London, one of the world’s most populous and economically vital cities, has long grappled with the health and environmental consequences of poor air quality. Historically, the city’s dense traffic, industrial legacy, and geographical basin have trapped pollutants, leading to levels of nitrogen dioxide (NO₂) and particulate matter (PM) that regularly exceed World Health Organization (WHO) guidelines. Since the creation of the Greater London Authority in 2000, the directly elected mayor has held significant powers over transport, planning, and environmental policy. This case study examines the effectiveness of London’s mayor—focusing primarily on the tenure of Sadiq Khan (2016–present) and his predecessor Boris Johnson (2008–2016)—in managing air pollution. We assess the key initiatives, measurable outcomes, persistent challenges, and the broader lessons for urban governance.

The Mayor’s Powers and Responsibilities for Air Quality

The mayor of London is responsible for producing and implementing the London Environment Strategy, which sets binding targets for reducing emissions and pollutant concentrations. This includes managing the city’s transport network via Transport for London (TfL), overseeing planning permissions that affect emissions, and coordinating with the 32 boroughs and the City of London Corporation. Crucially, the mayor has the authority to introduce road-user charging schemes and low-emission zones, powers that were used to create the Congestion Charge zone in 2003 and later expanded into the Ultra Low Emission Zone (ULEZ) in 2019.

While the mayor cannot directly control all sources of pollution—such as emissions from Heathrow Airport (which is run by a separate authority) or from the national power grid—they can influence traffic flows, public transport investment, cycling infrastructure, and green space creation. The effectiveness of mayoral action is thus tied to both the specific policies adopted and the degree of cooperation with national government and local councils.

Historical Context: From the Congestion Charge to ULEZ

London’s modern air quality management began under Mayor Ken Livingstone (2000–2008), who introduced the Congestion Charge in 2003. While primarily aimed at reducing traffic volume, it also brought modest improvements in NO₂ and PM levels. However, by the mid‑2010s, it became clear that the congestion charge was insufficient to address the growing problem of diesel vehicle emissions, which had been inadvertently encouraged by past European policies promoting diesel fuel for lower CO₂ output.

Under Mayor Boris Johnson (2008–2016), the Low Emission Zone (LEZ) was introduced in 2008, targeting older heavy goods vehicles and buses. Yet the LEZ was not strict enough to bring London into compliance with legal NO₂ limits. Mayor Johnson also oversaw the introduction of the first *Ultra Low Emission Zone* concept, but it was his successor, Sadiq Khan, who took the decision to implement and then radically expand the scheme.

Key Initiatives Under the Current Mayor

The Ultra Low Emission Zone (ULEZ)

Launched in April 2019, the ULEZ initially covered the same area as the Congestion Charge zone in central London. Vehicles not meeting Euro 4 (petrol) or Euro 6 (diesel) standards must pay a daily charge of £12.50 (for cars, vans, and motorcycles) and £100 for heavier vehicles. The charge operates 24/7, 365 days a year. The immediate effect was a sharp reduction in the number of non-compliant vehicles entering the zone. In October 2021, the ULEZ was expanded to cover the area inside the North and South Circular roads—an area roughly 18 times larger than the original zone. This expansion brought about 3.8 million Londoners into the zone and was widely considered Mayor Khan’s signature environmental policy.

Data from TfL show that in the first year of the expanded ULEZ, the proportion of vehicles meeting the standards rose from 85% to over 95%. In central London, NO₂ concentrations fell by an estimated 26% between 2019 and 2022, compared to a counterfactual scenario without the ULEZ. The scheme has been backed by health experts, but it has also provoked significant opposition from some residents, small businesses, and motoring groups who argue that the costs are regressive and that public transport alternatives are insufficient in outer London boroughs.

Investment in Cleaner Public Transport

The mayor has committed to making London’s bus fleet entirely zero-emission by 2034 (or earlier). As of 2024, over 1,000 zero-emission buses (electric or hydrogen) are in operation, representing roughly 20% of the fleet. TfL has also invested in retrofitting older buses with cleaner engines and installing charging infrastructure at bus depots. The London Underground is powered almost entirely by renewable energy, and the mayor has overseen the introduction of the Elizabeth Line, which provides a high-frequency, electric-powered service across the capital, reducing reliance on diesel trains.

These investments have helped reduce PM₂.₅ emissions from the transport sector, though non-road mobile machinery, such as construction equipment, remains a smaller but still significant source.

Expanding Cycling and Walking Infrastructure

The mayor’s Healthy Streets approach prioritizes active travel. During the pandemic, the “Streetspace for London” programme rapidly delivered over 100 km of new or upgraded cycle lanes, many on main roads. The cycle network now exceeds 300 km in total length, and cycling levels in central London have more than doubled since 2016. The mayor has also introduced a cycle hire scheme (Santander Cycles) and invested in secure bike parking. Walking improvements include widening pavements, creating more pedestrianised zones, and planting trees to provide shade and air filtration. Studies indicate that increased cycling and walking not only reduce vehicle emissions but also improve public health through greater physical activity.

Greening the City and Reducing Construction Emissions

Green spaces act as natural pollution sinks. The mayor has planted over 500,000 trees since 2016, with a target of 1 million by 2030. Parks and green corridors are being expanded, and new developments are required to meet stringent air quality standards. The London Plan mandates Air Quality Neutral and Air Quality Positive benchmarks for new buildings, and construction sites must follow a Code of Practice to minimise dust and diesel emissions.

