How Is the AQI Calculated? Breakpoints, the Highest-Pollutant Rule, and NowCast

The AQI is a conversion, not a measurement

Monitors don't measure "AQI." They measure concentrations — micrograms of fine particles per cubic meter of air, parts per billion of ozone, and so on. The AQI is the conversion layer that turns those raw concentrations into one 0–500 number the public can use. The full method is defined in the EPA's technical assistance document for reporting the daily AQI, which is the primary reference this page summarizes. (If you just want to interpret the final number, start with how to read the AQI — this page is about where that number comes from.)

Step 1: Each pollutant gets its own sub-index

The AQI covers the major criteria air pollutants — ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. For each one, the EPA publishes a table of breakpoints: concentration ranges that map onto AQI ranges.

The calculation is linear interpolation within a breakpoint range. In plain terms: if a pollutant's concentration falls a third of the way through the range that corresponds to AQI 51–100, its sub-index lands roughly a third of the way between 51 and 100. Each pollutant's measured concentration is run through its own table, producing a separate sub-index for each pollutant.

Two things about the breakpoints are worth knowing:

• They are anchored to the health standards. An AQI of 100 is designed to line up with the pollutant's short-term national standard, so "over 100" means "worse than the benchmark" for that pollutant.
• Each pollutant uses its own averaging time. Ozone is typically averaged over 8 hours, PM2.5 over 24 hours, carbon monoxide over 8 hours, and so on — matching the averaging times in the NAAQS table. You cannot compare a raw ozone reading to a raw PM2.5 reading, which is precisely the problem the index exists to solve.

Step 2: The highest sub-index wins

Here is the rule that surprises most people: the reported AQI is not an average of the pollutants. It is the maximum — the single worst sub-index at that location. If ozone works out to 62 and PM2.5 to 105, the reported AQI is 105, and PM2.5 is named as the "primary pollutant."

This is a deliberate design choice. Averaging would let one bad pollutant hide behind several good ones. Taking the maximum guarantees the headline number reflects the most pressing problem in the air, whatever it is. It also explains a common confusion: two cities can post the same AQI for completely different reasons — one because of summer ozone, the other because of wintertime particle pollution.

Step 3: Reporting in real time — the NowCast

There's a timing problem baked into the averaging times. A true 24-hour PM2.5 average isn't known until the day is over, which is useless when a smoke plume arrives at 2 p.m. For current conditions, AirNow therefore uses the NowCast, a weighted calculation that leans on the most recent hours of data — weighting recent hours more heavily when conditions are changing fast. That is how the current AQI on AirNow can respond to a sudden smoke event within hours, while the official daily value is still computed from the full averaging period. It is also why a "current" AQI and the final daily AQI for the same date can differ; they are answering slightly different questions.

Where the input data comes from

The concentrations feeding all of this arithmetic come from regulatory-grade monitors operated by state, local, tribal, and federal agencies. The EPA publishes both the current and historical measurements through its outdoor air quality data portal, so anyone can pull the underlying concentrations for a given monitor and date and check the math against the breakpoint tables. Whether a monitor exists near you — and which pollutants it measures — determines which sub-indexes can even be calculated for your area, which is one reason neighboring towns sometimes report different AQI detail.