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Area research7 min read

Air quality and NO₂: what the numbers mean when you're buying a home

Almost every home in London is within the legal limit for NO₂ and almost none meets the WHO's health guideline. What the two numbers mean, and how to check a street.

You've found a flat you like on a road you'd describe as "fairly busy". Someone mentions pollution. You look it up, find a number in micrograms per cubic metre, and have no idea whether it's good or bad.

Here's the plain version. There are two numbers that matter for nitrogen dioxide, and they're four times apart. The UK's legal limit is 40 µg/m³ as an annual average. The World Health Organization's guideline is 10. Almost everywhere you're likely to buy in and around London passes the first and fails the second. So "within legal limits" is true, and it tells you very little.

This is about England. Scotland sets some of its own objectives, for particles rather than NO₂.

What it actually is

Nitrogen dioxide is the pollutant you'll find street-level figures for, because it's the one councils act on most. Defra's summary of declared Air Quality Management Areas, checked in October 2026, lists 368 for NO₂ across the UK against 56 for PM10. And it tracks traffic closely: when the number of UK zones over the NO₂ limit fell from 33 in 2019 to five in 2020, Defra attributed it to the reduced road traffic of the lockdowns.

The legal number is old. The UK's national objective for NO₂ is 40 µg/m³ as an annual mean, to be met by 31 December 2005, and the same figure became a legal limit value from 1 January 2010 under the Air Quality Standards Regulations. There's also an hourly limit of 200 µg/m³, not to be exceeded more than 18 times a year.

The health number moved. The WHO's 2005 guideline for annual NO₂ was also 40. In its global air quality guidelines of 22 September 2021, it cut that to 10, citing "a marked increase in the quality and quantity of evidence that shows how air pollution affects different aspects of health." In other words, the UK's legal line is the WHO's old line. The evidence moved and the law didn't.

What our data shows

We hold a modelled NO₂ figure for every listing we track, and we ran the numbers again in October 2026.

Across active listings in and around London, not one sits above the 40 µg/m³ legal objective. The highest is 34.7. The median is 17.2. And 96.7 per cent sit above the WHO's 10.

That isn't a quirk of this year's market. Across our archive of delisted listings going back to September 2018, it's the same picture: none above 40, and 91.2 per cent above 10. Between the two, that's over 340,000 listings and not a single one over the legal line.

Some more texture from the active set, same population, same date:

  • About 3 per cent are at or below the WHO guideline.
  • Roughly three in ten are above 20, double the guideline.
  • Fewer than 2 per cent are above 30.

Now the caveat, which matters more than usual here. Our figures come from Defra's modelled background maps, produced each year on a 1 km grid. Background means the general level across an area, and Defra describes these maps as excluding local traffic and point sources. So a listing's figure is its neighbourhood's background NO₂, not a reading at the front door, and a flat directly on a main road will usually be breathing more than its background number says. That's exactly the gap the next section is about.

The official picture backs up our numbers. Defra assesses compliance across 43 zones of the UK, and in its 2024 report, published September 2025, 38 met the annual NO₂ limit value and five didn't: the West Midlands, Greater Manchester, Liverpool, Bristol and Coventry/Bedworth urban areas. Greater London was compliant. As recently as 2019, 33 zones were over.

That's real progress. It also tells you why a legal pass is a weak signal when you're choosing a street. A zone is a whole urban area. The legal line is a figure the WHO abandoned in 2021. And Defra itself notes that councils' local monitoring is often "sited differently to national monitoring in order to target local pollution hotspots", and can show different results to the national assessment.

So the useful question isn't "is it legal". It's how far above 10 the area sits, and how much worse the specific street is than its area.

One more pollutant worth a line. Fine particulate matter, PM2.5, has its own legal target in England: 10 µg/m³ as an annual mean by 2040, with an interim target of 12 by January 2028. In 2024, one monitoring station in England was over 10: London Marylebone Road. NO₂ is the one you'll find street-level figures for, but it isn't the whole story.

What an AQMA tells you

An Air Quality Management Area is the closest thing to a formal admission that a place has a problem.

Under section 83 of the Environment Act 1995, where it appears that air quality standards or objectives "are not being achieved", the local authority shall designate that part of its area as an AQMA. Section 83A then requires it to prepare an action plan with measures and dates. An AQMA can be a couple of streets or a whole borough.

Read it both ways. Inside an AQMA means the council has found, or modelled, a spot that misses the legal objective, and it's working on it. Outside one tells you less than it sounds, because the trigger is the 40 µg/m³ line. Given that nothing we track gets anywhere near 40 at background level, "not in an AQMA" is the expected answer almost everywhere, not a clean bill of health.

What to do about it

In the order you'd actually do it.

  1. Get the background figure for the area. Defra's background maps are on the UK-AIR website as open data, by 1 km square and by year. Hold the number against 10, not against 40.
  2. Check whether the street is in an AQMA. UK-AIR has the national list and maps, and the council will have the boundary. If it is, find out which pollutant and what the action plan says.
  3. Find the council's Annual Status Report. Councils report their air quality monitoring to Defra every year, and the report includes their diffusion tube results, which are often placed at specific roadsides and junctions. Search the council's website for "air quality annual status report". If there's a tube on or near your street, that's the closest thing to a measurement at your door you'll get.
  4. Assume the main road is worse than the map. The background figure leaves out local traffic by design, so if the front door opens onto a busy road, treat the number as a floor. Where you'd sleep, and which side the windows face, is worth thinking about at the viewing.
  5. Weigh it against the rest. For most buyers in London, air quality is a reason to prefer one street over another, not a reason to rule out an area. If you've got asthma in the household or a young child, give it more weight, and give the side-street flat a second look.

Nookaly shows each listing's modelled NO₂ in µg/m³ alongside noise, crime and flood risk. It's the background figure described above, with the same caveat, so do the comparison against 10 yourself.

The honest summary

The UK's legal limit for NO₂ is 40 and the WHO's guideline is 10. Across more than 340,000 listings in and around London since 2018, none breaks the first and the large majority break the second, and the figures are modelled background levels, so a busy road will be worse. An AQMA is a real signal; the absence of one isn't.

Legal is the floor. Ten is the number to compare against.

Worth reading next: noise, from railways to pubs, and what you can check, how to read crime stats without scaring yourself, and how to check flood risk properly.

This is general information, not health or legal advice. Figures and regulations are for England and are as at October 2026. Our NO₂ figures are modelled background concentrations from Defra's 1 km maps, assigned to each listing by area, and are not measurements at any property.