Vehicle exhaust filters failing to prevent the emission of the smallest, most dangerous particles

New research lead by Professor Roy Harrison at the University of Birmingham has found that filters designed to prevent the emission of particulate matter from vehicle exhausts are less than adequate at preventing the emission of ultrafine particles. 

The Department of Transport have this to say about the filter, which became a legal requirement in new cars from 2011 and lorries from 2013: ‘Diesel particulate filters (DPF) are devices fitted to diesel vehicles which filter particulate matter from exhaust gases. Diesel particulate filters perform an essential role in reducing air pollution and its health effects, their removal is almost always illegal.’

a street filled with lots of traffic next to tall buildings

Experiments showed that while the filters effectively removed larger particulate matter, they are not as effective at stopping smaller, liquid particles.

Ultrafine particles are particles with a size smaller than 0.1µm, compared to the 10µm and 2.5µm we normally discuss. They are a by-product of the incomplete combustion of organic materials such as wood and diesel and in June 2012 The WHO confirmed that ultrafine diesel engine emissions are carcinogenic and the leading cause of premature death from environmental causes. They are in the same category of risk as arsenic and asbestos, which have been banned in the EU for many years.

The team examined data from a monitoring station in Marylebone Road, London which has been sampling air quality since 2010. This revealed a steep decline in larger particles such as black carbon which declined by 81% between 2014 and 2021.

In contrast however, the number of particles described as ‘ultrafine’ – smaller than 100 nanometres (1 nanometre = 0.000001mm) fell by only 26%. The smallest group of particles, measuring less than 30 nanometres, did not reduce at all, giving a clear indication that filters are not effective against these types of particle. WHO guidelines define concentrations of ultrafine particles above 10,000 per cubic cm as ‘high’ and the concentrations measured at the Marylebone Road site were double this.

Professor Harrison, said: ‘Our research shows clearly that current, widely-used filters are not effective against these smaller particles and 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’.

‘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.’



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Toby S-C
Toby S-C
1 year ago

The conclusion drawn from the data is flawed in this instance. Data is taken from monitoring stations which sample the air around it. The only way to determine the effectiveness of a Diesel filter is to actually test the Diesel filter. There are many more factors not accounted for, such as the vehicle mix (Diesel cars make up a smaller proportion of sales in 2021 – 12% than they did in 2014 – 36.2% REF: Vehicle licensing statistics: 2021 – GOV.UK ( / Vehicle licensing statistics: 2014 ( and doesn’t account for an increase in ultra fine particles released from increased brake pad and tyre wear resulting from increasing average car weights (REF: Brake and tire particles measured from on-road vehicles: Effects of vehicle mass and braking intensity – ScienceDirect). A hybrid or battery electric car weighs more than the equivalent petrol or diesel.

Last edited 1 year ago by Toby S-C
Andy Eastlake
Andy Eastlake
1 year ago

Our ongoing work and data shows the particulate filters (both OEM and retrofitted) are indeed effective at reducing the ultrafine PN levels. We must remember that petrol vehicles from Euro 4 and 5 (all eligible for ULEZ) did not have a PN limit and in Euro 6 it only applied to GDI (direct injection) petrol, so the changing mix of vehicles may be a more significant source of PN than Diesel filters which were mandatory for all Euro 6 / VI diesel vehicles (but not necessarily for gas ones)

1 year ago

This isn’t good, is it? Thank you for the article, Paul. Can the filters be improved upon perhaps? If diesel fuel emits these ultrafine particles, what about biomass boilers and ecodesign wood stoves? Do they do the same? Can Professor Harrison tell us about this too, please? And what about Drax at Selby?

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