Today's launch of the Ultra Low Emission Zone (ULEZ) in central London is a welcome step on the road to cleaning up urban air quality but will not stop 'dirty diesels' from entering the capital despite controlling access based on emissions, says publishers of the AIR Index. The AIR (Allow Independent Road-testing) Index is an independent vehicle emissions rating for cars from A (the best) to E (the worst) that provides the on-road urban NOx emissions using the same independent test on every vehicle, to show the relative impact a specific vehicle has on air quality. However, AIR's own testing has found that the Euro emissions standards bear little resemblance to what comes out of a car's tailpipe in urban driving. There is huge variation in Euro 6 diesels, and although the latest are very clean there are still millions of dirty diesels on European roads that can emit in excess of Euro standards for urban NOx, yet still gain free access to ULEZ. The AIR Index is calling on policy makers in London and across Europe to make clean air zones more effective by tackling the issue of dirty Euro 6s - using ratings such as the AIR Index- to bring air quality into legal compliance as soon as possible. To learn more, we spoke to Nick Molden, co-founder of the AIR Index.