After the crystal clear days we've had in Chicago this week, it's hard to believe that the EPA has put us on a national list of smog offenders, but yep, they did. In 1997 new rules were enacted as part of the Clean Air Act and cities across the nation were given seven years to comply. Apparently that wasn't enough time for us.
To be technical for a minute, the new rules affect soot, which are the particulates released by big engines and smokestacks, as opposed to smog, which is the nitrogen oxide and sulfur oxide molecules (greenhouse gasses) that harm the ozone, create acid rain and make it hard to breathe.
Fortunately Chicago is 14th on the list of offending cities, just behind Weirton, West Virginia. That's not saying much, since Chicagoist has been to Weirton, and it isn't much more than a steel mill nestled in a valley. Kind of a natural pollution maker.