London’s ULEZ cut air pollution — high vehicle compliance left little room for post-expansion gains

People living, working and visiting London have seen substantial reductions in air pollution following the introduction of the Ultra Low Emissions Zone (ULEZ) introduced in 2019, according to a new research paper.

In a study published in npj Clean Air, researchers from the University of Birmingham have created a sophisticated model for assessing the direct impact of ULEZ on air pollution in the Greater London area.

The team found that there were significant reductions in nitrogen-based pollutants NO₂ and NO_x following the introduction of ULEZ in 2019 that extended beyond the geographical boundaries of the zone, including areas that were covered by the ULEZ expansion in 2023.

The study found that:

• NO₂ fell by 19.6% at roadside sites in central London within three months of ULEZ1 in April 2019,

• NOx fell by 28.8% in the same period for the same area,

• No significant impact was detected on NO₂ or NO_x following ULEZ expansion in 2023, and

• NO₂ and PM₂.₅ (fine particles that can enter into our lungs) pollution remains well above WHO guidelines across London.

The team analysed Transport for London data on non-compliant vehicles - those that do not meet emission standards and must therefore pay a daily charge to drive within the zone - which shows that the proportion of such vehicles operating in Central London fell from 39.1% at the time of ULEZ introduction in 2019 to 27.5% within three months of its implementation.

The composition of London’s vehicle fleet continued to change in the following years. By the time the ULEZ expansion was introduced in 2023, only 7.4% of vehicles on the road across London were classified as non-compliant. Three months later, this figure had fallen further to 4.2%.

Chengxu Tong, a PhD student from the University of Birmingham and first author of the study said: “The introduction of ULEZ in central London in 2019 has been effective in improving air quality. Importantly, our analysis reveals that these benefits are not confined to the designated zone, but extend beyond its boundaries, indicating a wider spill-over effect.”

Machine learning

Using machine learning, the team were able to remove the potential impacts of the weather on variations in air pollution during the time of the study. 124 sites across London captured hourly air pollution data across multiple years that enabled the research team to look at the introduction of ULEZ (called ULEZ1 in the paper) in 2019, and the major expansion of ULEZ (called ULEZ3) in 2023.

Professor Zongbo Shi from the University of Birmingham, who oversaw the study, said: “When ULEZ was introduced in central London, there was a rise in the number of compliant vehicles on the road. This contributed to the spillover effect on air quality beyond central London. Furthermore, the commitments to expansions may have encouraged earlier transitions to cleaner vehicles, which likely explained the limited additional impacts of 2023 ULEZ expansion on air quality. This is known as “anticipation effect” – where the benefits of a policy are already being seen before its formal implementation.

“Here, we showed that ULEZ is an important step, but it is not enough on its own. London still faces air pollution levels well above WHO health-based guidelines, requiring coordinated actions across multiple sectors, including from industrial, commercial, residential and agricultural sources.”

Dr Suzanne Bartington, an Associate Professor from the University of Birmingham and a senior co-author of the study said: "While it is encouraging that ULEZ did reduce NO₂ and NO_x pollution across London over the study period, it is an ongoing cause for public health concern that London and many major cities around the world.

“It’s important to highlight that the current ULEZ approach does not fully address significant traffic related public health issues, such as PM_2.5 pollution. As a result, we need to see a modal shift to more active travel and public transport to reduce the number of vehicles on the road, which could reduce non-tailpipe-related PM_2.5 emissions and improve public health.

The study was partially funded through WM-Air, supported by the Natural Environment Research Council’s Regional Impact from Science of the Environment (RISE) initiative. WM-Air has been working with partners to bring research organisations together with businesses, policy bodies and other actors contributing to economic development specific to their location, to deliver significant regional impact from NERC environmental science. Previous studies from WM-Air have highlighted major contributions of domestic woodburning to PM_2.5 emissions, and that air pollution in the West Midlands has caused up to 2300 premature deaths each year.