Climate change

A warming climate will increase pressures on air quality.

Australia’s air temperatures have increased by 1.44 °C since 1910. In tandem with a similar temperature rise in the oceans, these dynamics have led to more extreme weather patterns experienced across Australia (BOM & CSIRO 2020). These include a rise in the frequency of extreme heat events, which increase the risk of fire and extend the length of the summer bushfire season. Seasonal rainfall between April and October has decreased by 16% since the late 1990s across large parts of southern Australia, and the outlook for the Australian climate is for longer periods in drought.

These changes will affect air quality in the following ways:

  • Increased temperatures will lead to
    • increased fire events, causing smoke pollution
    • increased occurrence of summertime smog due to increased biogenic emissions and faster chemical formation of secondary pollutants.
  • Increased drought will lead to
    • increased dust, causing dust storms.

Prescribed burning and bushfires

Fire, whether from prescribed burning or bushfires, produces smoke. Smoke is a mixture of particles and gases resulting from the incomplete combustion of materials burned. From a human health perspective, smoke irritates eyes and airways, and causes respiratory problems. These problems can worsen long-term conditions such as asthma and chronic obstructive pulmonary disease (Reid et al. 2016).

The effect and behaviour of smoke depend on a range of factors:

  • The composition of smoke depends on the material being burned and the intensity (heat) of the fire.
  • The amount of smoke produced depends on the moisture content, area burned and fuel density (Williamson et al. 2016).
  • The height and distance travelled by the smoke depend on the conditions of the atmosphere.

Prescribed burns

Prescribed burns are an important part of fire management (Morgan et al. 2020). The aim of prescribed burns is to reduce the vegetative fuel load in the landscape to prevent large, out-of-control bushfires.

However, the smoke from prescribed burns can be a public nuisance and can also have health effects. Broome et al. (2016) estimated that 14 deaths were attributable to smoke from prescribed burning across 6 days in the Sydney area in 2016. Firefighters are also exposed to high levels of carbon monoxide (CO) and other toxic chemicals during prescribed burns (MacSween et al. 2020b).

Summer 2019–20 bushfires

Bushfires are common in Australia, and many Australian species have evolved to respond to fire or smoke. But large bushfire events can have severe negative impacts on the human and natural environment, and on air quality.

The summer of 2019–20 produced one of the worst bushfire seasons on record (Davey & Sarre 2020). More than 24 million hectares of forest burned across the Australian Capital Territory, New South Wales, Queensland and Victoria during the ‘Black Summer’ of the 2019–20 fire season (Binskin et al. 2020).

Smoke blanketed towns and cities in Australia for many weeks, with concentrations of PM2.5 (particulate matter with a diameter of 2.5 μm or less) well above air quality limits (Figure 23). Johnston et al. (2020) estimated that about 80% of the Australian population was affected by bushfire smoke at some point during the season.

High levels of smoke outside meant that some was able to leak indoors. There were reports of smoke alarms going off inside city buildings in Sydney and Melbourne (SBS News 2019). Canberra experienced the worst outdoor air quality measurements anywhere in the world (Filkov et al. 2020), with air quality classed as ‘poor’ or worse on 45 days during December 2019 and January 2020 (Figure 24). Daily PM2.5 concentrations on 1 January 2020 were 38.5 times the 24-hour National Environmental Protection Measure (NEPM) standard, which was the worst day during the period (see case study: ACT indoor air quality during the summer 2019–20 bushfires). Smoky deposits could be found on indoor surfaces and required regular cleaning.

Figure 23 The ‘Dish’ at Parkes, New South Wales, 19 December 2019 shrouded in bushfire smoke and dust

Sales of N95 facemasks (able to filter out 95% of all airborne particles) rocketed as residents sought methods to protect themselves from the smoke (Powell 2020). However, unless masks were correctly fitted over the nose and mouth, their benefits were not realised. The health advice was to avoid exercising and find a location with better air quality (Laumbach et al. 2015, Yu et al. 2020).

When the wind direction changes to blow smoke away from a town, or if rain removes smoke from the atmosphere, the air quality improves (although rain-out of smoke particles can contaminate waterways). Hence, intermixed with these ‘extremely poor’ air quality days were days with much better air quality (e.g. the fourth week in January; Figure 24).

Figure 24 Measurements of 24-hourly PM2.5 concentrations in calendar format, Canberra, December 2019 and January 2020

μg/m3 = microgram per cubic metre; PM2.5 = fine particulate matter

Note: PM2.5 measurements have been coloured by the air quality categories (AQC, see Approach). An additional maroon ‘extremely poor’ category has been added to the AQC colour bar to emphasise concentrations that are more than double the National Environment Protection Measures limit for PM2.5.

