expatwhy.

best of, air quality

Cities with the cleanest air, ranked by verified PM2.5

The WHO 2021 guideline puts the safe annual mean PM2.5 at 5 µg/m³. Anything above 35 µg/m³ counts as “Interim Target 1”, the floor for the most polluted countries. We pull hourly PM2.5 from Copernicus CAMS for the last 12 months and average it per city. CAMS is a model: it includes sea salt, so coastal cities read high against ground stations. Ranking within the list is sound; treat the absolute figure as modelled. Lower is better.

frequently asked

Quick answers.

Where does this air-quality data come from?
Hourly PM2.5 readings from the Copernicus Atmosphere Monitoring Service (CAMS), accessed via Open-Meteo's free Air Quality API. CAMS is the European Union's atmospheric monitoring program, same models used in official EU air-quality reports. CC-BY 4.0.
Why is PM2.5 the metric here?
PM2.5 (fine particulates under 2.5 µm) penetrates deep into lung tissue and is the single best-correlated air-quality measure with long-term health outcomes. The WHO 2021 guideline lowered the annual safe limit to 5 µg/m³.
Does the ranking change seasonally?
Yes, that's the point of showing 12 monthly means rather than one annual number. Chiang Mai is clean nine months of the year but spikes 40-55 µg/m³ in Feb-April burning season. Each city's detail page shows the 12-month curve.

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