There is no single air-quality index. Different jurisdictions and different agencies have chosen different mathematics for the same goal: condense several measurements into one number a person can act on. The four most relevant for North American and European users are the EPA AQI, the WHO 2021 guidelines, the Canadian AQHI, and the European CAQI.
EPA AQI (US): a 0 to 500 number representing the worst of five regulated pollutants (ozone, PM2.5, PM10, CO, NO2, SO2). Threshold-based bands (Good, Moderate, Unhealthy for Sensitive, Unhealthy, Very Unhealthy, Hazardous). Designed for regulatory enforcement first, public communication second. WHO 2021: not an index in the same sense. Per-pollutant health-based threshold values, tighter than EPA, with no single integrated score. Used as health-protection targets rather than for daily decision-making.
Canadian AQHI: a 0 to 10+ number designed by Environment and Climate Change Canada to represent combined health risk, computed from ozone, NO2, and PM2.5 using empirically-derived health-impact coefficients. Lower numbers, additive across pollutants rather than worst-of-N. European CAQI (Common Air Quality Index): a 0 to 100 number used by the European Environment Agency. Designed for hour-by-hour public reporting; pollutant selection differs slightly from EPA.
For Terrestream users, the dashboard surfaces the AQI scale appropriate to your region by default and offers the WHO 2021 stricter alternative. The math difference is non-trivial: a "moderate" outdoor day on EPA AQI may be "high risk" on Canadian AQHI for the same air. The interpretation layer reads your selected scale when deciding what counts as actionable. See AQI vs IAQ, WHO vs EPA, and Canadian AQHI for per-index detail.
References
- EPA - AirNow: AQI Basics www.airnow.gov
- WHO Global Air Quality Guidelines (2021) www.who.int
- Environment and Climate Change Canada - AQHI www.canada.ca
- European Environment Agency - Common Air Quality Index www.eea.europa.eu
