Greenpeace activists demonstrate in Stuttgart for cleaner air. A new Greenpeace study shows that air pollution in German cities is more problematic than previously known. Greenpeace measured high levels of nitrogen oxide in several cities, which is mainly caused by diesel motors. With a banner hanging near one of Germany's busiest roads, Neckartor, the environmental activists demand a ban for diesel-run vehicles in cities. The banner reads 'Cities want to breathe- ban dirty diesel'.