In honor of Earth Day week, here's some downer news for all of the air breathers out there: The American Lung Association's recent ranking of the Most Polluted Cities in America puts the Houston-Baytown-Huntsville area as No. 7 for worst ozone, and Harris County as No. 10 among counties for the worst air quality.

(Dallas-Fort Worth falls just behind at No. 8 and Beaumont-Port Arthur ranks No. 20 on the list. Los Angeles, of course, took the highest honor, followed by five other California spots.)

This bad air news comes on the heels of Houston being named (somewhat surprisingly) one of the Greenest Cities in America.

Before we go to pointing fingers at the nearby refineries for the bad air, it's prudent to also take an long, hard look at our commuter culture. In an interview with KUHF, Air Alliance Houston executive director Adrian Shelley points to cars as a main culprit.

"We are issuing, say, 30 to 40 ozone warnings every year, and those are days in which the ozone levels are getting high enough to be of concern to the general public," he says, acknowledging that even an increase in fuel-efficient autos won't make a different if the vehicle volume is higher on the roadways.

Want to know whether it's safe to step outside? Check out the Houston Clean Air Network's website or mobile ozone map app (available on iTunes and Android).