Assessing Effectiveness: Data and Evidence

Multiple independent studies and government monitoring stations show that air quality in London has improved significantly over the past decade. According to the Greater London Authority’s annual Air Quality Report, average annual mean NO₂ concentrations at roadside monitoring sites in central London fell from 98 µg/m³ in 2016 to 44 µg/m³ in 2023—a reduction of 55%. Throughout the entire city, the number of monitoring sites exceeding the legal limit of 40 µg/m³ dropped from over 90% in 2016 to around 30% in 2023.

PM₂.₅ levels have also declined, though more modestly: from 16 µg/m³ (2016 average) to 12 µg/m³ in 2023. This remains above the WHO guideline of 5 µg/m³, indicating that significant work remains. Notably, reductions have not been uniform: inner London has seen larger improvements, while some outer London hotspots, particularly near major roads and airports, remain problematic.

Public Health Impact

The health benefits of the mayor’s policies are increasingly visible. A study by Imperial College London estimated that the central London ULEZ alone reduced NO₂ concentrations by 13% in its first year, preventing an estimated 1,900 cases of lung problems in children. Another analysis projected that the expansion of the ULEZ could result in 200,000 fewer cases of asthma symptoms among London children over the next decade. However, critics point out that the health benefits tend to accrue more to wealthier, centrally located residents, while lower-income communities in outer boroughs still face higher exposure and the financial burden of upgrading vehicles.

Economic and Social Criticisms

The most persistent criticism of the mayor’s air quality policies is their regressive impact. The ULEZ daily charge of £12.50 for non-compliant vehicles falls disproportionately on small businesses that rely on vans, and on low-income drivers who cannot afford to replace older cars. Mayor Khan has introduced a scrappage scheme offering up to £2,000 for eligible Londoners to scrap non-compliant vehicles, but many argue the funding is insufficient. In 2023, the expansion of the ULEZ to all London boroughs triggered legal challenges from several Conservative-led councils and a by-election loss for Labour in Uxbridge and South Ruislip, where the issue was a major factor. The mayor has since been forced to allocate additional funds for scrappage and improve public transport in outer areas.

Another criticism is that the mayor’s focus on transport ignores other major sources of pollution, such as wood burning (a significant contributor to PM₂.₅) and construction. While the mayor has taken steps to regulate wood stoves and issue better guidance, enforcement remains weak. Additionally, the mayor has limited control over Heathrow Airport, a major source of NO₂ and PM emissions, particularly for communities living under flight paths.

Comparison with Other Global Cities

London’s approach is often compared to that of other megacities. Paris has implemented a Crit’Air sticker system and low-emission zones, but enforcement is less consistent. Beijing achieved dramatic reductions in PM₂.₅ through factory closures and coal bans, but at high economic cost. Berlin and Stockholm have congestion charges similar to London’s, though London’s ULEZ is the most stringent in terms of geographic coverage and hours of operation. One key lesson from London is that bold, unpopular measures can be implemented when backed by credible health data and phased in gradually to allow adaptation.

Future Directions: What Remains to Be Done

Despite measurable progress, London’s mayor will face several critical challenges in the coming years:

  • Further tightening of ULEZ standards: Currently, ULEZ standards align with Euro 4/Euro 6. As the vehicle fleet modernizes, the mayor may need to consider zero-emission zones for parts of the city by 2030.
  • Addressing non-transport sources: PM₂.₅ from wood burning, construction, and industrial sources require more robust regulation and public awareness campaigns.
  • Reducing inequality in air quality: Policies must be designed so that low-income communities are not disproportionately burdened. This could include expanded scrappage schemes, subsidized public transport, and investment in green infrastructure in deprived areas.
  • Integrating with climate goals: Air quality and carbon reduction are linked. The mayor’s plan to make London net zero by 2030 will require a radical reduction in fossil fuel use, which will also improve air quality.
  • Improving monitoring and transparency: While the current monitoring network is good, more real-time sensors in schools, hospitals, and residential areas could empower communities and inform targeted action.

The mayor’s office is currently developing the next iteration of the London Environment Strategy, expected in 2025, which will set new targets for NO₂ and PM₂.₅ aligned with 2030 WHO guidelines. Achieving these goals will require not only mayoral leadership but also support from the national government, which has been inconsistent in its own environmental policies.

Conclusion

London’s mayor has demonstrably moved the needle on air pollution through a combination of bold regulatory measures, significant investment in public transport and active travel, and a long-term strategic vision. Reductions in NO₂ and PM levels, while uneven, are real and have been validated by independent research. The ULEZ, in particular, has proven to be one of the most effective urban air quality interventions in the world, inspiring other cities to adopt similar schemes. Yet the social costs of these policies, particularly for outer London residents and small businesses, underline the need for greater equity in implementation. The mayor cannot solve the problem alone; national action on vehicle standards, airport emissions, and renewable energy is essential. Nonetheless, the case of London shows that mayoral power, when wielded with political courage and backed by scientific evidence, can produce tangible improvements in one of the most challenging urban environments on earth.

As London’s population grows and the effects of climate intensity, the next mayoral term will be crucial in determining whether the city can meet its ambitious air quality goals. The lessons learned here—the importance of comprehensive data, phased implementation, robust scrappage schemes, and cross-party consensus—offer a valuable roadmap for any city serious about cleaning its air.