Is reverse graffiti the same as conventional graffiti, since removing the images can be difficult? It ought to depend on whether paint, or just car exhaust, is removed, and whether the reverse writing is potentially offensive. Police in the fast-growing Brazilian city of Porto Alegre seemed to agree when they came upon these reverse taggers at work in a traffic tunnel. Vehicles are the biggest source of air pollution in Porto Alegre, so the message is right on target.