Methodology / Air quality

Last updated: May 2026

Air Quality Methodology

How we compile the air quality section of a Home-Checker report — sources, scope, limitations, and the professional instruments this report is not a substitute for.

1. Data sources, licence, and last update

The air quality section of a Home-Checker report is generated at the moment you run the report. We do not pre-compute or cache pollutant readings — each report fetches the most recent measurements for the resolved coordinates of the postcode you entered.

  • Primary live data: Open-Meteo Air Quality API (open-meteo.com/en/docs/air-quality-api), which serves the European Copernicus Atmosphere Monitoring Service (CAMS) ensemble forecast on a roughly 11 km grid. CAMS itself assimilates measurements from European reference networks — including the UK's DEFRA Automatic Urban and Rural Network (AURN) — into its modelling. Licence: free for non-commercial and commercial use under Open-Meteo terms; CAMS source data is licensed under the Copernicus licence.
  • Reference standards: the World Health Organisation 2021 Global Air Quality Guidelines, Table 0.1. Every guideline level is an average: annual mean for PM2.5 (5 µg/m³), PM10 (15 µg/m³) and NO₂ (10 µg/m³); a six-month peak season for O₃ (60 µg/m³); a 24-hour mean for SO₂ (40 µg/m³) and CO (4 mg/m³).
  • Index: the European Air Quality Index scale, as exposed by Open-Meteo.
  • Last methodology update: May 2026. Data freshness: live at the moment of report generation (instantaneous CAMS forecast value for the latest hour available, typically less than three hours behind real time).

The DEFRA AURN is the canonical UK reference instrument for outdoor air quality and we name it explicitly here because Home-Checker's reading does not come directly from an AURN station — it comes from the European modelled forecast that ingests AURN data among other inputs.

2. What we do

For the lat/lng resolved from your postcode, the Home-Checker air quality module fetches the latest available hourly CAMS forecast for the following pollutants and surfaces them in your report:

  • PM2.5 — fine particulate matter, in µg/m³
  • PM10 — coarse particulate matter, in µg/m³
  • NO₂ — nitrogen dioxide, in µg/m³
  • O₃ — ground-level ozone, in µg/m³
  • SO₂ — sulphur dioxide, in µg/m³
  • CO — carbon monoxide, in µg/m³
  • The European Air Quality Index value for the location

Alongside each reading we show the WHO 2021 guideline for that pollutant and the period WHO measures it over. We do not tell you whether a reading is above or below its guideline, and we do not count “exceedances”. The reason is arithmetic rather than caution: the reading is one instantaneous hourly value, while the shortest WHO guideline is a 24-hour mean and most are annual. One hour of data cannot establish compliance with an average, in either direction, so a verdict either way would be a comparison between two different quantities.

The judgement we do give you is the European Air Quality Index band. Our data source calculates it from these same readings and publishes a value for each hour, so it is a figure this snapshot supports. The narrative commentary references only the figures we have fetched — nothing is invented or extrapolated.

The reading time, the lat/lng we resolved, and the WHO guidelines shown for reference are all visible in the report so you can audit the basis of what you are reading.

3. What we don't

We are deliberate about what falls outside the scope of this section so you can fill in the gaps with the right professional instrument when needed:

  • We do not measure indoor air quality, radon ingress, mould, volatile organic compounds, or any pollutant inside the building envelope.
  • We do not read directly from a specific AURN, AQE or local authority monitoring station. The CAMS grid is approximately 11 km wide; we cannot resolve street-level micro-environments.
  • We do not compare against the UK Government's Daily Air Quality Index (DAQI) scale; we use the European AQI which uses different banding.
  • We do not assess Air Quality Management Areas (AQMAs), Clean Air Zones, or Low Emission Zones designated by local authorities. Designation status is a separate planning concern and is not visible in our pollutant readings alone.
  • We do not provide annual mean concentrations, and we do not compare your reading against a WHO guideline. A single hourly value sitting above or below an annual mean tells you nothing about the annual mean at that location, so we show the guideline as a labelled reference and leave it at that.
  • We do not attribute pollution to specific sources (a particular road, factory, port, or landfill).

4. Known limitations

You should weight the air quality section with the following caveats in mind:

  • Resolution. CAMS is a regional model. A property next to a busy urban A-road and a property half a mile away on a quiet side street will share the same CAMS grid cell and therefore the same headline figure. Real-world local variation is masked.
  • Time-of-snapshot bias. Pollutant levels vary diurnally, with weather, and with traffic patterns. A reading taken at 03:00 on a calm Sunday looks very different from a reading taken at 09:00 on a Tuesday rush hour. The report shows the time of the reading; treat it as a snapshot, not an average.
  • Modelled, not measured. CAMS is an ensemble forecast, not a direct sensor reading. It is calibrated against measurement networks (including AURN) but the value you see is a model estimate at the grid cell, not a station reading.
  • Ozone and instantaneous values. The WHO ozone guideline (100 µg/m³) is an 8-hour mean. Open-Meteo returns instantaneous values, so we deliberately do not flag ozone as exceeding or meeting the WHO guideline — the comparison would be apples-to-oranges. The same applies to SO₂ and CO, which we report without a WHO comparison.
  • Pollens, allergens, asbestos, lead-in-paint and other building-specific airborne risks are not within the scope of this dataset.
  • Trends. A single report does not tell you whether air quality is improving or worsening over time. For trend data, consult the DEFRA UK-AIR data archive directly.

5. What this is NOT a substitute for

The Home-Checker air quality section is not a substitute for site-specific air quality monitoring or a professional environmental health assessment.

If you have a specific concern — a planning objection, a health-related relocation decision, a workplace exposure question, or a property next to a known industrial or traffic source — the right instruments are:

  • A site-specific air quality survey using calibrated reference-grade or indicative monitoring equipment, commissioned through a consultancy accredited under the Institute of Air Quality Management (IAQM) or the Chartered Institute of Environmental Health (CIEH).
  • For a planning or development context: an air quality assessment carried out to IAQM and EPUK guidance.
  • For health-related concerns: advice from a GP and, where needed, referral to the local authority Environmental Health team, who can act on statutory nuisance and review AQMA designations.
  • For raw measured data at the nearest reference station: the DEFRA UK-AIR portal.

See our Report Disclaimer for the full position on what our reports do and do not constitute.