What Units Is Air Quality Measured In? µg/m³, ppm, and ppb (and Why Not psi)

The Air Quality Index has no units at all — it is an index number, typically reported on a 0 to 500 scale, not a measured quantity. The measurements underneath it do have units: particle pollution is reported in micrograms per cubic meter of air (µg/m³), and gases like ozone and carbon monoxide are reported in parts per million (ppm) or parts per billion (ppb). Pounds per square inch (psi) is a pressure unit used for things like tires and plumbing; it never appears in air quality reporting, and when you see "PSI" in a pollution context it almost always means Pollutant Standards Index.

Why an index has no unit

The AQI is a translation layer. It takes a pollutant concentration, compares it against a table of breakpoints, and returns a number on a common scale so that ozone and fine particles — measured in completely different units — can be described with one figure. Because the output is a position on a scale rather than an amount of anything, it is dimensionless. AirNow's AQI basics page describes the index in terms of categories and colors rather than units for exactly this reason.

That is also why "converting" an AQI number into any physical unit is a step backwards through the breakpoint table, not a multiplication. See how the AQI is calculated for the mechanics.

Micrograms per cubic meter (µg/m³)

Particulate matter is measured as mass per volume of air. A µg/m³ figure says how many millionths of a gram of suspended particle material are floating in each cubic meter of air. This works for particles because they are a physical mass of varied material — soot, dust, sulfates, organics — rather than a single molecule you could count as a fraction of the air.

PM2.5 and PM10 both use µg/m³, and both are sampled by drawing air through a size-selective inlet so only particles below the stated diameter are collected. EPA's particulate matter hub covers what those size cutoffs mean; the practical difference between the two is laid out in PM2.5 vs. PM10. Lead is also a criteria pollutant reported in µg/m³.

Parts per million and parts per billion

Gaseous pollutants are usually reported as a mixing ratio: how many molecules of the pollutant exist per million or per billion molecules of air. Ozone, nitrogen dioxide, and sulfur dioxide are typically discussed in ppb; carbon monoxide, which occurs at much higher concentrations, is reported in ppm. EPA's pages on ground-level ozone and carbon monoxide use these units throughout.

Mixing ratios have a useful property: they do not change when air warms or is compressed, because both the pollutant and the surrounding air expand together. Mass-per-volume units do change with temperature and pressure, which is why converting ppb to µg/m³ for a gas requires knowing the molecular weight of the gas plus the temperature and pressure conditions assumed. There is no single fixed conversion factor that works for every gas.

Where to see the units side by side

The cleanest place to see which pollutant uses which unit is the EPA NAAQS table, which lists every standard with its level, unit, and averaging time. Ozone appears in ppm, carbon monoxide in ppm, nitrogen dioxide and sulfur dioxide in ppb, and the particle and lead standards in µg/m³. Our walkthrough of how to read the NAAQS table explains the columns.

So where does psi come from?

Two separate places. One is simple unit confusion — psi is a familiar abbreviation from pressure gauges, so people reaching for "the unit of air" sometimes land on it. The other is genuine: Singapore and several other jurisdictions historically reported air quality using the Pollutant Standards Index, abbreviated PSI, which is a different index built on similar ideas to the AQI. If you arrived here trying to reconcile two abbreviations, AQI vs. PSI covers what that index is and why national scales differ.

The short version: an air quality reading is a concentration in µg/m³, ppm, or ppb; the AQI is an index derived from it; and psi belongs on a tire gauge.