Technologies to clean up traffic exhaust emissions may not be effective for some types of particle pollution, a study has found.

If you stand at the edge of any major road, each cubic centimetre of air that you inhale will typically contain tens of thousands of tiny particles, far smaller than the wavelength of light. The small size of these ultrafine particles (UFPs) makes them impossible to see. They are also missed by conventional techniques that measure air pollution, they are not covered by the UK’s air pollution laws and the new study suggests they are evading systems installed in even the most modern vehicles.

The researchers from the University of Birmingham analysed more than 10 years of air pollution measurements from alongside London’s busy Marylebone Road. They found some clear successes for cleanup technologies. There were big reductions in larger particles, including diesel soot, as newer vehicles with exhaust filters began to dominate. But the smaller UFPs showed no sign of improvement between 2014 and 2020.

The lack of improvement in UFPs from traffic stems from a mismatch between the way vehicles are tested before they can be sold – the vehicle approval test – and their real world use.

For particle pollution, the tests were designed about a decade ago to require manufacturers to fit the best cleanup technology available at the time. UFPs are difficult to measure reliably in a vehicle test laboratory since many of them only form as exhaust cools and mixes with the air. The approval system therefore excluded the smallest particles in the exhaust to get a reliable test.

Prof Roy Harrison from the University of Birmingham, who led the new study, said: “The low cleanup efficiency with respect to these small exhaust particles was not a surprise, but we had to wait to get several years of good data to be sure of what was happening. The size of the effect – essentially no reduction in the smallest particles – was a surprise.”

We cannot afford to ignore the UFPs. In 2021, the Dutch Health Council and the World Health Organization (WHO) highlighted the growing evidence that UFPs are damaging our health. This includes 75 studies; mostly relating to lung inflammation, blood pressure and heart problems, along with risks to foetal growth.

In 2021 the WHO also defined what should be considered high concentrations of UFP. This threshold is frequently breached at Marylebone Road and in locations worldwide with a lot of diesel traffic. Sales of fossil fuel cars are likely to continue beyond 2030 and we are still waiting for mass-market alternatives to diesel power for heavier vehicles. Exposure to UFPs alongside major roads looks set to continue unless governments mandate tighter exhaust standards, until we can transition away from internal combustion engines or reduce traffic.

The improvement in diesel soot particles in London was also seen throughout Europe. One example is Leipzig, Germany where there were reductions due to a combination of new vehicles and the city’s low emission zone.