RM: Most of it is more particulates in the air, but the other X factor is weather. The pollution sources are always there; it’s the weather that changes. And with changes in weather patterns we can get a lot more of those alert days, and that that can definitely change the grade.

Ramsey County has made pretty good improvements. Had they gotten one less alert day they would have earned a C. So you might call this year’s grade a D+.

And it’s kind of unfair to single out Ramsey County. I’ve talked with David Thornton at the Minnesota Pollution Control Agency and he can’t really give a good explanation why Ramsey County has a D here and the same trend hasn’t necessarily shown up in the other counties. It might just be an issue of where the air monitors are placed.

I know that Ramsey County and the city of St. Paul take air quality very seriously. They’ve been working, and we’ve been working with them, on a number of initiatives to make it better.

MP: How much of a contributor is that increasingly controversial source, home wood burning in fireplaces, stoves and backyard pits?

RM: Just looking at the science of what comes off of wood smoke, there’s no denying it’s a fairly polluting fuel source. It puts a lot of different substances into the air.

What we’re seeing in the Twin Cities is more of a social phenomenon, a marketing phenomenon — hardware stores and others have pushed fire rings, patio fires and things like that, and more people are doing that.

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People say, what’s the big deal with one little fire in one back yard, but multiply that by thousands of people — and a lot of them are not burning good, seasoned firewood the way they should. I can speak from personal experience, from a neighbor of mine who liked to burn garbage and old bits of pallets in his fire ring.

I think there’s a fair amount of abuse that takes place with city ordinances allowing for recreational fires. But how do we deal with that?

I think it will be a combination of public education and then grassroots efforts, like the indoor smoking ordinances. I think we’ll see it change city to city, county to county, before anything is mentioned on a state level.

MP: Generally speaking, Minnesota’s location has seemed to spare us the ozone problems that other parts of the country experience, but some of the metro-area ozone grades are going down, too. What’s driving that?

RM: The grades for this year, for ozone, are a B for Anoka and Washington, down from an A. Scott County had an A both years.

We do not have data for Dakota, Hennepin and Ramsey, and that’s not unusual, because ozone forms more often not in the urban core but in the suburbs. There’s no monitor at all in Carver County.

So both Anoka and Washington have slipped down a bit, which in practical terms means there was one air alert for ozone in a three-year period. That’s all it takes to go from an A to a B.

We’re never going to have ozone problems of the severity that other places will, because weather is such a factor. But if we see changes in our climate, even slight changes in general temperature, what effect that will have I can’t really say but I don’t think it bodes well.

I often tell people that I worry more about good grades than really bad ones sometimes because they give people a false sense of security — you know, mission accomplished, we’ve got the problem solved, no need to do anything.

The status quo is really our enemy here.

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Bonus link: Home page of the Donora Smog Museum and its sponsor, the Donora Smog Commemorative Committee.