People living, working and visiting London have seen substantial reductions in air pollution following the introduction of the Ultra Low Emissions Zone (ULEZ) in 2019, according to new research published today.
A study published in npj Clean Air today reveals that the capital experienced significant drops in nitrogen-based pollutants after ULEZ was first introduced, with benefits extending well beyond the zone’s geographical boundaries. However, the research found no measurable improvements following the scheme’s major expansion in 2023, presumably because vehicle compliance had already reached high levels by that point.
Researchers from the University of Birmingham created a sophisticated model to assess ULEZ’s direct impact on air pollution across Greater London. Their analysis found that nitrogen dioxide (NO₂) fell by 19.6% at roadside sites in central London within three months of ULEZ introduction in April 2019, while nitrogen oxides (NOx) decreased by 28.8% over the same period.
The reductions extended beyond the initial ULEZ boundaries into areas that would later be covered by the 2023 expansion. However, when that expansion was implemented, no significant impact on NO₂ or NOx levels could be detected. The study notes that NO₂ and PM₂.₅ pollution remains well above World Health Organization guidelines across London.
The research team analysed Transport for London data on non-compliant vehicles, which must pay a daily charge to drive within the zone. These figures reveal a dramatic shift in London’s vehicle fleet composition. At ULEZ’s 2019 launch, 39.1% of vehicles operating in central London were non-compliant. Within three months, this had fallen to 27.5%.
By the time of the ULEZ expansion in 2023, only 7.4% of vehicles across London were classified as non-compliant, dropping further to 4.2% three months after expansion. This high level of compliance before the expansion may explain why no additional pollution reductions were detected.
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’
The research team used machine learning techniques to analyse hourly air pollution data from 124 sites across London over multiple years, controlling for weather variations that could affect pollution levels. This methodology allowed them to isolate the specific impact of both the initial ULEZ introduction and its subsequent expansion.
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 NOx 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 PM2.5 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 PM2.5 emissions and improve public health.’
The full research can be read here.
Leave a Reply