London pollution has improved with evidence for small initial ULEZ effect: study

by Caroline Brogan

16 November 2021

London’s ULEZ reduced the city’s nitrogen dioxide levels by a few per cent during the first few weeks of its implementation.

This is according to a study by Imperial College London researchers who say their findings highlight that ULEZs are not a silver bullet and that sustained improvements in air pollution require multiple measures.

Between 2016 and 2020, the number of Londoners living in areas with illegally high levels of nitrogen dioxide fell by 94 per cent, and alongside this there were other reductions in London’s air pollution. New research from Imperial has found that changes in air pollution around the introduction of the ULEZ in April 2019 were small in comparison to these longer-term improvements.

The researchers used publicly available air quality data to measure changes in pollution in the twelve-week period from 25 February 2019, before the ULEZ was introduced, to 20 May 2019, after it had been implemented. They controlled for the effects of weather variations, and then used statistical analysis to look for and quantify changes in pollution.

ULEZ...works best when combined with a broader set of policies that reduce emissions across sectors like bus and taxi retrofitting, support for active and public transport Dr Marc Stettler Department of Civil and Environmental Engineering

The researchers behind the study, published in Environmental Research Letters, say it highlights the importance of combining a wide set of effective clean air measures that could encompass local actions like reducing transport emissions and use of wood burning stoves, and regional policies that reduce, for example, agricultural emissions. This is particularly important for cities that are considering introducing an ULEZ, and it comes as London’s ULEZ was expanded to create a larger zone in October 2021.

Corresponding author of the research Dr Marc Stettler, from Imperial’s Department of Civil and Environmental Engineering and Centre for Transport Studies, said: “Our research suggests that a ULEZ on its own is not an effective strategy to improve air quality – the case of London shows us that it works best when combined with a broader set of policies that reduce emissions across sectors like bus and taxi retrofitting, support for active and public transport, and other policies on polluting vehicles.”

Air pollution caused 40,000 deaths in the UK in 2019 – around 4,000 of which were in Greater London. Worldwide, outdoor air pollution accounts for around 4.2 million deaths per year due to conditions such as stroke, heart disease, lung cancer, and acute and chronic respiratory diseases.

In April 2019 the Mayor of London introduced the ULEZ, an area in which drivers of more polluting vehicles must pay a daily charge, with the aim of reducing air pollution emissions from road transport and accelerating compliance with EU air quality standards. The ULEZ is one of several London air pollution policies introduced since 2016 like the Low Emission Zone, Low Emission Bus Zones, and bus and taxi electrification.

Monitoring air quality

To carry out the study, the researchers analysed air quality data from roadside and non-roadside air quality monitors across London, comparing data over a twelve week period from before and after the ULEZ was introduced.

They found that, compared to the overall decrease in London’s air pollution levels, the ULEZ caused only small improvements in air quality in the weeks following its start date: an average reduction of less than 3 per cent for nitrogen dioxide concentrations, and insignificant effects on ozone and particulate matter (PM2.5) concentrations.

They also found that the biggest improvements in air quality in London in fact took place before the ULEZ was introduced in 2019.

Cities considering air pollution policies should not expect ULEZs alone to fix the issue as they contribute marginally to cleaner air. Dr Marc Stettler Department of Civil and Environmental Engineering

They detected changes in levels of nitrogen dioxide and ozone at 70 per cent and 24 per cent of the monitoring sites around the time that the ULEZ was introduced, respectively. Among these sites, changes in air pollution varied quite significantly and at some sites pollution actually worsened, with relative changes ranging from -9 per cent to 6 per cent for nitrogen dioxide, -5 per cent to 4 per cent for ozone, and -6 per cent to 4 per cent for PM2.5.

The researchers suggest that other cities considering implementing these schemes should consider them alongside a combination of other measures. Dr Stettler said: “Cities considering air pollution policies should not expect ULEZs alone to fix the issue as they contribute marginally to cleaner air. This is especially the case for pollutants that might originate elsewhere and be blown by winds into the city, such as particulate matter and ozone."

“Since the London ULEZ was introduced, similar schemes have been introduced in Bath, Birmingham and Glasgow, yet on a much smaller scale. Several other cities have plans to implement clean air zones and our findings could contribute to the development of their policy.”

“Has the Ultra Low Emission Zone in London improved air quality?” by Liang Ma, Daniel J. Graham and Marc E. J. Stettler, published 16 November 2021 in Environmental Research Letters.

The researchers looked at the Ultra-Low Emissions Zone’s effect on air quality over a 12-week period around the implementation of the first ULEZ scheme in April 2019. The findings pertain to the short-term changes in air pollution that resulted from the ULEZ becoming active. This approach enabled them to isolate the effects of the ULEZ from other changes or policies that may have affected air pollution in London over a longer period of time.

They found that at some locations air pollution did improve in immediate response, with reductions of up to 9 per cent for nitrogen dioxide (NO2). However, when they looked at the changes across all monitoring sites in London, the changes for NO2 were less than 3 per cent overall and insignificant for PM2.5.

The researchers say this suggests that road users perhaps anticipated what they had to do before the ULEZ came into effect (for example upgrade their vehicle or use an alternative mode of transport), and others responded only after the scheme was implemented.

The study looked at the implementation of the zone itself but also shows that there have been longer-term improvements in air pollution in London, with about a 6 per cent reduction in NO2 per year in the period from 2016 to 2019.

The researchers say that London has been relatively successful at improving air quality because of a coordinated set of policies, some of which have been wrapped up into the ULEZ messaging and enforcement. Examples of other policies introduced during this time, as part of the Mayor of London’s wider actions on air pollution, include the upgrading of the bus and taxi fleets in London. They say the ULEZ has helped raise the issue of air quality up the agenda and been a catalyst for change that may not have otherwise happened.