Methodology

1. Data sources

Three primary streams flow into every PlainAirQuality page:

  • EEA Air Quality e-Reporting database — the authoritative European register of regulatory air-quality observations, maintained by the European Environment Agency. Member states are required under Directive 2008/50/EC and Decision 2011/850/EU to report hourly observations from regulatory monitoring stations. Stations are categorized as traffic (kerbside/roadside), background (urban / suburban / rural), or industrial. We use the Validated data (Dataflow E1a), published annually each September for the preceding calendar year.
  • Copernicus Atmosphere Monitoring Service (CAMS) — the European Centre for Medium-Range Weather Forecasts produces a 0.1° gridded reanalysis combining satellite (TROPOMI, MODIS, AATSR), aircraft (IAGOS), and ground-station observations. CAMS provides the trans-boundary attribution and population-weighted exposure metrics used in our country-level premature deaths attributable to PM2.5 estimates.
  • National agency feeds — UBA (Germany), ADEME / Atmo (France), ISPRA / ARPA (Italy), GIOS / WIOS (Poland), Defra (UK), APA (Portugal), CHMI (Czech Republic), HSY (Finland), SLB-analys (Sweden), DCMR (Netherlands), and 18 equivalent national bodies. These are used for the most recent station-level data where the EEA validated set lags — we mark such pages with a "provisional" data vintage banner until the EEA validates them.

2. Pollutants and benchmarks

PlainAirQuality covers the seven pollutants regulated under the EU Ambient Air Quality Directive 2008/50/EC and revised by the 2024 EU directive: PM2.5, PM10, NO2, ground-level O3, SO2, CO, and benzene (C6H6).

On every detail page we show two benchmarks side by side:

  • EU limit value — the regulatory limit currently in force under Directive 2008/50/EC. For PM2.5 the annual limit is 10 μg/m³; for NO2 it is 40 μg/m³. Member states must take measures when limits are exceeded.
  • WHO 2021 guideline — the World Health Organization Global Air Quality Guidelines (2021 update), which describe concentrations below which no significant adverse health effect is expected. For PM2.5 the WHO guideline is 5 μg/m³ (half the EU limit); for NO2 it is 10 μg/m³ (one-quarter of the EU limit).

3. Measurement methods

All measurement methods cited on this site follow the European reference methods mandated by Annex VI of Directive 2008/50/EC:

  • PM2.5 / PM10: Gravimetric reference (EN 12341). Equivalent automated methods (TEOM-FDMS, beta-attenuation, optical particle counters) are permitted when calibrated against the gravimetric reference.
  • NO2: Chemiluminescence with photolytic converter (EN 14211).
  • O3: UV photometry (EN 14625).
  • SO2: UV fluorescence (EN 14212).
  • CO: Non-dispersive infrared spectroscopy / NDIR (EN 14626).
  • Benzene: Active or diffusive sampling on Tenax cartridges with thermal desorption / gas chromatography (EN 14662).

4. Aggregation

Hourly station observations are aggregated to:

  • Annual mean — arithmetic mean of all valid hourly values for the calendar year. Data capture ≥ 75% required per Directive 2008/50/EC.
  • Daily mean — for PM10 and SO2, where the EU 24-hour limit applies.
  • 8-hour maximum — for O3 (EU target value 120 μg/m³ over an 8-hour rolling mean, not to be exceeded more than 25 days per calendar year averaged over three years).
  • Exceedance day count — the number of calendar days on which the daily mean exceeds the WHO guideline (PM2.5: 15 μg/m³; PM10: 45 μg/m³).

5. Country-level statistics

Country averages on PlainAirQuality use the EEA-published population-weighted exposure metric — not a simple arithmetic mean across stations. This means a metropolitan area with millions of residents and dense monitoring contributes proportionally more to the national figure than a sparse rural region with a single background station. The population weighting follows the EEA's 1-km Gridded Population of Europe (GHS-POP) methodology.

Mortality attribution — the "premature deaths attributable to PM2.5" figure on each country page — uses the EEA's published health-impact assessment methodology (Health Risks of Air Pollution in Europe, HRAPIE, project recommendations), applied to the WHO 2021 concentration-response functions. We report the central estimate; the underlying uncertainty range is roughly ±30%.

6. Update frequency

The EEA publishes the validated dataset (Dataflow E1a) annually each September for the preceding calendar year. We refresh the entire site within two weeks of each release, and a banner appears on every page showing the data vintage (e.g. "Latest validated year: 2024, published September 2025"). Provisional national-agency data is used only where flagged as such.

7. Limitations

Air quality at any street, building, or specific moment may differ substantially from the station-level annual means we publish. Personal exposure depends on local micro-meteorology, indoor sources, commuting patterns, and short-duration episodes (e.g. wildfire smoke, Saharan dust intrusions, agricultural ammonia events). We publish station-level annual means because they are the regulatory metric; we do not publish hourly real-time data, real-time AQI, or hyperlocal exposure forecasts. For real-time alerts consult the European Air Quality Index or your national agency.

8. Corrections

If you find an error on PlainAirQuality — an incorrect station identifier, out-of-date capital city, mis-attributed operator, or any data-source discrepancy — please email hello@plainairquality.com. We respond within two business days and publish a corrections log linked from the footer once a correction has been verified against the underlying source.