Methodology
How we calculate and present air quality data on AQI Now.
AQI Standards
We compute two AQI standards for every city:
- US EPA AQI — The international standard used by the United States Environmental Protection Agency. Covers 6 pollutants: PM2.5, PM10, O₃, CO, NO₂, SO₂.
- India NAQI— The National Air Quality Index defined by India's Central Pollution Control Board (CPCB). Covers 7 pollutants (adds NH₃). Used as the primary index for Indian cities.
Calculation Method
For each pollutant, we use linear interpolation between standard breakpoint values to convert concentrations to sub-index values. The overall AQI is the maximum of the active pollutant sub-indexes (PM2.5 and PM10 for EPA; all pollutants for NAQI).
Breakpoint Tables
EPA breakpoints follow the AirNow technical specification. NAQI breakpoints follow the CPCB NAQI documentation.
Unit Conversion
Some upstream APIs return gas concentrations in ppb (parts per billion). We convert these to µg/m³ using molecular weight at standard temperature and pressure (25°C, 1 atm):
µg/m³ = ppb × (molecular_weight / 24.45)
Health Metrics
- Cigarette equivalents: PM2.5 concentration ÷ 22 (based on Berkeley Earth research)
- Life expectancy impact: (PM2.5 − 5) × 0.098 years (based on AQLI methodology)
AQI Categories
| AQI Range | Category | Health Message |
|---|---|---|
| 0–50 | Good | Air quality is satisfactory |
| 51–100 | Moderate | Acceptable for most people |
| 101–150 | Unhealthy for Sensitive Groups | Sensitive individuals should limit outdoor exertion |
| 151–200 | Unhealthy | Everyone may experience health effects |
| 201–300 | Very Unhealthy | Health alert — serious effects for everyone |
| 301–500 | Hazardous | Emergency conditions |
Data Freshness
Rankings are refreshed 3× daily (6 AM, 2 PM, 10 PM IST). City-level live AQI is fetched on demand from Open-Meteo CAMS and cached for up to 15 minutes. Google Air Quality API is supported as an optional source via a runtime flag.