Understanding the data

Learn how the air quality data readings are intrepreted to give a regional air quality value.

Air quality index

Data readings

In order to provide more detailed information than is available in the AQI (air quality index), measurements are also published as hourly data readings, and daily data readings, in scientific units rather than the derived values provided in the AQI.


Data readings are the actual scientific measurements for each air pollutant. The data readings are recorded in different units of measure, depending on the type of pollutant.

PollutantUnits used for air quality data
Ozone pphm (parts per hundred million)
Nitrogen dioxide pphm (parts per hundred million)
Visibility (as Bsp) 10-4 m-1
Carbon monoxide ppm (parts per million)
Sulfur dioxide pphm (parts per hundred million)
Particles µg/m3 (micrograms per cubic metre)

At each monitoring site, air quality
Data reading
are taken for each of 
the six pollutants.

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AQI (air quality index values)

Date readings are converted into AQI values by using the formula:

teh AQI for each pollutant = dta reading divided by the standard mutluplied by 100

This means the AQI is a derived value (based on the data reading). Because data readings have different underlying units of measure, it is difficult to compare the various pollutants. After we derive the AQI values we have a standardised set of values that we can then compare and present.

The data reading / standard x 100
is the
air quality index (AQI) 

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Site AQI

There are 24 monitoring sites around NSW where the air quality index (AQI) values are calculated. The AQIs are compared at each site. The highest AQI value at each site becomes the site AQI. In the example below, the highest AQI for pollutant 3 would be the site AQI

Grpah of site pollutant showing that the highest pollutant reading is the site AQI


The highest air quality index (AQI)
value is taken as the
site AQI
for that particular site

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Region AQI

All the site AQIs in a given region are compared. The highest site AQI for a region is taken as the region AQI. In the example below, the reading for site RST would be the region AQI 

Graph of site AQIs showing that highest site AQI is the region AQI

The highest site AQI 
for a region is taken as the 
region AQI

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Page last updated: 13 November 2015