This is clever: to drive home the point of just how hard at work the heavily polluted air in Chicago is at the business of killing everyone, even The Oprah, stencil graffiti is created with a power washer on the sidewalks. Where the dirt isn't creates the message of where the dirt is. Some people in the Treehugger thread we're getting this from speculate that this is a Photoshop hoax, but it seem plausible and look okay to us.

Air Pollution Guerrilla Marketing in Chicago [Treehugger]