The Air Quality Index (AQI) is a scale that helps to convert measurements of different air pollutants, including particulate matter and gaseous pollutants, into a single readily understandable scale. The AQI scales are often divided into air quality categories (such as good, poor, very poor,) with associated health information.
Historically, the limits between each category have been scaled to the equivalent air quality standard for each pollutant. The standards for each pollutant vary across countries, making comparison of air quality difficult. Additionally, as instrumentation has improved and knowledge on the short-term impacts of particulate matter have become apparent, the use of existing health-based 24-hour standards have become a limiting factor in providing current air quality information. As such, agencies responsible for air quality reporting have created varied, short-term air quality categorisation scales, leading to further differences worldwide.
As NSW DPIE was looking to implement the reporting of hourly-averaged measurements for particulate matter, a desktop review was undertaken to understand the approaches used elsewhere for reporting short-term averaged particulate matter concentrations. The specific focus of the review was to identify how shorter-term particulate matter measurements are accounted for when deriving AQIs, the universal indicator for air quality.
Review on approach to Air Quality Index (AQI) reporting
During early 2020, New South Wales (NSW) commissioned independent air quality experts to review the definitions and approaches to deriving AQIs applied by various Australian jurisdictions and other countries. Additional input to this review was provided by air quality and health professionals from NSW and other jurisdictions in Australia.
The review investigated how Air Quality Indices (AQIs) are calculated for particulate matter, specifically PM2.5 and PM10 (particles less than 2.5 and 10 micrometres in diameter respectively). Specifically, the review sought to identify:
- the averaging periods used for reporting particulate measurements
- whether an Air Quality Index or any system of air quality categorisation is used at all
- the number of air quality categories used, and the scales used to define such categories
- how the scales presented related to the equivalent national standards for PM2.5 or PM10
- health messaging associated with air quality, including if a public alert process was used.
The review scanned the publicly available literature on systems used by both Australian states and territories, as well as selected overseas jurisdictions. The specific jurisdictions studied were:
- each of the 8 Australian states and territories
- New Zealand, Canada, United States, Japan, United Kingdom, Colombia
- the European Union.
The published report Review of international air quality indices (PDF 3.4MB) describes in detail, the approaches used by different agencies when reporting air pollution in Australia and around the world. Key findings are:
- The review showed that as of early 2020 there was no consensus, either locally or internationally, on how particulate matter pollution should be calculated and reported. Most jurisdictions were found to calculate an AQI for each pollutant (particulates and gases) and calculate them against a scale where a value of 100 or greater was equivalent to poor. In some jurisdiction, a scale of 10 was used. Some jurisdictions did not report an AQI at all but did report equivalent air quality category, along with specific health or air quality information related to that category.
- Averaging periods used for particulate matter measurements when calculating the AQI varied significantly as well. All jurisdictions had standards which were calculated over 24 hours, although the averaging periods for particle concentrations used for AQI calculations were different between jurisdictions, and not necessarily 24-hour averages. For website reporting in particular, some jurisdictions varied their reporting to reflect more current measurements, by using short-term averages for particulate matter scaled to a ‘threshold’ value (where the traditional approach scales 24-hour measurements to a 24-hour national air quality standard). Averaging periods used for the calculation of AQI categories varied from hourly average to rolling 24-hour average measurements. Some jurisdictions used weighted averages which were designed to provide averages of up to 24-hours, but which were weighted to be more representative of the most recent hourly measurements of particulate matter concentration in the atmosphere.
This review has been a preliminary step for New South Wales and other Australian jurisdictions, towards defining and adopting a nationally consistent approach for air quality reporting in Australia. Between 2018 and 2020, bushfires and prolonged drought resulted in significant air quality issues across many parts of the country. Public feedback received by jurisdictions during this time highlighted the difficulty in comparing air quality conditions across Australia due to the use of many AQI scales and categories, as identified in the review report. There is a need for adopting a nationally consistent approach for communicating air quality and associated health information – the information must be presented in a format that is simple, easy to comprehend and provide sufficient guidance for people to take steps to protect their health.
A broad general agreement is being determined between environmental health and air quality experts across the federal and state jurisdictions of Australia, in how best to develop and apply a nationally consistent approach, using the information from this review as well as other health studies.