London, United Kingdom Air Quality & Smog

Dominant pollutants, when pollution and smog are worst, a notable historical episode, and the official monitoring agencies for London, United Kingdom — plus how to check the current reading. London reports air quality on the UK Daily Air Quality Index (DAQI) — our full guide explains how that scale works and how it compares to the US AQI.

Dominant pollutants

Nitrogen dioxide (NO₂, traffic), PM2.5, summer ozone.

Seasonal pattern: when smog is worst

London's defining pollutant is NO₂, concentrated along busy road corridors like Marylebone Road and the Euston Road where diesel traffic dominates. Winter brings PM2.5 episodes during cold, still, high-pressure spells, sometimes amplified by pollution imported from continental Europe; spring can add a particulate spike from agricultural ammonia and occasional Saharan dust. Summer ozone rises on hot, sunny days.

A notable air-quality episode

In spring 2014, a combination of local traffic pollution, continental import, and Saharan dust pushed the DAQI to "Very High" (band 10) across much of London. In a landmark 2020 ruling, air pollution was formally recorded as a cause of death for the first time in the UK, in the case of nine-year-old Ella Adoo-Kissi-Debrah, who lived near the South Circular Road.

The local index and who runs it

London reports air quality using the UK Daily Air Quality Index (DAQI), not the US AQI — so the same air can read as a different number than you may be used to. Defra's UK-AIR publishes the national DAQI, and London Air (run by Imperial College London) provides a dense borough-level monitor network. London also operates the Ultra Low Emission Zone (ULEZ), expanded London-wide in August 2023, to cut roadside NO₂.

• Defra (UK-AIR national network)
• Imperial College London — London Air
• Greater London Authority

How to check air quality in London

For the official live reading, the agency portals above are the canonical source. To understand what the numbers mean, start with our guide to the UK Daily Air Quality Index (DAQI) and the broader comparison of world air-quality indices. Smog Report puts glanceable air quality on your iPhone — widgets, Live Activities, Siri, and Apple Watch — free and with no account.

Common questions about London air quality

Why does London have air-quality and smog problems?

London's defining pollutant is NO₂, concentrated along busy road corridors like Marylebone Road and the Euston Road where diesel traffic dominates. Winter brings PM2.5 episodes during cold, still, high-pressure spells, sometimes amplified by pollution imported from continental Europe; spring can add a particulate spike from agricultural ammonia and occasional Saharan dust. Summer ozone rises on hot, sunny days.

What are the main air pollutants in London?

London's dominant pollutants are nitrogen dioxide (NO₂, traffic), PM2.5, summer ozone. The reading on any given day is usually driven by whichever of these is highest.

Has London had a major air-quality or smog event?

In spring 2014, a combination of local traffic pollution, continental import, and Saharan dust pushed the DAQI to "Very High" (band 10) across much of London. In a landmark 2020 ruling, air pollution was formally recorded as a cause of death for the first time in the UK, in the case of nine-year-old Ella Adoo-Kissi-Debrah, who lived near the South Circular Road.

What air-quality index does London use?

London reports air quality on the UK Daily Air Quality Index (DAQI) (DAQI), not the US AQI — so the same air can read as a different number than you may be used to. For live, glanceable readings on your iPhone, the free Smog Report app auto-selects the right local index for wherever you are.

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