AirQuality.city switches to the European index, which rates the same air more strictly

From today AirQuality.city rates air quality on the European Air Quality Index rather than the US scale it has used until now. The air itself is no different. The European index judges it more strictly, so the map and the station pages will show a worse rating more often than before.

The gap is widest over whole days. Between mid-August and early November, roughly one day in five at the official monitoring stations was rated "Poor" or worse.^[1] Over the same period the old scale showed an unhealthy day only twice, across all stations put together. Days are rated by the site's own daily rule, because the European index gives its rating hour by hour.^[2] A station used to appear on the warnings page once its air turned unhealthy; it now appears there at "Poor" or worse.

Much of the difference comes from gases. The old rating was worked out from particles alone, while the European one is set by the worst-rated pollutant among those a station measures.^[2] In late summer that was most often ground-level ozone, which decided nine in ten of the "Poor" or worse days in August.^[1] By early November most daily ratings were already being set by fine particles, the dust and smoke small enough to reach deep into the lungs.

The page on the air quality index explains why we moved to the European Environment Agency (EEA) index, and the page on the scale sets out the ratings and the thresholds for each pollutant.