How Do I Convert AQI to PSI? Why There's No Conversion Factor

The short answer

There is no conversion factor, and there cannot be a reliable one. An AQI value is not a measurement with a unit — it is a score produced by running a measured pollutant concentration through a country-specific lookup table, so converting one index into another is not arithmetic in the way that converting miles into kilometers is. The only quantity that genuinely travels between air quality scales is the underlying pollutant concentration, in µg/m³, ppm, or ppb.

An index number is not a unit

The U.S. Air Quality Index starts with monitored concentrations of specific pollutants. EPA's technical assistance document for reporting the daily AQI defines a breakpoint table for each pollutant: a concentration range is mapped, by linear interpolation, onto an index range. Every pollutant measured gets its own sub-index, and — as AirNow's AQI basics page explains — the AQI reported for a location is the highest of those sub-indices, not an average of them. Our page on how the AQI is calculated walks through that procedure step by step.

Three consequences follow, and together they kill the idea of a conversion factor:

• The mapping is piecewise, not proportional. Each category band has its own slope, so an AQI of 80 is not "twice" an AQI of 40 in concentration terms.
• The number hides its own cause. A reported AQI tells you the value of the worst sub-index, but not on its own which pollutant produced it. Without that, you cannot run the table backward to recover a concentration.
• The tables are jurisdictional. Breakpoints reflect each authority's own standards, pollutant list, and averaging times. Two indexes built on different tables answer different questions.

What the PSI is, and what to check before comparing

The Pollutant Standards Index is an air quality index published by certain national environment agencies. Which pollutants it covers, what averaging periods it uses, and where its category boundaries sit are all set by the agency that publishes it — so the authoritative description of any PSI figure is that agency's own documentation, not a U.S. reference. Our background page on AQI vs. PSI covers why different countries run different scales in the first place.

Before you compare any two index readings, check three things on each side:

• Which pollutants feed the index. If one scale includes a pollutant the other omits, the two numbers are not measuring the same air.
• The averaging time. A value built on a 24-hour average behaves very differently from one built on a rolling short-term average.
• Whether the number is a forecast or an observation. Forecast values and monitored values can differ for the same day and place.

Compare concentrations, not scores

The practical workaround is to drop down a level. Most reporting agencies publish the raw concentration alongside the index value; that concentration is the thing you can carry from one system to another. Fine particle pollution is reported in micrograms per cubic meter, while gases such as ozone and carbon monoxide are reported in parts per million or parts per billion — our page on air quality units explains which pollutant uses which.

From there, you can compare a concentration to whichever legal standard you care about. In the United States those limits live in the EPA's NAAQS table, which lists the level, averaging time, and form for each of the criteria air pollutants. That is a like-for-like comparison. Index-to-index is not.

One more source of confusion

"PSI" is also the everyday abbreviation for pounds per square inch, a unit of pressure used for tires, pipes, and pressure vessels. It has nothing to do with air quality, and no air quality quantity is ever expressed in it. If a search brought you here from a tire-pressure question, that is the collision at work — the two uses share three letters and nothing else.