The Air Quality Index (AQI) is calculated by measuring the concentration of specific pollutants in the air, then converting those raw numbers into a standardized scale from 0 to 500. The higher the number, the more polluted the air and the greater the health risk. Each pollutant is measured separately, and the highest individual score becomes the overall AQI reported for that area.
The Six Pollutants Behind the Number
The EPA sets air quality standards for six “criteria” pollutants under the Clean Air Act. These are the pollutants that monitoring stations track to produce AQI values:
- Ground-level ozone, formed when vehicle exhaust and industrial emissions react with sunlight
- Fine particulate matter (PM2.5), tiny particles 2.5 micrometers or smaller that penetrate deep into the lungs
- Coarse particulate matter (PM10), larger dust and debris particles up to 10 micrometers
- Carbon monoxide, a colorless gas mainly from vehicle exhaust
- Sulfur dioxide, released primarily by power plants burning fossil fuels
- Nitrogen dioxide, produced by cars, trucks, and power plants
Lead is also a criteria pollutant but is not typically included in daily AQI reporting. In most U.S. cities, the AQI is driven by either ozone or PM2.5, since those two tend to reach unhealthy levels more often than the others.
How Monitors Collect the Data
Regulatory monitoring stations use different technologies depending on the pollutant. For particulate matter, the gold-standard method is gravimetric sampling: air is pulled through a filter for 24 hours, and the filter is weighed before and after to determine exactly how much particle mass was collected. This gives a precise measurement but only produces one reading per day.
For continuous readings, stations use instruments that shine a beam of light through a sample of air. The amount of light scattered by particles correlates with how much particulate matter is present. These optical instruments provide hourly updates but need regular calibration against the gravimetric reference to stay accurate.
Gaseous pollutants like nitrogen dioxide are measured through chemical reactions inside the instrument. Nitrogen dioxide monitors work by first converting the gas into nitric oxide, then reacting that nitric oxide with ozone inside a chamber. The reaction produces a faint glow, and the intensity of that glow is proportional to the concentration of the gas. The instrument runs the air sample both with and without the conversion step, then subtracts one reading from the other to isolate the nitrogen dioxide concentration. Ozone, sulfur dioxide, and carbon monoxide each have their own detection methods tuned to their specific chemical properties.
Turning Concentrations Into an AQI Number
Raw pollutant concentrations don’t mean much to most people. Knowing that PM2.5 is at 22 micrograms per cubic meter doesn’t tell you whether it’s safe to go for a run. The AQI solves this by mapping concentration ranges to a simple scale with color-coded categories.
For PM2.5, the breakpoints work like this: a 24-hour average concentration of 0 to 9.0 micrograms per cubic meter corresponds to an AQI of 0 to 50, labeled “Good” (green). Concentrations of 9.1 to 35.4 map to an AQI of 51 to 100, or “Moderate” (yellow). From there, 35.5 to 55.4 hits “Unhealthy for Sensitive Groups,” 55.5 to 125.4 reaches “Unhealthy,” 125.5 to 225.4 is “Very Unhealthy,” and anything above 225.5 enters the “Hazardous” range, which can push the AQI above 300.
Each pollutant has its own set of breakpoints based on health research. The EPA calculates a sub-index for every pollutant being measured, and the highest sub-index becomes the reported AQI. So if ozone scores 85 and PM2.5 scores 140, the AQI for that hour is 140, and PM2.5 is listed as the “responsible pollutant.”
Real-Time Reporting vs. 24-Hour Averages
There’s a timing problem built into the AQI. The health research behind particulate matter standards is based on 24-hour exposure, which means the “true” daily AQI can only be calculated after a full day of data is collected. That’s not helpful if wildfire smoke rolls in at 2 p.m. and you need to decide whether to keep your windows open.
To solve this, the EPA uses a system called NowCast. Instead of waiting for a complete 24-hour average, NowCast takes the most recent 12 hours of readings and creates a weighted average. When air quality is changing rapidly, the algorithm gives much more weight to the most recent hours, so the number reflects current conditions rather than yesterday’s clean air dragging the average down. When conditions are stable, each of the 12 hours counts roughly equally. This is the number you see on apps like AirNow, and it updates every hour.
Why Consumer Sensors Often Disagree
If you own a portable air quality monitor or check a crowd-sourced network like PurpleAir, you may notice readings that differ from the official AQI. The reason comes down to how the sensors work. Low-cost monitors use small optical particle counters that estimate particle mass by counting individual particles and measuring how much light they scatter. Regulatory stations, by contrast, rely on gravimetric methods or more sophisticated optical instruments that are regularly calibrated.
Optical particle counters are particularly sensitive to changes in larger particles. A small increase in the number of coarse particles can cause a disproportionately large jump in the reported mass concentration, because bigger particles weigh more. Regulatory-grade instruments that measure total scattered light don’t react as strongly to the same shift. Studies comparing the two approaches consistently find different scaling factors, which is why the EPA emphasizes that consumer sensors need site-specific calibration against reference monitors to produce reliable readings. Many apps now apply correction factors to low-cost sensor data, but accuracy still varies depending on the type of particles in the air, humidity, and temperature.
Recent Changes to PM2.5 Standards
In February 2024, the EPA tightened the annual standard for fine particulate matter from 12.0 to 9.0 micrograms per cubic meter. This is the threshold used to determine whether a region meets federal air quality standards over the course of a year. The change reflects newer health research showing that long-term exposure to PM2.5 causes harm at lower concentrations than previously thought.
For daily AQI reporting, this revision shifted the breakpoints. The “Good” category now tops out at 9.0 micrograms per cubic meter instead of 12.0, which means areas that previously registered as “Good” may now show up as “Moderate” on days with slightly elevated particle levels. The practical effect is that the AQI has become more sensitive to fine particle pollution, and more communities will see days that trigger health advisories for sensitive groups like children, older adults, and people with asthma or heart disease.