Air quality index (AQI)

An air quality index is a scale, not a measurement. Monitors report concentrations — micrograms of fine particles per cubic metre, parts per billion of ozone — and the index converts each of those onto a single unitless number, usually reporting whichever pollutant scores worst, in colour-coded bands from good to hazardous. The US EPA anchors its scale to national standards: an AQI of 100 generally corresponds to the national ambient air quality standard for that pollutant, across the five pollutants regulated under the Clean Air Act.

Because each scale is anchored to its own country's standards, indices are not comparable. India's AQI, Indonesia's ISPU and Mexico's IMECA use different pollutants, breakpoints and averaging times, so a headline putting one city's index next to another's is comparing two different rulers. What the number does varies even more. In Delhi it has legal force: since 2017, defined AQI bands trigger construction bans, freight restrictions and school closures. In Jakarta, where a court ordered the city to measure and the network grew from five reference stations to 111 monitoring points, no ISPU value triggers anything at all.

One trap is worth naming: consumer apps publish a daily index for cities with no working ground station, derived from a satellite model. Algiers has had no operating reference network since August 2009, and a number for it still appears every day. Find the station before you trust the reading.