Source: Health Protection Service, ACT Government

Smoke from the bushfires travelled many thousands of kilometres. Satellite images from the Suomi National Polar-orbiting Partnership/Visible Infrared Imaging Radiometer Suite from 20 December 2019 show smoke from fires in New South Wales travelling to northern Victoria, and strong westerly conditions at the beginning of January 2020 sending smoke towards New Zealand (Figure 25). Smoke deposits were observed on the Tasman Glacier of New Zealand’s South Island (BBC News 2020b). By mid-January, NASA (the National Aeronautics and Space Administration) observed that the Australian bushfire smoke had circumnavigated the entire globe (Jenner 2020).

Figure 25 Suomi National Polar-orbiting Partnership/Visible Infrared Imaging Radiometer Suite images on 20 December 2019 showing smoke cutting around the Victorian Alps towards Melbourne (left); and 4 January 2020, showing smoke travelling towards New Zealand (right)

Public information

The 2019–20 bushfires highlighted the importance of getting air quality information to the public, to ensure that they can act to protect their health. Limits to monitoring and varied messaging across jurisdictions caused confusion, but more sensors and standardised reporting are improving the information available to the public (see Air Quality Index).

Whereas residents in cities and large towns are generally near a fixed air quality monitoring station, this is not the case for residents living outside these zones. No measurements of their exposure to the smoke exist. During the bushfires, New South Wales was able to deploy emergency air quality monitoring capabilities to 6 locations, and Victoria deployed smoke sensors to 15 of the worst-affected locations (Binskin et al. 2020).

The use of sensors as a low-cost method of expanding the air quality network is being explored, although the quality of measurements can be variable (see case study: Low-cost sensors). A larger network of air quality monitoring will help to improve air quality forecasting systems such as AQFx from CSIRO, AirRater from the University of Tasmania, and the New South Wales Air Quality Forecasting Framework.

Case Study ACT indoor air quality during the summer 2019–20 bushfires

A mobile particulate monitor was placed inside a Canberra house during one of the worst smoke episodes of the summer 2019–20 bushfire period. The house is probably quite ‘average’ in Australia, built around the 1960s. It is not very well sealed, and does not use any kind of filtration system such as a high-efficiency particulate air filters. The occupant describes the house as being hard to heat in winter and keep cool in summer, which suggests that it is ‘leaky’. Five-minute average measurements were taken over a short period during the peak bushfire event. These measurements compared indoor and outdoor concentrations of fine particulate matter (PM2.5) when the smoke was particularly bad (Figure 26). The outdoor measurements are from the nearest monitoring station at Civic, approximately 5 km from the house.

Figure 26 (a) Comparison of PM2.5 measured indoors with outdoor PM2.5 from the nearest fixed air quality monitoring station (Civic) during the summer 2019–20 bushfire season. Both datasets are 5-minute averaged PM2.5. (b) Fraction of total particles 2.5 μm diameter or less

μg/m3 = microgram per cubic metre; µm = micrometre; PM2.5 = fine particulate matter

Note: The red arrow denotes when the indoor sensor was moved to a room with slightly better sealing ability, late in the morning of 1 January 2020.

Source: Health Protection Service, ACT Government

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At 10 pm on 31 December 2019, the outdoor peak measurement of PM2.5 was 1,633 micrograms per cubic metre (μg/m3). As the outdoor monitoring station is 5 km away, the times that the smoke plume reached the station and the house would differ. But given the extreme nature of this air quality event and how widespread these smoke plumes were, we can assume that all of Canberra was exposed at approximately these levels. Around 7 am the next morning, levels of PM2.5 inside the house had reached 316 μg/m3 as a 5-minute average. Although the PM2.5 concentration inside the house was around 5 times less than outdoor levels, the 24-hour indoor average was still well above the National Environmental Protection Measures limit of 25 μg/m3. The red arrow in Figure 26 shows when the sensor was moved into a different room, which the occupant was able to seal a little better. Concentrations of PM2.5 decreased by around 70 μg/m3 during this 2-hour period. When the smoke outside passed (shown in the plot at noon on 30 December), the householder benefited from ventilating the house quickly to get the smoke out.

Given the extreme levels outside, the occupant was definitely better off inside. During this elevated smoke period, the particles indoors were almost entirely 2.5 micrometres (μm) or less, which are more likely to travel deeper into the lungs than the larger particles.

The 5-minute average measurements (Figure 26) also highlight how rapidly concentrations outdoors can increase. It took just 50 minutes for PM2.5 concentrations outdoors to increase from around 13 μg/m3 to 1,000 μg/m3 on the evening of 31 December. Doors and windows needed to be sealed before 8 pm to keep the worst of the smoke out of the house.

