Fog, light winds and smoke from fireworks and wood burning pushed readings into “unhealthy” territory earlier this week. Government monitors now show low pollution across the capital.
London’s air quality has returned to low levels after a high air pollution alert earlier this week, according to Defra’s national monitoring network. Dense fog and light winds let harmful particles build up across the capital, and the Mayor of London issued the alert on Tuesday on advice from forecasters at Imperial College London. It was the fourth such alert this year. Defra’s Daily Air Quality Index (DAQI) read 2 out of 10 at both roadside and urban-background sites at 7am on Thursday.
What caused the build-up
Forecasters said smoke from Bonfire Night and Diwali fireworks, together with wood burning, was likely to have contributed to the high readings. Vulnerable residents were urged to take particular care. The alert applied to London on Tuesday, and levels have fallen since, though some areas still had readings that could cause moderate health concerns for unusually sensitive people.
The fog also disrupted air travel. Several flights from Heathrow and London City were cancelled on Monday.
What the maps showed
The World Air Quality Index Project runs an interactive map that it says gathers data from more than half a million monitoring stations worldwide. According to its data, much of London was in the “unhealthy” band between Sunday and Tuesday. At that level, everyone could begin to feel health effects, and sensitive groups could be more seriously affected. The site advises active children and adults, and people with respiratory disease, to avoid prolonged outdoor exertion in such conditions.
Maps from the Apple Weather app gave a borough-by-borough picture earlier in the week. Westminster, Wandsworth, Lambeth, Merton, Ealing, Brent, Tower Hamlets, Greenwich and Bexley were all rated 10. Most other boroughs, including Croydon, Richmond upon Thames and Kensington and Chelsea, were rated 8 or 6. North London had somewhat cleaner air, with Haringey and Enfield mainly at 2 or 4.
A UK score of 10 does not mean the same as a US AQI reading of 10, according to the UK Health Security Agency (UKHSA). The UK index runs from 1 to 10, while the US scale uses different numbers and pollutant thresholds, so different systems can give apparently conflicting results for the same place. The UK index takes the highest score among five pollutants: PM2.5, PM10, nitrogen dioxide, ozone and sulphur dioxide. One pollutant can therefore drive a borough’s overall rating.
The latest readings
Defra’s records show pollution had already fallen substantially by Wednesday morning, with roadside sites at index 3 and other urban sites at index 2 at around 7am. By the end of Wednesday, both categories were at index 2. The department’s website lists no active national air pollution alerts, though this is separate from the Mayor’s local warning system and does not by itself confirm that every local warning has ended.
Modelled data from the Copernicus Atmosphere Monitoring Service, published via the site Pollution Alert, puts the peak earlier than the alert. It estimated a London-wide daily average for PM2.5 of 34.36 µg/m³ on 7 November, about three times the 11.56 µg/m³ estimated for 5 November. Nitrogen dioxide reached 60.93 µg/m³ on 8 November, more than double the 28.06 µg/m³ of 5 November. By 11 November, modelled PM2.5 had fallen to 16.60 µg/m³, about 52% below the peak, while PM10 dropped from 40.63 µg/m³ to 19.86 µg/m³. These are model estimates, not station readings. The hourly estimates for 2am on Thursday were 7.4 µg/m³ for PM2.5 and 14.8 µg/m³ for nitrogen dioxide. Defra’s PM2.5 index uses a rolling 24-hour average, so hourly model figures and daily index scores are not equivalent.
Advice from the Mayor and health officials
The Mayor, Sadiq Khan, urged Londoners to help reduce pollution by walking, cycling or taking public transport, avoiding unnecessary car journeys and not burning wood or garden waste. Information was also shared with GPs and emergency departments to help them support vulnerable patients.
Mete Coban, the Deputy Mayor for Environment and Energy, said: “Toxic air is responsible for thousands of premature deaths in London each year, which is why the Mayor has done everything in his power to tackle it, which includes launching the world’s largest clean air zone. It is really important we all do our bit to protect those who are most vulnerable as we continue working to build a safer and greener London for all.”
UKHSA guidance, updated in April 2026, warns that symptoms can appear hours or days after exposure. They include coughing, chest tightness, eye and throat irritation, worsening asthma, breathlessness and heart palpitations. During high pollution, the Met Office says people with heart or lung conditions should reduce strenuous outdoor activity, while the wider population is advised to consider cutting back on exertion if they have symptoms. Stricter restrictions apply at very high levels. UKHSA’s updated thresholds put a daily PM2.5 level of 54 to 70 µg/m³ in the high band and 71 µg/m³ or more in the very high band.
The long-term picture
Imperial College London research commissioned by City Hall and Transport for London, published on 24 June 2026, estimated that long-term exposure to air pollution was linked to the equivalent of 3,800 to 5,100 premature deaths in Greater London in 2024. That is a statistical estimate of the health burden, not a count of death certificates. It put the burden at 6,400 to 8,000 equivalent deaths in 2019, a fall of about 40% by 2024, and estimated the annual economic cost at £3.8bn to £5.1bn. City Hall says no part of London currently meets the World Health Organization’s final guidelines for both PM2.5 and nitrogen dioxide.
City Hall’s reports on the Ultra Low Emission Zone (ULEZ) estimate that roadside nitrogen dioxide was 27% lower across London in 2024 than it would have been without it, with a 54% effect in central London, 29% in inner London and 24% in outer London. It estimated that exhaust PM2.5 emissions from cars and vans in outer London were 31% lower because of the London-wide expansion, preventing about 9.1 tonnes of fine particulate exhaust emissions in 2024. That figure covers vehicle exhaust only, not fireworks or domestic fires. City Hall says about 97% of vehicles seen in London now meet ULEZ standards, and the number of non-compliant vehicles seen on an average day has fallen by 58% since June 2023.
Where to check local readings
Imperial College London’s London Air Quality Network lets users select monitoring stations, view individual pollutants and compare readings with national limits. It separates automatically checked data from validated historical data. Defra publishes a national map with historical daily maximum scores for chosen dates, which allows a more reliable comparison of this week’s episode than a screenshot of a constantly updating map.
