London's ULEZ and the NO₂ Problem

Last updated May 29, 2026 · 6 min read

London's air-quality story is, above all, a story about nitrogen dioxide. And its most consequential policy response — the Ultra Low Emission Zone — is one of the largest urban clean-air interventions anywhere in the world. This guide explains why NO₂ dominates London's air, how the ULEZ works, and what the evidence says.

Why NO₂ is London's defining pollutant

Nitrogen dioxide (NO₂) is a combustion gas, and in a dense city the dominant source is road traffic — historically diesel vehicles in particular. London's NO₂ is highly localised: concentrations spike at the kerbside along busy corridors like Marylebone Road and the Euston Road, where they have long exceeded legal limits, even as the citywide background sits lower. NO₂ inflames the airways, worsens asthma, and is associated with reduced lung development in children.

How the ULEZ works

The Ultra Low Emission Zone charges older, more-polluting vehicles a daily fee to drive within its boundary. Vehicles that meet the emissions standard (broadly, newer petrol and the cleanest diesels) pay nothing; those that don't pay a daily charge. The aim is to accelerate the turnover of the vehicle fleet toward cleaner engines and discourage the dirtiest vehicles from entering.

The expansion timeline

• April 2019 — ULEZ launches in central London, replacing the earlier T-Charge.
• October 2021 — expanded to inner London, up to the North and South Circular roads.
• August 2023 — expanded again to cover all London boroughs, making it one of the largest clean-air zones in the world.

A separate Low Emission Zone targets heavy goods vehicles across a wider area.

The evidence on impact

Greater London Authority assessments have reported meaningful reductions in roadside NO₂ within the zone following each phase, alongside a shift toward compliant vehicles, though disentangling the ULEZ's effect from broader fleet renewal and other trends is genuinely difficult. The expansions have also been politically contentious, with debate over costs to drivers in outer boroughs.

A landmark legal moment

In 2020, a coroner ruled that air pollution contributed to the death of nine-year-old Ella Adoo-Kissi-Debrah, who lived near the heavily trafficked South Circular Road — the first time air pollution was formally recorded as a cause of death in the UK. The ruling sharpened the legal and political pressure behind measures like the ULEZ.

How NO₂ shows up in the DAQI

The UK reports air quality on the Daily Air Quality Index (DAQI), a 1–10 scale where each pollutant has its own breakpoints and the overall index is the worst of them. On a typical London day NO₂ or particulate sets the DAQI band; near a busy road, NO₂ often dominates. Defra's UK-AIR and Imperial's London Air publish the readings.

Primary sources: Transport for London — Ultra Low Emission Zone · UK Defra — Daily Air Quality Index · London Air (Imperial College London)