Diesel cars cause more than twice the air pollution of buses and lorries

In fact, small private diesel-run vehicles give off more than double the about of Nitrous Oxide (NOx) as multi-person communal transport vehicles using the same type of fuel.

When the ICCT carried out research which showed discrepancies between emission data recorded in laboratories and 'real' emissions on the roads, the results sparked further investigations which unmasked the Volkswagen 'Dieselgate' scandal.

Data from Germany and Finland were used to show that cars using the 'Euro 6' type of diesel – which complies with current air-pollution legislation – generated between 480 and 560 milligrams (mg) of NOx per kilometre, whilst buses and lorries using the exact same diesel type only let off 210mg per kilometre.

The ICCT report concludes by stressing the need for road tests to calculate emissions, in addition to the standard laboratory tests, as the only way of assessing true NOx levels generated in order to prevent or reduce them.

These emissions tests will start to be introduced into European Union countries from September this year for private vehicles, and are already compulsory for lorries.

The German government declined to comment on the ICCT's report, but recalled that it has been working on a 'cross-European level' since 2011 to install emission-readers on roads to complement the tests already carried out during the manufacturing process.

The present emissions limit for cars using the Euro 6-type diesel is just 80mg per kilometre, meaning those measured by the ICCT are up to six or seven times over the legal maximum and would not have been allowed onto the forecourt if on-road NOx tests had been carried out when they were manufactured.