Full compliance with WHO’s ‘interim target’ for NO2 of 20 ug/m3 by 1 January 2030
Dear Sir Sadiq
I am writing on behalf of Clean Air in London (CAL) to you as Mayor of London. You are also Co-Chair of C40 Cities which is due to hold the World Summit of Mayors in Brazil 3-5 November 2025[1].
It is almost exactly 20 years since CAL wrote its first ever letter on 9 April 2006. In it, I urged Mayor Livingstone to go further faster on his proposed first phase of the low emission zone to comply with limit values for nitrogen dioxide (NO2) (of 40 micrograms per cubic metre (ug/m3) to be achieved by 1 January 2010) and particulate matter (PM10) (to be achieved by 1 January 2005). Annual mean concentrations of NO2, a toxic gas, were 112 ug/m3 in Marylebone Road in calendar 2005[2].
Nearly 20 years later, with the World Health Organisation (WHO) having slashed its air quality guideline for NO2 in September 2021[3], I am urging you as Mayor of London to commit to London complying with the WHO’s interim target for NO2 of 20 ug/m3 by 1 January 2030 i.e. as you have committed to achieving the equivalent PM2.5 interim target of 10 ug/m3. All your C40 European counterparts are already legally required to do the same.
Please make a public commitment before Friday 21 November 2025 i.e. the scheduled second reading of the Clean Air (Human Rights) Bill[4]. Oxford City Council did so on 27 October 2025[5].
NO2 concentrations between 2005 and 2015
As you know, the WHO published its ‘2005’ air quality guidelines in October 2006 with an NO2 annual mean guideline of 40 ug/m3. An NO2 limit value at this level had applied since 1999[6].
During this period, CAL lobbied (successfully) for powerful new EU air quality laws which entered into force on 11 June 2008 as Directive 2008/50/EC on ambient air quality and cleaner air for Europe. Among other things, the Directive (which was transposed as Air Quality Standards Regulations 2010) included limit values to be ‘attained within a given period and not to be exceeded once attained’. The NO2 annual mean limit value of 40 ug/m3 had to be attained by 1 January 2010. The UK government tried but failed to obtain a time extension of up to five years for London because it could not ‘demonstrate how conformity will be achieved with the limit value[s] before the new deadline’. This is all detailed on the CAL website.
NO2 annual mean concentrations were 98 ug/m3 in Marylebone Road in 2010 and 88 ug/m3 in 2015. Concentrations in Brixton Road (Lambeth) were 128 ug/m3 in 2015 with 848 exceedances of the hourly limit value of 18 times over 200 ug/m3.[7] Ella Roberta Adoo-Kissi-Debrah died tragically during this period.
A prominent article in the Sunday Times on 6 July 2014 was titled ‘Diesel fumes choke Tox ford Street’. It reported that Oxford Street’s NO2 annual mean of 135 ug/m3 was more than three times the EU limit and that levels had exceeded the hourly limit of 200 ug/m3 1,568 times. It then quoted David Carslaw, a highly regarded scientist, as saying “To my knowledge this is the highest in the world in terms of both hourly and annual mean. NO2 concentrations [in Oxford Street] are as high as they have ever been in the long history of air pollution”. David posted a blog evidencing his views on 9 July 2014[8]. Mayor Johnson characteristically dismissed claims about Oxford Street as ‘B*ll*cks’ on 16 July 2014.[9]
Cleaner air since 2015
Your manifesto for the Mayoral election in May 2016 included the following among your ‘priorities’ (page 8)[10]:
“Restore London’s air quality to legal and safe levels, with action to make travel greener and pedestrianise Oxford Street, while protecting the green belt.”
Fortunately for London’s air quality, you were elected decisively in 2016, 2021 and 2024. Please remember this pledge now. Science and WHO guidelines have moved on since 2016, so must you.
I will not recap here your many achievements in reducing NO2 concentrations in London during more than nine years in office. I have documented much of this in articles on the CAL website including about the importance of the expansion of the Ultra Low Emission Zone (ULEZ) to outer London i.e. the ninth phase of low and ultra low emission zones (LEZs and ULEZs). Suffice it to say that I respect and thank you for what you have done for cleaner air. Your colleagues and others have also played a valuable part which must not be ignored. In the end, this has mainly been a story about ‘diesel’ and so the 10th and final stage of the LEZs and ULEZs should be the phasing out of diesel vehicles in London by 2030, with limited exceptions. This would be consistent with your ‘net zero’ pledge.
