The Air Quality Index is calculated using a straightforward formula that converts a pollutant’s measured concentration into a number on a 0-to-500 scale. You measure the concentration of a pollutant (like fine particulate matter or ozone), find which range it falls into on a standardized breakpoint table, then plug the values into a linear equation. The highest individual pollutant score becomes the reported AQI for that location.
The AQI Formula
The EPA uses one core equation for every pollutant:
AQI = ((AQI_high – AQI_low) / (C_high – C_low)) × (C – C_low) + AQI_low
Here’s what each variable means:
- C is the actual measured concentration of the pollutant (truncated to one decimal place for most pollutants).
- C_low is the concentration breakpoint that is equal to or just below your measured value.
- C_high is the concentration breakpoint just above your measured value.
- AQI_low is the AQI value corresponding to C_low.
- AQI_high is the AQI value corresponding to C_high.
This is a simple linear interpolation. You’re finding where your pollutant concentration sits within a defined range and mapping it to the corresponding AQI range. The formula works the same way for every pollutant; only the breakpoint numbers change.
The Six AQI Categories
The 0-to-500 scale is divided into six color-coded health categories:
- Green (Good): 0 to 50. Air quality is satisfactory with little or no health risk.
- Yellow (Moderate): 51 to 100. Acceptable, though unusually sensitive individuals may notice effects.
- Orange (Unhealthy for Sensitive Groups): 101 to 150. People with asthma, heart disease, or other vulnerabilities may experience symptoms.
- Red (Unhealthy): 151 to 200. Some members of the general public begin experiencing effects.
- Purple (Very Unhealthy): 201 to 300. Health risk increases for everyone.
- Maroon (Hazardous): 301 and above. Emergency conditions where the entire population is at risk.
Each of these ranges has specific concentration breakpoints for each pollutant. Those breakpoints are what make the formula work.
Breakpoint Tables by Pollutant
The EPA publishes breakpoint tables for five major pollutants: fine particulate matter (PM2.5), coarse particulate matter (PM10), ground-level ozone, carbon monoxide, and nitrogen dioxide. Each pollutant is measured over a specific averaging period. PM2.5 uses a 24-hour average, carbon monoxide uses an 8-hour running average, and nitrogen dioxide uses a 1-hour measurement.
For carbon monoxide (measured in parts per million), the breakpoints look like this: 0.0 to 4.4 ppm maps to Good (0–50), 4.5 to 9.4 ppm maps to Moderate (51–100), 9.5 to 12.4 ppm maps to Unhealthy for Sensitive Groups (101–150), and 12.5 to 15.4 ppm maps to Unhealthy (151–200). The pattern continues through Very Unhealthy and Hazardous.
For nitrogen dioxide (measured in parts per billion), the Good range is 0 to 53 ppb, Moderate is 54 to 100 ppb, and Unhealthy for Sensitive Groups is 101 to 360 ppb. You can find the complete breakpoint tables for all pollutants on the EPA’s AQI breakpoints page.
A Worked Example
Say your local monitor reports a 24-hour average PM2.5 concentration of 35.9 µg/m³. You’d look up the PM2.5 breakpoint table and find that 35.5 to 55.4 µg/m³ falls in the Unhealthy for Sensitive Groups category, which spans AQI values 101 to 150.
Plugging into the formula: C = 35.9, C_low = 35.5, C_high = 55.4, AQI_low = 101, AQI_high = 150.
AQI = ((150 – 101) / (55.4 – 35.5)) × (35.9 – 35.5) + 101
= (49 / 19.9) × 0.4 + 101
= 2.46 × 0.4 + 101
= 101.98
You’d round that to 102. The AQI for PM2.5 that day is 102, which falls in the orange “Unhealthy for Sensitive Groups” range.
How Multiple Pollutants Become One Number
You calculate a separate AQI value for every pollutant measured at a given monitoring station. The overall AQI reported for a location is simply the highest of those individual values. The pollutant responsible for that highest number is labeled the “main pollutant” for the day.
So if PM2.5 produces an AQI of 102, ozone produces 78, and carbon monoxide produces 42, the reported AQI is 102 with PM2.5 as the main pollutant. There’s no averaging or combining across pollutants. The worst one wins.
Real-Time AQI and the NowCast Algorithm
The standard AQI calculation uses 24-hour or 8-hour averages, which means it reflects yesterday’s air quality more than what’s happening right now. For real-time reporting, the EPA developed an algorithm called NowCast that estimates the AQI hourly using the past 12 hours of particulate matter measurements.
NowCast adjusts how much weight each of those 12 hours gets. When air quality is changing rapidly (like during a wildfire), the most recent hours count much more heavily. When conditions are stable, all 12 hours are weighted almost equally. The algorithm works by calculating a weight factor based on the range of measurements over those 12 hours:
- Find the minimum and maximum hourly readings from the past 12 hours.
- Divide the range (max minus min) by the maximum to get a scaled rate of change.
- Subtract that rate from 1 to get the weight factor.
- If the weight factor drops below 0.5, it’s set to 0.5 as a floor.
Each hour’s reading is then multiplied by the weight factor raised to a power equal to how many hours ago it was recorded. The most recent hour gets the weight factor raised to the power of 0 (which equals 1, so it always counts fully). Two hours ago gets the weight factor raised to the power of 1, three hours ago gets it squared, and so on. This means older readings fade out faster when conditions are volatile. The weighted average that results is then run through the standard AQI formula.
Why Other Countries Report Different Numbers
If you’ve ever compared AQI readings between U.S. and Chinese sources for the same city, you’ve probably noticed they don’t match. That’s because different countries use different breakpoint tables and averaging methods, even though the basic formula structure is similar.
China’s AQI system, for instance, uses PM2.5 breakpoints that are less strict than the U.S. system at AQI values below 200. A concentration that registers as “Unhealthy” on the U.S. scale might still read as “Moderate” on China’s scale. China also uses 24-hour average concentration breakpoints for hourly particulate matter calculations, while the U.S. uses the NowCast method for real-time readings. This difference can make China’s hourly AQI appear to overstate pollution severity during rapidly changing conditions, since a 24-hour breakpoint is being applied to what’s essentially a shorter snapshot.
India, the European Union, and other regions each have their own scales, some with different numbers of categories or different pollutant lists. When comparing air quality across countries, always check which standard is being used. An AQI of 100 in the U.S. does not represent the same concentration as an AQI of 100 in China.