The EQUA Air Quality Index contains 450 petrol and diesel vehicles, of which around a third are Euro 6 models and the rest Euro 5, and it will keep adding results in the coming months.

Vehicles are grouped into eight ratings based on NOx emissions in grams per kilometre, as below, and users can sort and search the results my manufacturer, fuel type and rating.

The data is independently funded, based on tests carried out using a Portable Emissions Measurement System (PEMS) attached to the car during a road route to analyse exhaust content including NOx emissions.

Emissions Analytics says it wants the index to complement forthcoming changes to the European test cycle, which come into force next year and are aimed at addressing the gap between laboratory testing and real-world performance.

To see the database, visit: equa.emissionsanalytics.com