Diesel cars emit more air pollution on hot days

Emissions from diesel cars – even newer and supposedly cleaner models – increase on hot days, a new study has found, raising questions over how cities suffering from air pollution can deal with urban heat islands and the climate crisis.

Research in Paris by The Real Urban Emissions (True) initiative found that diesel car emissions of nitrogen oxides (NOx) rose by 20% to 30% when temperatures topped 30C – a common event this summer.

Emissions from a range of vehicles were found to be many times higher than those declared by manufacturers in laboratory tests, confirming earlier findings following the 2015 Dieselgate scandal, in which Volkswagen cars were found to emit 40 times more NOx on the road than during laboratory tests.

The latest findings will come as no surprise to Parisians, who returned to work last week after the long summer holiday. Car lobbies dismiss residents’ complaints about asthma and allergies as anecdotal evidence and contest city hall’s measures to discourage car use in the city centre.