In passing, CAL wishes to record its unique perspective on the timing of ULEZ expansion two years after the event. ULEZ expansion did not get the credit it deserved at the time because:
- the Government used its control of TfL funding support (post-COVID) to force you to introduce the ULEZ expansion at the most politically challenging time i.e. in the febrile political environment that existed in the months before the Mayoral and General Elections and days after the byelection in Uxbridge and South Ruislip (20 July 2023) following Boris Johnson’s resignation as Prime Minister and an MP;
- you had to implement ULEZ expansion with nine months’ notice i.e. far less than half the usual time[11]; and
- the public were (and remain) largely unaware that the WHO had slashed their air quality guidelines in September 2021 or that ULEZ expansion was targeting NO2 at roadsides rather than carbon dioxide per se across the whole of London.
In the end, a ‘perfect storm’ of domestic publicity around ULEZ expansion suffered from a political ‘timing’ problem, of sorts, as the government had hoped (and was a missed opportunity for you on ‘communication’ around the significance of the new WHO air quality guidelines and other benefits).
CAL’s media strategy for ULEZ expansion was to go ‘high and wide’ (largely ignoring domestic politics). It was quoted over 300 times in 19 countries after contacting many major international media organisations. CAL was quoted most often as saying “Big problems need big solutions”[12] and “Low emission zones work”.
Of course, the Conservative Government’s plan backfired because the subsequent Mayoral election energised Londoners wanting a clean, healthy and sustainable environment to support you and avoid a major setback in environmental and other policies[13]. This certainly contributed, perhaps in large part, to your emphatic victory on 2 May 2024. Your Deputy Mayor said (rightly) on X (formerly Twitter) that “It was #ULEZ wot won it!”[14] (4 May 2024). A Mayoral candidate who called for all congestion and emission schemes in London to be cancelled, lost their election deposit.
Please remember the above. By explaining the need for measures and giving appropriate notice of them you can reduce ‘political risk’ while protecting public health and achieving the full ‘political benefit’. The political costs of ‘inaction’ on diesel are far greater than those of ‘action’ in London.
According to Sprout Social, the volume of monthly activity on X (formerly Twitter) rose from 1,800 in January 2023 to 3,378 in June 2023 and 16,020 in August 2023 before falling to 1,365 in October 2023, 1,084 in January 2024 and 1,276 in May 2024 and 321 in June 2024. It blipped briefly to 4,292 in April 2024 i.e. the month immediately before the Mayoral election. ULEZ and #ULEZ rarely get 100 mentions per month globally now. CAL would like to see much more coverage of the many benefits and lessons from the nine phases of LEZs and ULEZs in London and the need for a tenth.
Defra claims London met its technical standards for compliance with NO2 limit values in 2024
On 30 September 2025, the Government published its annual Compliance Assessment Summary that showed that London had met the Defra technical and reporting criteria for compliance with annual mean concentrations of NO2 of 36 ug/m3 in Marylebone Road in 2024. This was 14 years after the legal deadline but more than six years earlier than Defra had expected in 2014. It was also achieved well ahead other major cities and parts of England, much to your credit. No hourly exceedances have been recorded there since 2018.
However, limit values apply everywhere (with few exceptions[15]), not only in Marylebone Road, and as your media release highlighted rightly on 30 September 2025, at least three official monitors (not on Defra’s monitoring network) reported exceedances in 2024 i.e. Brixton Road, Euston Road and Loampit Vale at 48 ug/m3, 42 ug/m3 and 46 ug/m3 respectively[16]. Modelling and NO2 diffusion tube data has confirmed other exceedances in statutory Annual Status Reports from the City of London Corporation and other local authorities. Exceedances of the 2005 guideline and 2008 limit value persist i.e. an annual mean for NO2 of 40 ug/m3.
You will also be acutely aware that the WHO slashed its air quality guideline for NO2 from 40 ug/m3 to 10 ug/m3 on 21 September 2021. The EU has worked hard to update its 2008 Directive for this new reality such that Directive (EU) 2024/2881 on ambient air quality and cleaner air for Europe entered into force on 10 December 2024. This includes a limit value for NO2 annual mean of 20 ug/m3 to be attained by 1 January 2030 and not exceeded thereafter i.e. the same standard I am asking you to meet.
