The photochemical smog that forms on a sunny day over car-infested cities bears little resemblance to the soot-laden 'pe-souper' fogs of coal-burning Victorian London. But in a report in nature (September 10), Dr. Markus Ammann and colleagues from the Paul Scherrer Institute, Villigen and the University of Bern in Switzerland point a finger at one often neglected pollutant in this brew - simple old-fashioned soot - as an essential ingredient in the chemistry that produces modern photo-chemical smogs.