Air quality index
EPA reports air quality measurements as an air quality index. The lower the index is, the better the quality of our air.
Calculating the index
Conversion of a pollutant concentration into an index
Pollutants, standard levels and calculation averaging times
Calculation of the index
Station index
Example
Five-colour coded categories
Conversion of a pollutant concentration into an index
An index for any given pollutant is its concentration expressed as a percentage of the relevant standard, or:
It is therefore easy to interpret the pollutant index; that is, assess whether the pollutant is at a level which may cause harm.
An index value of 100 means the pollutant is currently at a concentration equal to an environmental standard level. These standards are designed to protect human health and the visual quality of the air environement, and are taken from either:
- The National Environment Protection Measure (Air NEPM), or
- The State Environment Protection Policy (Ambient Air Quality) (SEPP AAQ), or
- Standards derived from the above levels.
Expressing air quality as an index makes it easier to compare at a glance pollutant levels and air quality at different monitoring stations. The raw data (actual measurements/concentrations) are also available on our website for anyone seeking more detail.
Pollutants, standard levels and calculation averaging times
| Pollutant | Standard level | Source | Averaging time | How calculated* |
|---|---|---|---|---|
| Ozone | 100ppb | Air NEPM | 1-hour | Note A |
| Nitrogen dioxide | 120ppb | Air NEPM | 1-hour | Note A |
| Sulfur dioxide | 200ppb | Air NEPM | 1-hour | Note A |
| Carbon monoxide | 9ppm | Air NEPM | 8-hour | Note B |
| Fine particulates (PM10) | Note C | Note C | Note C | Note C |
| Visibility (airborne particle index) | 2.35 | SEPP | 1-hour | Note A |
*How calculated
| A: | For daily and weekly reports: Uses the maximum
of the preceeding 24 one-hour averages For hourly updates: Uses the selected hourly average (defaults to the latest available reading) |
| B: | For daily and weekly reports: Uses the maximum
of the preceding 16 rolling 8-hour averages For hourly updates: Uses the 8 hours up to the selected hour (defaults to the latest 8-hour average) |
| C: | For daily and weekly reports: Uses an average of
the preceeding 24 hours of PM10 readings, divided by the 24-hour Air
NEPM standard of 50 µg/m3. For hourly updates: Uses the selected hourly average PM10 (defaults to the latest available reading), divided by a derived 1-hour standard of 80 µg/m3. |
Calculation of the index
The data obtained from EPA's monitoring stations are averaged over various averaging times depending on pollutant, as listed in the above table. These averages are calculated every hour by EPA.
The calculation is done separately for each station. There are two stages in the calculation. First, each pollutant measurement is converted into a pollutant index, and second, all the pollutant index values are combined together to form a station index.
Station index
In order to get an overall appreciation of air quality at a particular EPA monitoring station, a station index is calculated as the maximum of all the pollutant index values for that station.
See also Determination of an overall station summary.
Example
Assume the values listed in the table below have been measured at an EPA monitoring station, and that EPA is preparing the 9am bulletin on 2/2/98.
| Pollutant | Maximum measured level | Averaging time | When did maximum occur? | Standard level* | Calculated index |
|---|---|---|---|---|---|
| Ozone | 75ppb | 1-hour | 3pm 1/2/98 | 100ppb | 75 |
| Nitrogen dioxide | 4ppb | 1-hour | 7pm 1/2/98 | 120ppb | 38 |
| Sulfur dioxide | 30ppb | 1-hour | 8am 2/2/98 | 20ppb | 15 |
| Carbon monoxide | 0.9ppm | 8-hour | 12pm-8pm 1/2/98 | 9ppm | 10 |
| Fine particulates (PM10) | 20µg/m3 | 24-hour | 9am 1/2/98-9am 2/2/98 | 50µg/m3 | 40 |
| Visibility (airborne particle index) | 1.20 | 1-hour | 5pm 1/2/98 | 2.35 | 51 |
| Overall station index (the maximum of the figures is taken to be the index) | 75 |
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Five-colour coded categories
The maximum of individual pollutant indices at a monitoring station is then taken as the overall index for that station.
An index is then assigned to one of five colour-coded air quality categories:
| Category | Index range |
| Very good air quality | 0–33 |
| Good air quality | 34–66 |
| Fair air quality | 67–99 |
| Poor air quality | 100–149 |
| Very poor air quality | 150 or greater |