Tukuk-OSAir quality indices do one job: convert raw pollutant concentrations into a single number where higher is worse. The US AQI runs 0–500 in six bands — 0–50 good, up to “hazardous” above 300 — computed separately for several pollutants, and the headline number is whichever pollutant hurts most, usually the driver of the day. It is a public-health alarm clock, deliberately coarse enough to act on without a degree in chemistry.
The pollutant that matters most for longevity is PM2.5: particles under 2.5 micrometres — around 30 of them side by side across a human hair. At that size, inhaled particles bypass every defence your airways have, lodge in deep lung tissue and cross into blood. Sources: combustion — vehicles, cookfires, cigarettes, crop burning, coal — not pollen, which is far bigger. The World Health Organization’s 2021 guideline for annual average exposure is 5 µg/m³, halving its previous target because the evidence kept hardening; much of the world’s population lives above 35.
Understand what a live ranking like our /air-quality page is measuring: satellite- and chemistry-transport-model estimates at city coordinates (the Copernicus CAMS service, via Open-Meteo), refreshed hourly. Models are brilliant for regional comparison and wrong for your street, where a busy road or a cooking fire can triple the local number. Treat the city figure like a fever chart, not a diagnosis.