London’s ULEZ led to just a small air quality improvement

London’s Ultra Low Emission Zone (ULEZ) led to only a small improvement in air quality in the first few weeks, new research suggests. 

Researchers at Imperial College London used publically available data to measure changes in pollution in the twelve-week period from February 2019, before the ULEZ was introduced to May 2019, after it had been implemented. 

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% for nitrogen dioxide 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.

They detected changes in levels of nitrogen dioxide and ozone at 70% and 24% 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% to 6% for nitrogen dioxide, -5% to 4% for ozone, and -6% to 4% for PM2.5.

The researchers say their findings highlight that ULEZs are not a silver bullet and that sustained improvements in air pollution require multiple measures. 

Corresponding author Dr Marc Stettler said: ‘Cities considering air pollution policies should not expect ULEZs alone to fix the issue as they contribute only 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.

‘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.’

Shirley Rodrigues, Deputy Mayor for Environment and Energy, said: ‘This study is very misleading and its findings have been queried by experts within the university. Imperial College’s world-leading Environmental Research Group have been clear that the ULEZ has had a significant and positive effect on London’s air pollution.

‘The ULEZ has already helped cut toxic roadside nitrogen dioxide pollution by nearly half and led to reductions that are five times greater than the national average. But pollution isn’t just a central London problem, which is why the expansion of ULEZ will benefit Londoners across the whole of the city and is a crucial step in London’s green recovery from this pandemic.’



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