Diesel vehicles produce lots of soot (particulate matter) that can cause respiratory problems and contribute to the risk of cardiovascular diseases. All diesel cars manufactured since 2009 are fitted with a Diesel Particulate Filter (DPF) in the exhaust to stop this soot passing into the atmosphere.
DPFs need to be cleared of soot on a regular basis in order to work properly. This happens automatically in a process known as ‘regeneration’ when the exhaust temperature is high enough, for instance, when driving on motorways or fast A-roads. The collected soot is burnt off, leaving only a small ash residue. The ash can’t be removed – unless the DPF is removed from the vehicle and sent away for specialist cleaning – but a DPF in a car used correctly should be good for well over 100,000 miles.
Regeneration brings its own problems. Initiated every 300 miles or so, it causes diesel vehicles to eject quantities of polluting particles into the atmosphere at over 1,000 times their normal rate.
According to the World Health Organisation (WHO), particle pollution affects more people than any other pollutant, with 77% of the inhabitants of European cities exposed to levels above WHO guidelines.
Ultrafine particles (smaller than the size of a typical virus) could be the most dangerous, as they can penetrate deep into the body. They are emitted in large amounts by internal combustion engines and have recently been linked with some types of brain cancer. Currently only solid particles which are larger than 23nm in diameter are regulated.
In 2019, European green campaigners Transport & Environment carried out tests on two popular diesel cars, taking smaller particles into consideration. When solid particles as small as 10nm were measured total emissions increased by between 11-184% compared to when only regulatory particles were measured. This means that large amounts of particle pollution are completely neglected from a regulatory point of view, despite potentially being the most harmful to human health. So even the newest diesel cars manufactured after the introduction of the latest Euro 6 emission testing regime are producing excessive levels of particulates during DPF regeneration.
Source: European Federation for Transport and Environment