October 22, 2025

expert reaction to the assessment of the direct impact of ULEZ on air pollution in the Greater London area

An assessment published in npj Clean Air looks at the direct impact of ULEZ on air pollution in the Greater London area. 

Dr James Weber, Lecturer in Atmospheric Radiation, Composition and Climate, University of Reading, said:

“This study shows one of the better ways to evaluate the impact of a policy, rather than just comparing what happened before and after. It uses a method that’s also been used to study other low-emission zones. The bigger drop in nitrogen oxides (NOx) compared to fine particles (PM2.5) makes sense because traffic is a major source of NOx. The smaller change in levels of fine particles suggests that they come from many different sources as well as traffic, so reducing them requires a more complex approach than just targeting emissions from vehicle exhausts. The differences between the results from the method used in this study, and those from earlier studies using other techniques, show why it is important to use more than one method when evaluating policies.”

Prof Chris Griffiths, Professor of Primary Care, University of Oxford and Queen Mary University of London (QMUL), said:

“This study is a welcome addition to the literature showing impacts of London’s ULEZ on air pollution levels in London. Differences exist between reports depending on the methods used. Most underestimate impacts by not including ‘pre-compliance’ (or anticipatory effects), whereby drivers start to change driving habits and move to less polluting vehicles well in advance of implementation of restrictions.

“The authors point make important points: NO2 and PM2.5 levels remain above the World Health Organisation’s target levels in outer London and further action is urgently needed to attain these levels. The ULEZ has wider societal effects that go beyond changes in pollution exposures, including switches to active travel. From other large scale analyses its clear there are no safe levels of air pollution.”

Dr Chloe Brimicombe, Climate scientist and public engagement manager, Royal Meteorological Society (RMetS), said:

“This paper demonstrates how the ULEZ has been a positive policy for reducing air pollution below international city air pollution standards.

“Going forward more can be done around indoor air pollution, including mould, inequities around who is the most exposed to higher levels of pollution and how air pollution effects long-term health conditions in the UK.

“The study makes use of a modelling framework built around a counter-factual where they compare what the air pollution was like before the policy Vs after. This is a standard modelling framework used in lots of studies but is only as accurate as an indication of the real world.

“We cannot conclude on if the new emissions zone was more successful than the previous policy from this study, this could be of interest for future work.” 

‘Further improvement in London’s air quality demands more than the Ultra Low Emission Zone policy’ by Chengxu Tong et al. was published in npj Clean Air at 10:00 UK time on Wednesday 22^nd October. 

DOI: https://doi.org/10.1038/s44407-025-00030-9

Declared interests:

Dr Chloe Brimicombe: No conflicts to disclose. 

Prof Chris Griffiths: I receive funding from NIHR to evaluate health impacts of London’s ULEZ. 

Dr James Weber: No conflicts of interest. 

For all other experts, no reply to our request for DOIs was received.