Most people rely on the smell of smoke to know when to close the house to keep smoke out, and open it for ventilation once the smoke has passed. Access to real-time air quality measurements can help people manage indoor air quality, and protect them from high levels of smoke indoors, when levels outside might be lower. Access to smoke forecasts could also help people prepare their houses for these ‘extremely poor’ category air quality events.


Biogenic emissions

Trees and plants provide social amenity (e.g. in the form of shade in urban areas), and shelter for flora and fauna. They are also culturally significant for Indigenous populations. Our trees and plants have both positive and negative impacts on air quality.

Vegetation can provide surfaces for particulate deposition, removing pollution from the air. However, in urban areas, vehicle pollution can be trapped underneath tree canopies, particularly if the street is narrow or the canopies are very wide. This increases the exposure of pedestrians underneath to poor air quality (Kumar et al. 2019).

Trees and plants also produce biogenic volatile organic compounds (VOCs) and pollen that affect air quality.

Biogenic volatile organic compounds

Biogenic VOCs are emitted by vegetation, and produce the pleasant smells we associate with plants (e.g. eucalyptus, citrus, pine). Eucalyptus trees, which are common throughout Australia, have the highest emission rates of VOCs of any plant genus in the world (Benjamin et al. 1996, Guenther et al. 2006).

Eucalypts are classed as broadleaf evergreen trees. The high number of these trees in the south-east of Australia makes the region a hotspot for biogenic VOC emissions (Figure 27). Cities in the south-east of Australia have a high proportion of biogenic VOCs in their emissions. The emissions inventories for the greater metropolitan areas of Melbourne and Sydney show that biogenic VOCs emitted to the atmosphere exceed the human-made VOCs in these urban areas (60% natural, 40% anthropogenic). The regions surrounding urban areas are often densely vegetated, and also emit biogenic VOCs that may be transported to the urban airsheds and contribute to summertime smogs.

Shrubs and grasses also emit biogenic VOCs. Although their emission rates are not as high as eucalypts, their large areal extent in the north of Australia means that they contribute substantial biogenic VOCs there.

Figure 27 Percentage of coverage of (a) broadleaf evergreen trees, (b) grasses and (c) shrubs in Australia

Emissions of biogenic VOCs increase in response to temperature and light, and more VOCs are emitted in summer than in winter. City airsheds can be dominated by these natural emissions in summer, contributing to the production of summertime smogs. Chemical reactions in the atmosphere convert the biogenic VOCs into ozone and secondary organic aerosols, causing a blueish haze above the tree canopy (Went 1960). Biogenic VOC emissions are the reason for the name of the Blue Mountains.

The temperature dependence of biogenic VOC emissions suggests that, in a hotter atmosphere, more biogenic VOCs will be emitted. Using climate projections of a summertime temperature increase of around 2 °C, for a city such as Sydney, estimates include:

  • a 120% increase in the most dominant biogenic VOC species, isoprene, by 2050
  • an additional hourly peak of 21 parts per billion (ppb) of ozone (20% of the current hourly ozone NEPM limit) (Emmerson et al. 2020).

The impact of increased vegetation emissions in a warmer climate should be managed by reducing anthropogenic emissions of nitrogen oxides, to curb the ozone and particle pollution in cities. Choice of tree species that emit low levels of biogenic VOCs when planting or planning new urban developments can also be part of this strategy (Paton-Walsh et al. 2019). However, because trees use biogenic VOCs to protect against heatwaves, the trees must be drought- and heat-tolerant too.

Pollen

Vegetation can be a source of aeroallergens such as pollen and fungal spores. These particles tend to be a few tens of micrometres in diameter (larger than coarse particulate matter, PM10). The introduction of highly allergenic species, such as cypress pine from the Cupressaceae family, contributes to hayfever and asthma. The Canberra pollen count site frequently reports extreme levels of Cupressaceae pollen (in excess of 500 grains per cubic metre) during the winter flowering season (Haberle et al. 2014). In other regions, such as Brisbane and Melbourne, flowering grasses contribute a large proportion of the allergens. Remote sensing is being used to monitor the exact timing of flowering to predict grass pollen levels (Tran et al. 2020).

Epidemic thunderstorm asthma refers to the acute occurrence of asthma in many people at the time of a thunderstorm (see the Extreme events chapter) (see case study: Thunderstorm asthma). The winds associated with a thunderstorm can lift pollen into the air, increasing the allergen load. It is thought that levels of air pollution found in cities can also exacerbate the human immune response to allergens, meaning that more of the population in cities is likely to suffer health impacts than are rural populations (D’Amato et al. 2002).

