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 25 μg/m³ (falling to 10 μg/m³ by 2030 under the 2024 revision, Directive (EU) 2024/2881); 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 (one-fifth of the current 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%.

5b. Station-level figures

The individual station pages on PlainAirQuality show a modeled estimate, not an independently re-derived aggregation of that station's own raw hourly readings. Each station's annual mean is calculated as its country's EEA-reported population-weighted average, adjusted by a multiplier reflecting the station's classification: traffic stations +15%, industrial +25%, background −15% (ozone uses a separate, smaller traffic adjustment). This means two stations of the same classification within the same country will show identical modeled values, even though their real-world readings would differ — our current dataset does not yet ingest each station's own independent time series from the EEA Dataflow E1a. Treat station pages as "what a typical station of this type looks like in this country," not a substitute for the station's actual reported reading (available directly from the EEA datahub linked on each station page). We are working toward ingesting genuine per-station historical series in a future data update.

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.