Heathrow expansion
The UK Government seems determined to ignore the above and is pressing ahead with Heathrow expansion that may struggle to meet the NO2 limit value set in 2008 never mind the new one that has applied across the EU since December 2024. Further, limit values must not be exceeded once attained. Perhaps the Government hopes to approve this project (and others) before aligning with EU environmental standards to achieve trading and many other benefits. This would be cynical and ignore public health and human rights. If expansion is approved, it is likely to be impossible to comply with Directive (EU) 2024/2881’s annual mean limit value for NO2 around Heathrow in the foreseeable future i.e. because of increased traffic and other emissions. You may be aware of the lack of similar recent airport approvals in the EU.
NO2 interim target of 20 ug/m3 by 1 January 2030
Your recent media release (30 September 2025)[17] said:
“The Mayor has delivered a comprehensive package of measures to reduce emissions from all sources in London and is committed to meet the World Health Organization (WHO)’s latest guidelines for NO2 and PM2.5 as soon as possible.” Note: These are annual means for NO2 and PM2.5 of 10 ug/m3 and 5 ug/m3 respectively.
I am writing to ask you to commit to full compliance with the WHO’s interim target for annual mean NO2 of 20 ug/m3 throughout Greater London by 1 January 2030 and not exceed it thereafter.
This would match your near identical commitment to the WHO interim target for PM2.5 of 10 ug/m3 (which is identical to the PM2.5 annual limit value applying across Europe to protect public health, climate and the environment).
You have committed often to complying with the WHO’s interim target for PM2.5 of 10 ug/m3 by 2030[18]. This no longer seems hard to achieve with Defra claiming in its CAS, that the monitoring station in Marylebone Road was the only one in England to exceed this target in 2024. As with NO2, we recognise that some local monitors still report PM2.5 annual mean concentrations over 10 ug/m3 and that air pollution must be reduced there.
Your large election win in May 2024 should not just have been about stopping two or more other political parties from reversing ULEZ expansion and other clean air and environmental policies. Nor should your third Mayoral term be a ‘victory lap’ for your remarkable clean air achievements in the first two terms. Your emphatic victory in 2024 showed that ‘clean air’ matters to Londoners.
I look forward to hearing from you before 21 November 2025.
Yours sincerely
Simon Birkett
Founder and Director
[1] https://www.c40.org/pt/events/c40-world-mayors-summit-2025/
[2] https://www.airqualityengland.co.uk/site/statistics?site_id=MY1
[3] https://cleanair.london/policy/new-who-air-quality-guidelines/
[4] https://bills.parliament.uk/bills/3998
[5] https://www.bbc.co.uk/news/articles/cly1qe4rjq0o
[6] https://eur-lex.europa.eu/eli/dir/1999/30/oj/eng
[7] https://www.londonair.org.uk/LondonAir/LATools/MonthlyAverages.aspx
[8] https://www.londonair.org.uk/london/asp/news.asp?NewsId=OxfordStHighNO2&StartIndex=31
[9] https://www.theguardian.com/environment/2014/jul/18/boris-johnson-bollocks-oxford-street-worlds-worst-pollution
[10] https://cleanair.london/app/uploads/CAL-325-Sadiq_Khan_Manifesto-090316.pdf
[11] https://www.london.gov.uk/ultra-low-emission-zone-will-be-expanded-london-wide
[12] https://cleanair.london/policy/ulez-expansion-is-an-important-step-on-the-path-to-phasing-out-diesel-in-london/
[13] https://cleanair.london/policy/clean-air-scorecard-2024/
[14] https://x.com/sabrodrigues61/status/1786777712142458958
[15] https://cleanair.london/app/uploads/CAL-269-Letter-of-clarification-from-the-Commission-190214_Redacted-1.pdf
[16] https://www.london.gov.uk/london-meets-legal-limits-toxic-no2-pollution-first-time-almost-200-years-earlier-predicted
[17] https://www.london.gov.uk/london-meets-legal-limits-toxic-no2-pollution-first-time-almost-200-years-earlier-predicted
[18] https://data.london.gov.uk/download/2lg5g/8bc8b62a-17d0-42e8-9588-7e20f55a3b13/LAEI%202022%20Summary%20Note%20August%202025.pdf
