Air Quality Index (AQI) Interpreter
Interpret AQI readings on the 0-500 scale for PM2.5, PM10, ozone, and NO2.
Returns health category from Good to Hazardous with activity guidance.
The Air Quality Index (AQI) is a standardized scale used by the US EPA (and adapted by most countries) to communicate how clean or polluted outdoor air is at any given moment. It is calculated separately for up to six pollutants, and the highest individual value becomes the reported AQI.
AQI formula (EPA piecewise linear):
AQI = [(I_high − I_low) / (C_high − C_low)] × (C − C_low) + I_low
Where:
- C = observed pollutant concentration
- C_low, C_high = breakpoint concentrations bracketing C
- I_low, I_high = AQI breakpoints corresponding to C_low and C_high
AQI categories and health guidance:
| AQI | Category | Who is affected |
|---|---|---|
| 0–50 | Good | No health risk |
| 51–100 | Moderate | Unusually sensitive people |
| 101–150 | Unhealthy for Sensitive Groups | Elderly, children, asthma sufferers |
| 151–200 | Unhealthy | Everyone may experience effects |
| 201–300 | Very Unhealthy | Everyone, significant risk |
| 301–500 | Hazardous | Emergency conditions; everyone affected |
Primary pollutants tracked:
- PM2.5: Fine particles (< 2.5 microns), penetrate deep into lungs
- PM10: Coarser particles, dust, pollen, mold
- Ozone (O₃): Ground-level smog formed from sunlight + NOx + VOCs
- CO: Carbon monoxide from combustion
- SO₂: Sulfur dioxide from industrial sources
- NO₂: Nitrogen dioxide from vehicle emissions
Worked example — PM2.5: Observed concentration: 55.5 µg/m³ (24-hour average) Breakpoints: C_low=55.5, C_high=150.4 | I_low=151, I_high=200 AQI = [(200−151)/(150.4−55.5)] × (55.5−55.5) + 151 = 151 → Unhealthy
Protective actions at AQI > 100: Stay indoors, use HEPA air purifiers, avoid outdoor exercise, and wear N95 masks if outdoor exposure is unavoidable.