Air Quality Index Calculator

Atmospheric Health Metrology & US EPA AQI Linear Interpolation

The US EPA Air Quality Index converts non-linear pollutant concentrations into an intuitive 0–500 scale:

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• EPA Linear Interpolation Formula (40 CFR Part 58)**:

I_p = I_high - I_lowC_high - C_low (C_p - C_low) + I_low

• I_p = Calculated Index for pollutant p

• C_p = Truncated concentration of pollutant p

• [C_low, C_high] = Pollutant concentration breakpoints

• [I_low, I_high] = AQI index breakpoints corresponding to C

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• US EPA PM2.5 Breakpoint Reference Table**:

• 0.0 - 12.0\ μg/m^3: AQI 0 - 50 (Good - Green)

• 12.1 - 35.4\ μg/m^3: AQI 51 - 100 (Moderate - Yellow)

• 35.5 - 55.4\ μg/m^3: AQI 101 - 150 (Unhealthy for Sensitive Groups - Orange)

• 55.5 - 150.4\ μg/m^3: AQI 151 - 200 (Unhealthy - Red)

• 150.5 - 250.4\ μg/m^3: AQI 201 - 300 (Very Unhealthy - Purple)

• 250.5 - 500.4\ μg/m^3: AQI 301 - 500 (Hazardous - Maroon)

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• Standard Sizing & Health Benchmarks**:

• PM2.5 = 6.0 µg/m³: AQI 25 | Good (Green) | No health restrictions

• PM2.5 = 22.0 µg/m³: AQI 72 | Moderate (Yellow) | Minor risk for sensitive individuals

• PM2.5 = 38.5 µg/m³: AQI 108 | USG (Orange) | Sensitive groups reduce outdoor exertion

• PM2.5 = 75.0 µg/m³: AQI 161 | Unhealthy (Red) | Everyone reduces outdoor exertion

• PM2.5 = 275.0 µg/m³: AQI 325 | Hazardous (Maroon) | Emergency condition; stay indoors

How to Use This Calculator

Enter your Pollutant Type & Averaging Time — choose from PM2.5 (Fine Particles, ≤ 2.5 µm - 24-Hour Avg in µg/m³), PM10 (Coarse Particles, ≤ 10 µm - 24-Hour Avg in µg/m³), Ozone (O3 - 8-Hour Avg in ppb), Measured Pollutant Concentration (e.g. 38.5 in µg/m³ / ppb / ppm), and Air Quality Benchmark — choose from Custom Pollutant Reading, Clean Mountain Air (PM2.5 = 6.0 µg/m³) → AQI 25 (Good - Green), Suburban Summer Haze (PM2.5 = 22.0 µg/m³) → AQI 72 (Moderate - Yellow). Results update instantly as you type — adjust any value to compare scenarios.

The Formula

Linear Interpolation: Ip = ((I_high - I_low) / (C_high - C_low)) * (Cp - C_low) + I_low

Worked Example

During a regional wildfire smoke plume, a local air sensor reads a 24-hr PM2.5 concentration of 38.5 µg/m³, which converts to an AQI of 108 (Orange - Unhealthy for Sensitive Groups), prompting school sports coaches to move soccer practices indoors.

Understanding Your Result

This calculation gives a precise result based on the inputs you provided. For practical decisions, consider whether the result changes meaningfully with slight variations in your inputs — if a 5–10% change in input causes a significant output change, gather more precise input data before deciding. Use this tool to quickly compare multiple scenarios side by side.