About the Air Quality Index

The Air Quality Index (AQI) was an air pollution scale that standardised pollutant concentrations and provided advice to modify activities if pollution levels were high.

Between 2008 and 2020, air quality data in New South Wales was presented using the Air Quality Index (AQI). It was replaced by the current Air Quality Categories (AQC) in November 2020 to provide more detailed health information and clarity on ambient air quality levels.

The AQI was, 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) helped us to understand air quality. The ASX200 is calculated from the values of 200 selected Australian shares. Likewise, the AQI was calculated from air quality data for 5 pollutants and visibility readings.

What the AQI numbers meant for you

For each pollutant, the Air Quality Index (AQI) was 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 corresponded to the NEPM national standard, and levels at or above 100 signified 'Poor' air quality.

A lower value indicated better air quality and a higher value, worse.

The AQI comprised of 6 categories and provided more limited health information than the current AQC. The AQC has 5 categories.

AQI

What action should people take?

Very good
0-33

Enjoy normal activities

Good
34-66

Enjoy normal activities

Fair
67-99

People unusually sensitive to air pollution should reduce or reschedule strenuous outdoor activities.

Others are not likely to be affected when the AQI is in this range.

Poor
100-149

Sensitive groups should reduce strenuous outdoor activities.

Other adults are not likely to be affected. Anyone who experiences symptoms should reduce outdoor activities.

Very poor
150-200

Sensitive groups should avoid strenuous outdoor activities

Other adults should reduce or reschedule strenuous outdoor activities.

Hazardous
200+

Sensitive groups should avoid all outdoor activities.

Other adults should avoid strenuous outdoor activities.

Steps to calculate AQI values

Because data readings have different underlying units of measure, it is difficult to compare the various pollutants. AQI values were calculated from data to have a standardised set of numbers to compare and share. The AQI was calculated using the following steps:

Data is collected on 5 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

Data readings were then used to calculate the index (AQI) values for each pollutant and visibility using the following formula. The data reading was 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 used in the formula for each major pollutant is specified by the Australian Government in the National Environment Pollution Measure (Ambient Air).

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 (210 Mm-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.

Interim web reporting approach for 1-hour particles

The following hourly threshold values were implemented for reporting particles since February 2020.

Pollutant Averaging period Interim web reporting threshold

PM10

1 hour

80.1 µg/m3

PM2.5

1 hour

62.1 µg/m3

After being derived from data readings, the AQI represented a standardised set of values that were then compared at each site. The highest AQI value at each site became the site AQI. 

AQI values were interpreted by categorising them according to the air quality they described. The AQI ratings, from very good air quality to hazardous, were labelled and colour-coded for ease of interpretation.

An AQI of 100 or greater indicated that pollutant readings 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+

Example

Bargo at 2pm on a certain day had an ozone measurement of 11 parts per hundred million (pphm). The national ambient air one hour standard for ozone is 10 pphm. The AQI formula would then have calculated an AQI for ozone of 11/10 x 100 = 110.

If Bargo’s AQI for ozone is highest at 2pm and ozone was higher than any other pollutant’s AQI at this time, the site AQI at Bargo would have been 110. If another pollutant had a higher AQI at Bargo, that would then become the site AQI.

To calculate the region AQI, all site AQIs in a region for the same time were compared. The highest site AQI for a region was taken as the regional AQI.

Example

Bargo in Sydney southwest recorded a site AQI of 110 at 2pm. Its site AQI was compared to others in the region for the same time and if its AQI was higher than that of other sites, then the Sydney southwest region AQI would have been 110 and therefore 'poor'.