Australia's Air Quality Index Explained
Australia's AQI looks like the US version — a number where higher is worse, with colour-coded categories — but it's built on a completely different idea. Instead of a fixed concentration-to-index lookup table, Australia's AQI expresses each pollutant as a percentage of its national standard. An AQI of 100 means "exactly at the standard." This guide explains the formula, the standards behind it, and why the categories differ from state to state.
The core idea: a percentage of the standard
Australia sets national ambient air quality standards under the National Environment Protection Measure (the "Air NEPM"). The AQI for each pollutant is simply:
AQI = (measured concentration / relevant standard) × 100So a pollutant sitting exactly at its standard scores 100; half the standard scores 50; twice the standard scores 200.
As with most systems, each pollutant is scored separately and the highest value becomes the site's overall AQI. The pollutants covered are PM2.5, PM10, ozone (O₃), nitrogen dioxide (NO₂), sulphur dioxide (SO₂), and carbon monoxide (CO).
The standards the index is built on
Because the AQI is a percentage of these values, the standards are the index. These are the Air NEPM reference values:
| Pollutant | Averaging period | Standard (AQI 100) |
|---|---|---|
| PM2.5 | 24-hour | 25 µg/m³ |
| PM10 | 24-hour | 50 µg/m³ |
| Ozone (O₃) | 1-hour | 0.10 ppm |
| Ozone (O₃) | 4-hour | 0.08 ppm |
| Nitrogen dioxide (NO₂) | 1-hour | 0.12 ppm |
| Sulphur dioxide (SO₂) | 1-hour | 0.20 ppm |
| Carbon monoxide (CO) | 8-hour | 9.0 ppm |
The category bands
A commonly used national category scheme maps the AQI value to plain-language bands like this:
| AQI | Category | In plain English |
|---|---|---|
| 0–33 | Very Good | Enjoy activities. |
| 34–66 | Good | Enjoy activities. |
| 67–99 | Fair | People unusually sensitive to air pollution should cut back or reschedule strenuous outdoor activities. |
| 100–149 | Poor | Sensitive groups should cut back or reschedule strenuous outdoor activities. |
| 150–200 | Very Poor | Sensitive groups should avoid strenuous outdoor activities; everyone should cut back. |
| Above 200 | Hazardous | Everyone should avoid outdoor activity. |
Why the categories vary by state
This is the wrinkle that surprises visitors. Air quality reporting in Australia is run by each state and territory's environment agency, not a single national operator, and they have not all adopted identical category labels or cut-points. Several jurisdictions (including NSW) moved to a revised scheme with bands such as Good / Fair / Poor / Very Poor / Extremely Poor defined directly on pollutant concentrations rather than on the older "% of standard" categories above; Tasmania uses its own finer-grained set. The underlying measurements and the NEPM standards are national, but the presentation you see depends on which state's network you're looking at. When in doubt, check your state EPA's own category key.
How Australia's AQI compares to the US AQI
The numbers are not interchangeable, because Australia's "100" is pinned to its national standard rather than to the US health-based breakpoints. Roughly:
- Australian AQI under 67 ("Good"/"Very Good") corresponds to clean air the US would also call "Good."
- Australian AQI around 100 (at the standard) lands near the US "Unhealthy for Sensitive Groups" boundary for particulates.
- Above 150–200, both systems agree the air is poor to hazardous.
For a stable cross-border comparison, the most reliable metric is still raw PM2.5 in µg/m³ against the WHO 24-hour guideline of 15 µg/m³.
Air quality on your home screen
Smog Report shows real-time air quality with widgets and Live Activities, and lets you switch between regional scales including Australia's AQI. Free on iOS.
Download for iOSPrimary sources: Australian Government — Ambient Air Quality standards · NSW — Air Quality Index