Air Quality Index (AQI) Formula
Convert pollutant concentrations to AQI values.
Understand air quality levels from good to hazardous.
The Formula
The AQI converts a pollutant concentration into a standardized index from 0 to 500. It uses breakpoint tables to interpolate linearly between defined AQI category boundaries.
Variables
| Symbol | Meaning |
|---|---|
| AQI | Air Quality Index value (0-500) |
| C | Measured pollutant concentration |
| C_low, C_high | Concentration breakpoints surrounding C |
| I_low, I_high | AQI breakpoints corresponding to C_low and C_high |
AQI Scale: 0-50 Good, 51-100 Moderate, 101-150 Unhealthy for Sensitive Groups, 151-200 Unhealthy, 201-300 Very Unhealthy, 301-500 Hazardous.
Example 1
PM2.5 concentration is 35.9 μg/m³. Find the AQI.
Breakpoints: C_low = 35.5, C_high = 55.4, I_low = 101, I_high = 150
AQI = ((150 - 101) / (55.4 - 35.5)) × (35.9 - 35.5) + 101
AQI = (49 / 19.9) × 0.4 + 101 = 2.462 × 0.4 + 101
AQI ≈ 102 (Unhealthy for Sensitive Groups)
Example 2
Ozone concentration is 0.060 ppm. Find the AQI.
Breakpoints: C_low = 0.055, C_high = 0.070, I_low = 51, I_high = 100
AQI = ((100 - 51) / (0.070 - 0.055)) × (0.060 - 0.055) + 51
AQI = (49 / 0.015) × 0.005 + 51 = 16.33 + 51
AQI ≈ 67 (Moderate)
When to Use It
Use the AQI formula when:
- Converting raw pollution measurements into health-relevant values
- Comparing air quality across different locations
- Deciding whether outdoor activities are safe on a given day
- Reporting air quality data to the public