In this chapter, we continue to use the Air Quality Index (AQI) defined in the previous 2 state of the environment reports (Table 8), so that the state and trends of Australia’s air quality reported here can be directly compared across years. The AQI takes each pollutant measurement and uses the National Environment Protection Measures (NEPM) standard as the basis to make the comparison: This means that, if a pollutant is measured at the limit set by the NEPM, the AQI calculation would yield a value of 100; if the measurement is at half the limit, the AQI would be 50. New NEPM standards for ozone, nitrogen dioxide and sulfur dioxide were agreed on 15 April 2021. As all air quality data for this report were collected before this date, any assessments will be made against the previous NEPM standards. Table 8 AQI used in this and previous state of the environment reports Category AQI range Colour Description Health advice (if any) for the general populationa Very good 0–33 Blue Air quality is considered very good, and air pollution poses little or no risk None Good 34–66 Green Air quality is considered good, and air pollution poses little or no risk None Fair 67–99 Yellow Air quality is acceptable. However, there may be a health concern for sensitive people None Poor 100–149 Orange Air quality is unhealthy for sensitive groups. The general population is not likely to be affected in this range Reduce outdoor physical activity if you develop symptoms such as cough or shortness of breath Very poor ≥150 Red Air quality is unhealthy, and everyone may begin to experience health effects. Sensitive people may experience more serious health effects Avoid outdoor physical activity if you develop symptoms such as cough or shortness of breath. When indoors, close windows and doors until outdoor air quality is better Extremely poorb ≥200 Maroon Air quality is very unhealthy, and everyone may begin to experience health effects. Sensitive people may experience more serious health effects Stay indoors as much as possible with windows and doors closed until outdoor air quality is better. If you feel that the air in your home is uncomfortable, consider going to a place with cleaner air (such as an air-conditioned building like a library or shopping centre) if it is safe to do so AQI = Air Quality Index Health advice from NSW (DPIE 2021b). Health advice for sensitive groups is likely to apply at earlier air quality categories. An additional ‘extremely poor’ maroon category was added in the summer 2019–20 bushfire case study to account for very high ambient fine particulate matter measurements. This category is likely to be retained by the jurisdictions. If air quality is extremely poor, residents are advised to find a room with clean air, such as a large public building with air-conditioning. Share on Twitter Share on Facebook Share on Linkedin Share this link The AQI also allows an easy comparison of levels of different pollutants across different locations. However, the use of different descriptions for the AQIs among the jurisdictions has been confusing (see Summer 2019–20 bushfires). The jurisdictions are now working towards unifying how they report air quality levels for clearer public messaging. To make the air quality assessments in this report, data from every day between 1 January 2015 and 31 December 2019 for ozone, coarse particulate matter (PM10) and fine particulate matter (PM2.5) from each jurisdiction were evaluated against the relevant air quality limit. The total numbers of ‘hits’ for each AQI category were added, and the percentage of the total number of days for the AQI category was determined. The overall assessment for each pollutant in each capital city is calculated by comparing the AQI category percentages with the rules in Table 9. For example, if a city has more than 50% of PM10measurements in the ‘very good’ category but 7% in the ‘poor’ and 2% in the ‘very poor’ categories, the overall assessment will still be ‘very good’. Table 9 Criteria for assigning overall AQI–based qualitative categories Overall category Annual distribution of AQI values Very good (%) Good (%) Fair (%) Poor (%) Very poor (%) Very good >50 >20 <10 <10 <5 Good >20 >30 <20 <20 <10 Fair <10 <20 >30 >20 <10 Poor <10 <20 <20 >30 >20 Very poor <5 <10 <10 >20 >50 AQI = Air Quality Index Share on Twitter Share on Facebook Share on Linkedin Share this link The trend in the assessment is calculated by comparing the results for 2015–19 with the results from the previous 2016 and 2011 state of the environment reports. For example, if the percentage of ‘very good’ AQIs has consistently increased, the trend will have improved. Population-weighted annual PM2.5 maps The United Nations Sustainable Development Goal (SDG) 11 is to make cities and human settlements inclusive, safe, resilient and sustainable. Relevant to air quality, SDG 11 has a target to reduce the adverse per capita environmental impact of cities by 2030. The key indicator for this target is the annual PM2.5 concentration, weighted by the population. As population-weighted PM2.5has not been previously evaluated for Australia, we need to make an initial baseline assessment with which future state of the environment assessments can be compared. In this way, we will be able to establish whether the SDG 11 target will be met in Australia by the 2026 state of the environment report. In this chapter, we mapped the annual mean levels of PM2.5in cities (population weighted). The PM2.5concentration data were taken from a satellite and land use regression model for 2018 (Knibbs et al. 2018, Knibbs 2020) at a resolution of 1 km. These concentration data, C, were weighted by the Australian population, P, resident in each of these 1 km × 1 km squares, summed across each Statistical Area Level 2 (SA2), i (ABS 2015). SA2 areas are defined as being at the suburb level containing 3,000–25,000 people, with an average of 10,000 people. For example, divide an SA2 region into squares 2 km on a side. Square X has 200 residents and an annual PM2.5 concentration of 2 micrograms per cubic metre (μg/m3). The adjacent square Y has 800 residents and an annual PM2.5 concentration of 6 μg/m3.The population-weighted PM2.5 in this example SA2 region is In producing the population-weighted maps, there were approximately 30–40 SA2 regions that had zero population – for example, in areas of wilderness. To avoid dividing the annual PM2.5 concentrations by zero population, the average PM2.5 across all the 1 km × 1 km grid cells in these SA2 regions was used instead.