Photograph of the "Particle Falls" art installation on the outside of the Wilma Theater. Created by environmental artist Andrea Polli, the colors broadcast onto the wall are determined by the amount and types of particulate matter in the air at any given moment. Taken at the opening for the 2013 Particle Falls installation at the Wilma Theater, Philadelphia, part of the Sensing Change exhibit organized by the Chemical Heritage Foundation, Philadelphia, PA, USA, 26 September 2013.