What Happens When the AQI Goes Above 500?
The U.S. Air Quality Index is formally defined on a scale of 0 to 500, and 500 is the top of the published breakpoint table. Readings that exceed the concentration mapped to 500 are described by EPA as "Beyond the AQI" — the air is still being measured, but the index no longer has a defined value for it. Some apps and dashboards nonetheless print numbers above 500 by extending the last linear segment of the formula, which is an extrapolation rather than an official index value.
How the top of the scale is built
The AQI maps pollutant concentrations onto index values using a piecewise-linear function. Each category has a concentration range and a corresponding index range, and the formula interpolates between them. The tables and the equation are published in EPA's technical assistance document for reporting the daily AQI.
Because the function is defined only over the listed breakpoints, it has a last segment. Once a concentration exceeds the upper concentration breakpoint of that segment, there is no further row in the table to interpolate against. The index runs out before the air does.
The categories themselves, in order, are Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy, and Hazardous — described in plain terms on AirNow's AQI basics page and on the American Lung Association's AQI guide. The Hazardous band occupies the top of the range. Older presentations of the index split the top into two separately labeled segments; current reporting generally treats the whole upper range as one Hazardous category with one color. If you see an old chart with different labels above 300, that is a versioning difference, not a different country's scale. Our page on how to read the AQI covers the current bands.
Why the extrapolated numbers vary
When concentrations run past the table — which in practice is almost always fine particles during severe smoke or dust episodes — different publishers handle it differently:
- Report "Beyond the AQI" and give the measured concentration instead of an index number.
- Cap the display at 500, so the number stops moving while conditions keep changing.
- Extend the last line segment mathematically, producing values like 600 or 800.
All three are defensible presentation choices, but only the first two stay inside the defined index. This is one reason two sources can disagree dramatically at the extreme end of the scale while agreeing closely at ordinary levels.
There is a second reason the numbers diverge: the value in a real-time app is usually a NowCast estimate, which weights recent hours more heavily so the display responds to fast-changing conditions, while an official daily AQI is computed from a completed averaging period. During a smoke episode those two can be far apart. How the AQI is calculated explains the NowCast weighting.
Does every country's scale stop at 500?
No — and this is where scale comparisons get confusing. Other national indexes use different ranges, different category counts, different colors, and different breakpoint concentrations, so a number from one system cannot be read as a number from another. The Pollutant Standards Index used in parts of Southeast Asia is one example; AQI vs. PSI covers why those scales are not interchangeable.
What to look at when the index maxes out
At the top of the scale, the index is compressing a very wide range of concentrations into a narrow band of numbers, so the raw measurement carries more information than the index does. Official monitoring data — including the concentrations behind the index — is published through EPA's outdoor air quality data portal, and the agencies that issue daily reports are covered in where daily air quality reports come from. For conditions and official notifications during an episode, your state or local air agency and the National Weather Service are the issuing authorities.