A fine mess
Exhaust filters failing to cut enough pollution
Car exhausts systems are still failing to meet set targets for removing particulates.
While the filters fitted remove the majority of larger, solid particles, they are less effective at removing ultrafine particles.
Filters, designed to mitigate this pollution, have been a legal requirement in new cars since 2011. However, new research reveals it is failing to achieve the cleaning of gases expected.
Airborne particles from vehicle emissions are a major contributor to air pollution levels. This new study, published in Environment International, shows the filters are less effective at removing smaller liquid particles.
So far, the World Health Organisation has not yet set a guideline for safe levels of ultrafine particles. However, it recognises that particulate pollution overall is associated with negative impacts on cardiovascular and respiratory health.
Local roadside pollution issues have become dominant in news headlines over recent years as the negative effects become more evident.
Air quality guidelines published by WHO in 2021 also outline concerns over ultrafine particles and their ability to be transported around the body.
Improving, but not fast enough
Professor Roy Harrison is the lead author on the study.
“Our research shows clearly that current, widely-used filters are not effective against these smaller particles,” states Harrison. “We welcome recommendations from the World Health Organisation that surveillance of these measurements increase and note with concern that current concentrations measured in London are classified as ‘high’.”
The team used data collected in from a monitoring station in Marylebone Road, in London. Air quality sampling at this site has produced a long-term dataset, containing data for particle mass and number dating back to 2010.
This data showed a steep decline in larger particles. Black carbon, for example, declined by 81% between 2014 and 2021. It shows there has been a positive impact from the introduction of exhaust filters.
In contrast, however, the number of particles described as ‘ultrafine’ – smaller than 100 nanometres – reduced by only 26%, analysis of the data suggests.
The smallest group of particles, measuring less than 30 nanometres, did not reduce at all, indicating that filters are not effective against these types of particle, says the study.
Zero emissions needed
WHO guidelines define concentrations of ultrafine particles above 10,000 per cubic cm as “high”.
Concentrations measured at the Marylebone Road site were around twice this level.
Professor Harrison added: “High concentrations of ultrafine particles are likely to be a widespread and persistent phenomenon.
“In order to meet WHO guidelines we are likely to need a much higher uptake of electric vehicles, as well as additional measures to reduce emissions from diesel vehicles.”