Modern diesel cars produce more toxic emissions than trucks and buses, according to European researchers. That’s because heavy duty vehicles in the EU have much stricter regulations than cars, and so even if they meet lab tests, cars end up producing much more nitrogen oxides (NOx) when driven on actual roads.

Nitrogen oxides are released into the air from motor vehicle exhaust or the burning of coal and fossil fuels; they cause tens of thousands of early deaths across Europe every year, with Italy and the UK having the largest number of deaths. Many of these toxic emissions in Europe are produced by diesel cars, which are extremely popular in the EU.

The new report, released by the International Council on Clean Transportation (ICCT), shows that trucks and buses tested in Germany and Finland emitted about 210mg NOx per kilometer driven, less than half the 500mg/km produced by diesel cars that meet the highest “Euro 6” emission standards.

That’s because heavy duty vehicles are tested under much stricter regulations that were put in place in 2011. NOx emissions from trucks and buses are measured through mobile devices while the vehicles are being driven on actual roads. EU tests for cars instead are limited to emission measurements in the lab for prototype vehicles. The problem is that once driven outside the lab, under real-world conditions, vehicles tend to pollute more. The same stricter testing standards should be applied to cars as well, according to the researchers.

Today’s report comes in the wake of the Volkswagen emissions cheating scandal that broke in September 2015, opening up a heated discussion about exhaust emissions of NOx from vehicles. Changes to the ways cars are tested in the EU are set to begin in September, but the new testing regimes won’t be fully implemented until 2019. In view of today’s findings, the implementation process should be expedited, the researchers say.