London’s 2019 emissions policy quickly cut air pollution, but may leave little room for further gains

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

In a study published in npj Clean airresearchers from the University of Birmingham have created a sophisticated model to assess 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.₂ and NO_x following the introduction of ULEZ in 2019 which extended beyond the geographical boundaries of the area, including areas covered by the ULEZ expansion in 2023.

The study found that:

• NO₂ fell by 19.6% at roadside sites in central London in the three months following ULEZ1 in April 2019.

• NO_x fell by 28.8% over the same period for the same area

• No significant impact was detected on NO₂ or NO_x following the expansion of ULEZ in 2023

• Pollution by NO₂ and PM₂,₅ (fine particles that can enter our lungs) remains well above WHO guidelines in London.

The team analyzed Transport for London data on non-compliant vehicles, i.e. those that do not meet emissions standards and therefore must pay a daily charge to operate in the area, showing that the proportion of these vehicles operating in central London fell from 39.1% at the time of ULEZ’s introduction in 2019 to 27.5% in the three months following its implementation.

The composition of the London car fleet continued to evolve over the following years. By the time the ULEZ extension was introduced in 2023, only 7.4% of vehicles traveling across London were classed as non-compliant. Three months later, this figure had fallen further to 4.2%.

Chengxu Tong, a Ph.D. A University of Birmingham student and first author of the study said: “The introduction of ULEZ in central London in 2019 was effective in improving air quality. Importantly, our analysis reveals that these benefits are not limited to the designated area, but extend beyond its boundaries, indicating a broader ripple effect.

Using machine learning, the team was able to eliminate the potential impacts of weather on variations in air pollution during the study. 124 sites across London captured hourly air pollution data over several years, allowing the research team to examine 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 an increase in the number of compliant vehicles on the road. This has contributed to the knock-on effect on air quality beyond central London. The so-called “anticipation effect”, i.e. the benefits of a policy are already visible before its formal implementation.

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

Dr Suzanne Bartington, Associate Professor at the University of Birmingham and co-lead author of the study, said: “While it is encouraging that ULEZ did indeed reduce NO₂ and NO_x pollution in London during the study period, it is a continuing cause of public health concern in London and many major cities around the world.

“It is important to highlight that the current ULEZ approach does not fully address traffic-related public health concerns, such as fine particulate matter._2.5 pollution. As a result, we need to see a modal shift towards more active travel and public transport to reduce the number of vehicles on the road, which could reduce non-tailpipe particles._2.5 emissions and improve public health.