These days, when we talk about emissions, it’s usually global warming and carbon dioxide we're talking about. But tailpipes and power plants also emit ozone, which is the main building block of smog and dramatically worsens local air quality. Have we licked that problem?

Nope, in fact Los Angeles — the poster child for bad air — had a very bad year in 2014, with 92 “unhealthful” days (up from 88 in 2013). That’s serious enough that you should avoid staying outside for very long. In places like Beijing and New Delhi, the air pollution has gotten so bad that even notoriously slow-to-act governments are taking notice. In Beijing, they're even taking cars off the streets.

air quality in beijingTrying to breathe in Beijing's Tiananmen Square. (Photo: Michael Davis-Burchat/flickr)

Some 23 million Americans suffer from asthma, and the condition is aggravated by breathing in bad urban air. Ozone is like sunburn on your lungs. The five smoggiest cities in the U.S. (and No. 8, too) are all in California. Other mega-bad places to breathe include Houston, Dallas, Las Vegas and Phoenix. Now you know why they’re really into zero-emission electric cars in California.

But we’re finding out something new about urban pollution. Professor James Donaldson of the University of Toronto has been conducting studies in his smoggy city and in grimy Liepzig (formerly East Germany). What he found is that the filth that turns statues (including the glorious sculptures of Rome) and buildings black — aka grime — isn’t staying put once it lands somewhere, as scientists had thought. When sunlight hits it, nitrogen-oxide compounds are released that go back into the air and make even more smog.

Trevi Fountain in RomeThe Trevi Fountain in Rome. You should have seen it before it was cleaned. (Photo: Christopher Chan/flickr)

Yes, it’s a particularly nasty feedback loop. “The current understanding of urban air pollution does not include the recycling of nitrogen oxides and potentially other compounds from building surfaces," says Donaldson “But based on our field studies in a real-world environment, this is happening. We don't know yet to what extent this is occurring, but it may be quite a significant, and unaccounted for, contributor to air pollution in cities.”

At a press conference, Donaldson announced he’s working with a professor in China, which has the world’s most polluted cities, to conduct similar experiments there. The tests involve putting out glass beads (which have a lot of surface area) to simulate windows, and then measuring the effect of sunlight on whether the grime stayed put or went airborne.

Donaldson said it’s not so much that we’ve been underestimating the extent of emitted nitrogen oxides (“the total amount is well captured”), but that we probably don’t understand how it performs on the ground.

Smog and ragweedThe relationship between smog and asthma is a close one. (Photo: Natural Resources Defense Council)

Either way, the solution doesn’t change: Stop emitting so much ozone. President Obama has proposed a major cleanup of power plant emissions, and has run into the expected wall of opposition. About 40 percent of the total emissions is from tailpipes, reports the EPA, and Obama also has a solution to that: improving auto fleet fuel efficiency to 54.5 mpg by 2025 and rigidly policing tailpipe emissions. Electric cars, of course, are zero emission, and that’s why so many governments — including California’s — are encouraging them.

In the meantime, if you live in one of those smoggy cities,the Natural Resources Defense Council recommends you should check air quality daily before venturing out, particularly if you or loved ones have allergies or asthma.

Here's a little mini-doc on smog in Los Angeles:

Jim Motavalli ( @jmotavalli ) writes about cars, technology and the environmental world to anyone curious enough to ask.