Air Pollution Index Calculator

PM2.5 Concentration Isn't the Same as the Number You See on an App

Weather and air-quality apps display a single AQI number, but that figure isn't the raw pollutant concentration — it's a piecewise-linear transformation of it, mapped onto a standardized 0–500 scale defined by the EPA. This calculator applies that exact mapping for PM2.5, the fine particulate matter that drives most day-to-day air quality concerns, converting a measured concentration in micrograms per cubic meter into the familiar AQI number and category.

The Formula

AQI = ((AQI_high − AQI_low) ÷ (BP_high − BP_low)) × (C_p − BP_low) + AQI_low

Where C_p is the measured PM2.5 concentration and BP_low/BP_high are the EPA breakpoint concentrations bracketing it, with AQI_low/AQI_high the corresponding AQI values for that bracket. The scale isn't a single straight line — each breakpoint range has its own slope, which is why the same 10 µg/m³ increase in concentration can shift the AQI by a very different amount depending on where you start.

Where AQI Matters

Outdoor activity decisions — health guidance for sensitive groups (people with asthma, older adults, children) typically kicks in once AQI crosses specific published thresholds.
Wildfire smoke tracking — PM2.5 spikes from wildfire smoke are one of the most common real-world drivers of AQI swinging rapidly into unhealthy territory.
Comparing locations — converting raw sensor readings from different monitors into the same standardized AQI scale makes air quality directly comparable across cities or neighborhoods.

EPA PM2.5 Breakpoints and Example Readings

EPA AQI breakpoints for PM2.5 (24-hour average) and category labels
PM2.5 range (µg/m³)	AQI range	Category
0.0 – 9.0	0 – 50	Good
9.1 – 35.4	51 – 100	Moderate
35.5 – 55.4	101 – 150	Unhealthy for Sensitive Groups
55.5 – 125.4	151 – 200	Unhealthy
125.5 – 225.4	201 – 300	Very Unhealthy
225.5 – 325.4	301 – 500	Hazardous

Example concentrations converted to AQI using the calculator's own formula
PM2.5 (µg/m³)	AQI
5	28
12	56
40	112
80	168
150	225

Breakpoints are the official EPA values for PM2.5; AQI outputs above are computed directly from the calculator's own formula.

How to Use This Calculator

Enter the PM2.5 Concentration in micrograms per cubic meter (µg/m³).
Select Calculate to get the corresponding AQI value and health category.

Related Calculations

Concerned about other forms of environmental exposure? See the Noise Pollution Calculator for a comparable exposure-based reference figure.