London’s ULEZ only reduced nitrogen dioxide levels by 3%, study says

Just a month after London’s Ultra Low Emissions Zone (ULEZ) was expanded, a new study suggests it isn’t making much of a difference to the city’s air pollution. 

Researchers at Imperial College London found that ULEZ only reduced nitrogen dioxide (NO2) levels by three per cent over a 12-week period, starting before and ending after the ULEZ was launched by Mayor Sadiq Khan in April 2019.   

They also found ‘insignificant’ reductions in levels of ozone (O3), which can damage the lungs, and tiny particles of dirt and liquid called PM2.5 that are thought to reach the brain. 

Amazingly, at some sites around the capital, air pollution actually worsened, despite the ULEZ coming into force. 

These new findings show that ULEZs are ‘not a silver bullet’ and that sustained improvements in air pollution require various measures, the authors say. 

ULEZ will cover central London from 2019, but under the new plans it was extended massively to all of inner London two years later

ULEZ now stretches to cover an area surrounded by the North and South Circular roads. The ULEZ is separate from the Low Emission Zone (LEZ), which implemented tougher emissions standards for heavy diesel vehicles from March 1, 2021

ULEZ now stretches to cover an area surrounded by the North and South Circular roads. The ULEZ is separate from the Low Emission Zone (LEZ), which implemented tougher emissions standards for heavy diesel vehicles from March 1, 2021


In April 2019, the Mayor of London introduced the ULEZ, an area in which drivers of more polluting vehicles must pay a daily charge.

It aims to reduce air pollution emissions from road transport and accelerating compliance with EU air quality standards. 

It previously only covered the same area of central London as the Congestion Charge. 

But since October 25 this year, it includes all areas within the North and South Circular roads in an attempt to boost air quality. 

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. 

Introduced in April 2019, ULEZ allows authorities to charge diesel vehicles for being in Central London, with the aim of reducing vehicle emissions in some of the city’s most polluted areas. 

The zone was expanded from Central London up to, but not including, the North and South Circular Roads from October 25, 2021 – although three in five motorists weren’t aware of the expansion, a recent poll found. 

Drivers of vehicles that do not comply face being landed with a £12.50-a-day charge, but the new study suggests the zone isn’t as effective as hoped. 

‘Our research suggests that a ULEZ on its own is not an effective strategy to improve air quality,’ said study author Dr Marc Stettler, from Imperial’s Department of Civil and Environmental Engineering and Centre for Transport Studies. 

‘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 levies on polluting vehicles.’  

For the study, the researchers used publicly available air quality data from the London Air Quality Network, which shows air quality around the city based on roadside and non-roadside (‘background’) sensor readings. 

Using the site, researchers measured changes in pollution in the 12-week period from February 25, 2019, before the ULEZ was introduced, to May 20, 2019, after it had been implemented. 

The Ultra Low Emission Zone in London is now 18 times larger due to the October expansion, meaning any drivers whose cars do not meet standards will face a £12.50 charge

The Ultra Low Emission Zone in London is now 18 times larger due to the October expansion, meaning any drivers whose cars do not meet standards will face a £12.50 charge


Londoners have blasted London Mayor Sadiq Khan’s ULEZ expansion because they say it has left them facing an extra five mile trip to avoid being ‘taxed to recycle’.

Residents in leafy Richmond say their local waste centre has been sucked into the ultra-low emission zone following its expansion on Monday.

It means residents living in the town and driving older vehicles face a £12.50 charge if they want to take their recycling to the local tip.

Read more: London drivers blast Mayor over new ULEZ scheme  

They controlled for the effects of weather variations, and then used statistical analysis to look for and quantify changes in pollution. 

They found the number of Londoners living in areas with illegally high levels of NO2 between 2016 and 2020 fell by 94 per cent, and alongside this there were other reductions in London’s air pollution, but the April 2019 introduction contributed only ‘minimally’ to these improvements.        

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

They detected changes in levels of NO2 and O3 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

NO2 inflames the lining of the lungs and can reduce immunity to lung infections while exacerbating respiratory problems. 

While particulate matter, or PM, comes from a variety of sources, including vehicle exhausts, construction sites and industrial activity, and may even have detrimental effects on the brain.  

The researchers suggest that other cities considering implementing these schemes should consider them only alongside a combination of other measures.           

‘Since the London ULEZ was introduced, similar schemes have been introduced in Bath, Birmingham and Glasgow, yet on a much smaller scale,’ said Dr Stettler. 

‘Several other cities have plans to implement clean air zones and our findings could contribute to the development of their policy.

‘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.’    

Air pollution caused 40,000 deaths in the UK, according to 2016 study by the Royal College of Physicians – 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. 

The study has been published today in the journal Environmental Research Letters.  

Will I be hit by Ulez and how much will it cost me? 

Many more motorists will be liable for a daily charge after London’s Ultra Low Emission Zone was expanded. Here are 12 key questions about the scheme:

– What vehicles are affected?  It depends how much nitrogen dioxide it emits, which is generally linked to its age. For diesel cars, most of those that meet the minimum standard were first registered after September 2015. Compliant petrol cars are typically those registered after 2005.

– How can I check the status of my vehicle?  Transport for London is urging people to use its online checker by visiting 

– What is the charge for non-compliant vehicles?  The daily fee is £12.50 for cars, motorcycles and vans up to 3.5 tonnes. Heavier vehicles, including lorries, buses and coaches, are charged £100.

– When do charges apply?  All day, every day of the year except on Christmas Day. 

– What is the Ultra Low Emission Zone (Ulez)? It is an area in London where drivers of older, more polluting vehicles are charged a daily fee.

– What has changed?  It previously only covered parts of central London, but on Monday it became 18 times larger.

– What are the new boundaries?  The Ulez has been extended to include all areas within the North and South Circular roads.

– Who has done this and why?  The change has been implemented by London mayor Sadiq Khan in an attempt to improve air quality in the capital.

– How quickly do I have to pay?  You have until midnight on the third day after the journey.

– How do I do it?  You can pay online through TfL’s website, using the Pay to Drive in London app, or by phone. Drivers also have the option of setting up automatic billing so they are charged monthly.

– What happens if I do not pay?  You could be handed a Penalty Charge Notice for £160, reduced to £80 if paid within a fortnight.

-Where does the money go?  TfL says the cash it receives from the Ulez is reinvested into improving London’s transport network and to improve air quality. It insists ‘we don’t make a profit’ from the scheme.