Part 1 — Website comparison: Lists global AQI map sites (IQAir, AQI.in, AirGradient, Aqicn, OpenAQ, Clarity, Air-quality.com) and their historical data access windows (ranging from 3 days to full history). Also notes country-specific maps for Japan (Soramame) and Ukraine (SaveEcoBot).
Part 2 — US AQI calculation method (per EPA technical guidance):
- Truncate each pollutant's raw concentration to a set precision (e.g., PM2.5 → 1 decimal, PM10/SO2/NO2 → integer, O3 → 3 decimals).
- Find the breakpoint interval (BPLo–BPHi) containing that value from the standard EPA table (categories: Good → Hazardous).
- Apply the linear interpolation formula: Ip = ((IHi−ILo)/(BPHi−BPLo))×(Cp−BPLo) + ILo.
- Round to nearest integer per pollutant; overall AQI = the highest of all pollutant sub-indices.
Worked example: PM2.5 = 4.234632 µg/m³ → truncated to 4.2 → AQI = 18.
About OpenAQ: Nonprofit (founded 2015 by Christ Hasenkopf & Joe Flasher) providing unified access to air quality data — combines government "reference monitor" data with private-company "air sensor" data (partners include Airgradient, Clarity, IQAir, Habitat Map, etc.). Individuals can't contribute data (orgs only), and without API access only the last 30 days are viewable per sensor.
Continent-by-continent sensor coverage tables (as of 14 July 2026), marking which countries have reference monitors and/or local air sensors, and flagging stale data (e.g., Russia's reference monitor last updated 9 years ago, Israel 7 years ago, Mongolia 7 years ago). Highlights:
- Europe: near-complete coverage; some gaps (Albania, Belarus, Liechtenstein, San Marino, Vatican City).
- Asia/Middle East: patchy — several countries have only stale reference data (Afghanistan 5yr, China 5yr, Turkey 3yr).
- Africa: sparse — mostly local air sensors only, with many countries lacking any current sensor.
- N/C America: US/Canada/Mexico well covered; most Caribbean islands lack sensors.
- S America: Argentina/Brazil/Peru only have one reference monitor city each; Chile is the only country with country-wide reference coverage.
- Oceania: mostly Australia/New Zealand only.
Data page structure: each sensor entry shows location/country, sensor type & measured pollutants, data source/ownership, last update & reporting-since date, and a "Show Details" view with hourly readings (last 24h–30 days) plus yearly hour-of-day pattern charts.