About the Air Quality Index

The Air Quality Index (AQI) is a scale of air pollution that helps us understand air quality and modify our activities if pollution levels are high.

The Air Quality Index (AQI) is, like any other index, a number. Just as the ASX200 index allows investors to understand the strength of the Australian stockmarket, so the air quality index (AQI) allows us to understand air quality.

Indices are calculated according to a formula, (eg the ASX200 is a number calculated from the values of 200 selected Australian shares). The AQI is a number calculated from air quality data for 5 pollutants and visibility readings.

What the AQI numbers mean for you

For each pollutant, the Air Quality Index (AQI) is the data value expressed as a percentage of the level specified by the National Environment Protection Measure for Ambient Air (NEPM) standard (or, in case of visibility, of the relevant NSW standard). An AQI of 100 corresponds to the NEPM level. A lower value indicates better air quality and a higher value, worse.

AQI

What action should people take?

Very good
0-33

Enjoy activities

Good
34-66

Enjoy activities

Fair
67-99

People unusually sensitive to air pollution:
Plan strenuous outdoor activities when air quality is better

Poor
100-149

AIR POLLUTION HEALTH ALERT
Sensitive Groups: Cut back or reschedule strenuous outdoor activities

Very poor
150-200

AIR POLLUTION HEALTH ALERT
Sensitive groups: Avoid strenuous outdoor activities
Everyone: Cut back or reschedule strenuous outdoor activities

Hazardous
200+

AIR POLLUTION HEALTH ALERT
Sensitive groups: Avoid all outdoor physical activities
Everyone: Significantly cut back on outdoor physical activities

Steps to calculate AQI values

Because data readings have different underlying units of measure, it is difficult to compare the various pollutants. We calculate AQI values from the data so we have a standardised set of numbers to compare and share. The steps are:

Monitoring and reporting air quality starts with data from a sophisticated network of air quality sensors and instruments that we manage at 25 sites across NSW.

Data readings are actual measurements; numbers with measurement units, from scientific instruments for each air pollutant. Data is collected on five major pollutants (in keeping with legislative requirements) and visibility.

Pollutant/Visibility

Symbol

Measurement unit

Ozone

O3

Parts per hundred million (pphm)

Nitrogen dioxide

NO2

Parts per hundred million (pphm)

Carbon monoxide

CO

Parts per million (ppm)

Sulfur dioxide

SO2

Parts per hundred million (pphm)

Particles - less than 2.5 micrometres diameter (small); less than 10 micrometres diameter (large)

PM2.5, PM10

Micrograms per cubic metre (µg/m3)

Visibility

Bsp

10-4 m-1

We provide hourly data readings for each pollutant

Data readings are then used to calculate the index (AQI) values for each pollutant and visibility using the following formula. The data reading is divided by the standard and multiplied by 100 to get the AQI for the pollutant. The formula is:

 AQIpollutant = 
pollutant data reading   X 100
 standard


The ‘standard’ value in the formula for each major pollutant is specified by the Australian Government in the National Environment Pollution Measure (Ambient Air).

For the gaseous pollutants (ozone, nitrogen dioxide and sulfur dioxide), monitored concentrations are reported as parts per hundred million (pphm).  To calculate the AQI, the national standards are converted from parts per million (ppm) to pphm.  For example, the national one hour standard for ozone (0.10 ppm) is used but converted to the NSW reporting format (10.0 pphm).

The AQI calculation for visibility uses the one hour NSW visibility standard of 2.1 10-4 m-1.

National Air NEPM air quality standards used to calculate the AQI

Pollutant

Averaging period

Air NEPM Standard

NSW reporting format

Carbon monoxide

8 hours

9.0 ppm

9.0 ppm

Nitrogen dioxide

1 hour

0.12 ppm

12.0 pphm

Ozone 

1 hour
4 hours

0.10 ppm
0.08 ppm

10.0 pphm

8.0 pphm

Sulfur dioxide

1 hour

0.20 ppm

20 pphm

PM10

1 day

50 µg/m3

50 µg/m3

PM2.5

1 day

25 µg/m3

25 µg/m3

NSW Visibility standard

Pollutant

Averaging period

NSW Standard

Visibility (as Bsp)

1 hour

2.1 10-4 m-1

Bsp = coefficient of light scattering due to particles. The lower the Bsp value, the lower the level of suspended particles and the better the visibility. The NSW OEH visibility standard of 2.1 10-4 m-1 corresponds to a visual distance of approximately 9 kilometres.

After we derive the AQI values we have a standardised set of values that we then compare and present. The AQIs are compared at each site. The highest AQI value at each site becomes the site AQI. 

AQI values are interpreted by categorising them according to the air quality they describe. The resulting ratings, from very good air quality to hazardous, are labelled and colour-coded for easy interpretation.

The categories provide at-a-glance information to help people plan their activities. An AQI of 100 or greater indicates that pollutant readings have reached or exceeded the relevant national or NSW standard.

VERY GOOD

GOOD

FAIR

POOR

VERY POOR

HAZARDOUS

0-33

34-66

67-99

100-149

150-199

200+

 

Learn more about health of air quality and what you can do 

Example

Bargo at 2pm one day has ozone measured as 11 parts per hundred million (pphm). The national ambient air one hour standard for ozone is 10 pphm. The AQI formula, calculates an AQI for ozone of 11/10 x 100 = 110.

If Bargo’s AQI for ozone is highest at 2pm and ozone is higher than any other pollutant’s AQI at this time, the site AQI is 110. If another pollutant gave a higher AQI at Bargo, that would be the site AQI.

All site AQIs in a region for the same time are compared. The highest site AQI for a region is taken as the region AQI.

Example

Bargo in Sydney southwest region records a site AQI of 110 at 2pm. Its site AQI is compared to others in the region for the same time and if it’s higher than other sites’ AQIs, the Sydney southwest region AQI is 110.

A regional AQI of 110 lies between 100 and 149 in the ratings scale. This rates Sydney south west’s air quality at 2pm as ‘poor’ and would cause an air pollution health alert to be issued for the region. Sensitive people would be advised to cutback or reschedule strenuous activity.