The Air Quality Index (AQI) is measured by monitoring the concentration of specific pollutants in the air, then converting those raw concentrations into a standardized scale from 0 to 500. The higher the number, the more polluted the air and the greater the health risk. In the United States, the EPA calculates AQI based on six major pollutants, and the single highest pollutant reading on any given day becomes the reported AQI value.
The Six Pollutants Behind the Number
The Clean Air Act requires the EPA to set air quality standards for six “criteria” pollutants: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. Each has its own concentration thresholds tied to known health effects. On most days, the AQI you see on a weather app is driven by just one or two of these, typically ozone in summer and particulate matter in winter or during wildfire events.
Particulate matter gets split into two categories: PM2.5 (fine particles smaller than 2.5 micrometers, roughly 30 times thinner than a human hair) and PM10 (coarser particles up to 10 micrometers). PM2.5 is the more dangerous of the two because particles that small can travel deep into the lungs and even enter the bloodstream.
How Raw Pollution Becomes a 0-to-500 Score
Each of the six pollutants has a set of “breakpoints,” specific concentration ranges that correspond to AQI ranges. The EPA measures the actual concentration of each pollutant, finds where it falls within those breakpoints, and uses a formula to scale it to a number between 0 and 500. This process is called linear interpolation: if a pollutant’s concentration lands halfway between two breakpoints, the AQI score lands halfway between the corresponding index values.
For example, a 24-hour average PM2.5 concentration of 0.0 to 9.0 micrograms per cubic meter maps to an AQI of 0 to 50 (“Good”). A reading of 9.1 to 35.4 maps to 51 to 100 (“Moderate”). The EPA calculates a sub-index for each pollutant separately, and whichever pollutant scores highest becomes the overall AQI for that location. That dominant pollutant is also reported so you know what’s driving the number.
What the AQI Categories Mean
The 0-to-500 scale is broken into six color-coded categories:
- Green (0 to 50), Good: Air quality is satisfactory with little or no health risk.
- Yellow (51 to 100), Moderate: Acceptable for most people, though unusually sensitive individuals may notice effects.
- Orange (101 to 150), Unhealthy for Sensitive Groups: People with asthma, heart disease, or other respiratory conditions may experience symptoms. Most others won’t.
- Red (151 to 200), Unhealthy: The general public may start to feel effects, and sensitive groups face more serious risk.
- Purple (201 to 300), Very Unhealthy: Health risk is elevated for everyone.
- Maroon (301+), Hazardous: Emergency conditions where everyone is likely affected.
Real-Time AQI vs. Daily AQI
The official daily AQI for particulate matter uses a full 24-hour average concentration, which means the true value can’t be calculated until the day is over. That’s not very useful when smoke rolls in at noon and you want to know whether it’s safe to go outside. To solve this, the EPA developed a method called NowCast that estimates AQI in real time using a weighted average of the past 12 hourly measurements.
NowCast adapts to changing conditions. When pollution levels are shifting rapidly, it weights the most recent hours more heavily so the number reflects what’s actually happening outside. When conditions are stable, it weights all 12 hours more equally to smooth out noise. The AQI number you see on apps like AirNow updates hourly and uses this NowCast approach, so it’s already an estimate rather than a locked-in daily figure. Ozone uses a similar concept but is typically based on an 8-hour running average.
How Pollutants Are Physically Measured
The gold standard for measuring particulate matter is called a federal reference method. These stations collect air on filters over a 24-hour period, then weigh those filters in a lab to determine exactly how much particulate mass was captured. The results are extremely precise, but the tradeoff is obvious: you don’t get data in real time.
For continuous monitoring, regulatory stations use instruments that measure particles using light scattering, vibration frequency changes, or other physics-based techniques that report concentrations every hour. These feed the real-time AQI readings you see online. Ozone and gaseous pollutants like carbon monoxide, sulfur dioxide, and nitrogen dioxide are measured with dedicated analyzers at these same monitoring stations, each using methods specific to the chemistry of that gas.
Consumer Sensors vs. Government Monitors
Low-cost air quality sensors, the kind built into home monitors or community networks like PurpleAir, use small optical particle counters. A tiny laser shines into a chamber, particles pass through the beam, and the sensor counts the flashes of scattered light. Algorithms then estimate particle mass from those counts.
These sensors correlate well with reference instruments in typical outdoor conditions. Lab testing has shown correlation coefficients above 0.96 for several popular sensor models in environmental settings. But accuracy drops in unusual conditions. Sensors tend to overestimate pollution in occupational settings with heavy dust or unique aerosol types, and their readings can drift depending on particle size and composition. Humidity is another complicating factor, since water droplets on particles make them look larger than they are.
If you’re using a consumer sensor, treat its AQI reading as a reasonable estimate rather than a precise measurement. It’s reliable enough to tell you whether air quality is good, moderate, or unhealthy, but it may not nail the exact number the way a government station would.
Recent Changes to PM2.5 Standards
In 2024, the EPA lowered the annual PM2.5 standard from 12.0 to 9.0 micrograms per cubic meter, reflecting newer evidence that long-term exposure to fine particles is harmful at lower levels than previously thought. This change also shifted the AQI breakpoints: a 24-hour PM2.5 reading of 9.0 micrograms per cubic meter now marks the top of the “Good” category, where it previously would have registered well within it. The practical effect is that some areas that used to report “Good” air quality will now show “Moderate” readings more often, giving residents a more conservative picture of their exposure.
Why AQI Scales Differ by Country
The 0-to-500 scale is specific to the United States. Other countries use their own systems with different breakpoints, different pollutant lists, and sometimes different scales entirely. China uses a similar 0-to-500 structure but sets its breakpoints at higher concentrations, meaning the same pollution level might register as a lower number in Beijing than in Los Angeles. India’s system runs from 0 to 500 as well but includes eight pollutants. The World Health Organization publishes air quality guidelines but doesn’t maintain a single index, leaving each country to define its own thresholds.
This means you can’t directly compare AQI numbers between countries. An AQI of 100 in the U.S. does not represent the same pollution level as 100 in India or China. If you’re traveling or comparing cities internationally, check which scale is being used and look at the raw pollutant concentrations if you want a true comparison.