AusPollen is a national initiative geared towards standardised pollen measurements and reporting (Davies et al. 2016). Pollen is currently counted and reported in 5 jurisdictions: the Australian Capital Territory, New South Wales, Queensland, Tasmania and Victoria. The observations are a vital component in the 5-day curated pollen forecasts.

Case Study Thunderstorm asthma

The world’s most severe incident of epidemic thunderstorm asthma (ETSA) occurred on 21 November 2016 in Melbourne, coinciding with the peak in the grass pollen season (Thien et al. 2018). At 5:30 pm local time, many people were commuting or outside enjoying temperatures above 30 °C when a line of dry thunderstorms swept across the city from west to east (Figure 28). The first calls to the emergency services for severe breathing difficulties started within minutes of the storm passing. Over the following 6 hours, 814 ambulances were called (DHHS 2017). Victorian health services were overwhelmed, with a 672% increase in emergency department presentations that night, and a 3,000% increase in intensive care patients with asthma (DHHS 2017). Unfortunately, 10 people died as a result of this storm.

Figure 28 Radar image showing the position of the thunderstorm system at 5:00 pm, and progression of the storm eastwards in half-hourly periods

The majority of ETSA patients tend to be allergic to grass pollen (Knox 1993), and in particular to ryegrass pollen. Ryegrass is a used in agricultural pasture and currently found in regions to the north and west of Melbourne. Distributions of pasture grass may change with climate and agricultural practices (see the Land chapter).

In laboratory studies, grass pollen grains have been observed to rupture when submerged in water, ejecting hundreds of highly allergenic sub-pollen starch grains (Suphioglu et al. 1992). In the atmosphere, it is thought that pollen grains can be swept high into thunderstorm clouds where they rupture, and the sub-pollen grains are then concentrated at ground level by the downdraft (Marks et al. 2001, Taylor & Jonsson 2004).

Epidemic thunderstorm asthma forecasting

ETSA is a relatively infrequent event (Silver et al. 2018), but its potential impact highlights the importance of developing a predictive system to better prepare emergency services for a large influx of patients. For people with hayfever or pollen-exacerbated asthma, a high grass pollen forecast can also influence the use of preventive medications, or encourage people to stay indoors.

In 2017, the Victorian Department of Health and Human Services funded the development of a pilot thunderstorm asthma service (Bannister et al. 2021). This provided 5 additional pollen monitoring sites in Victoria. A statistical model to predict daily mean grass pollen concentrations up to 5 days ahead was developed based on weather data and 21 years of grass pollen measurements at the University of Melbourne (Figure 29). The ambient temperature, relative humidity and wind speed all affect how pollen is emitted and dispersed in the Melbourne atmosphere (Emmerson et al. 2019). The Bureau of Meteorology then developed a simple decision matrix that estimates relative thunderstorm asthma risk by combining forecast pollen concentration (low, moderate, high, extreme) with forecasts of wind gust speed and coverage, and gives a rating to each region (Figure 30).

Figure 29 Example of the Melbourne grass pollen forecast at the peak of the season (mid-November 2020)

Figure 30 Example of the thunderstorm asthma forecast for 23 November 2020

The pilot warning system was implemented in September 2017, in time for the spring pollen season. It successfully identified the increased risk associated with 2 localised asthma events during November 2017, and verification against hospital data showed that the system has some ability to discriminate days with elevated ETSA risk. However, the process of pollen rupturing triggered by high relative humidity caused frequent false alarms of ETSA, as the relative humidity levels during night-time in spring are usually very high (Emmerson et al. 2021).

Research is continuing to understand how weather and pollen interact to cause conditions that lead to ETSA events, which will allow us to refine and improve the system.


Dust storms

In relation to air quality, the term ‘dust’ refers to particles of soil lifted by the wind. Particles can be swept into the atmosphere from land regions with little or no vegetation cover. The process is driven by low soil moisture and high wind speeds. The amount of vegetation cover varies throughout the year, peaking after winter rains. Vegetation can also decrease suddenly after a bushfire.

Increased droughts and bushfires associated with climate change are likely to increase the number of dust days. Dust days are defined as days with total particle concentrations in the air above 10 micrograms per cubic metre where a fog- or smoke-related source can be eliminated. When a dust storm passes through, hourly concentrations of total particles can be thousands of micrograms per cubic metre (Figure 31).

The Community DustWatch program has been observing dust in southern Australia since 2005 (Leys et al. 2008). Dust concentrations had been declining since the dust storms of 2009; however, November 2019 was the dustiest month on record (Community DustWatch 2019). This coincided with low rainfall and high winds: there were twice the number of expected strong wind gusts above 40 km/h, and most of New South Wales remained in the lowest 10% of rainfall recorded. The Australian Capital Territory has also seen an increase in frequency of dust storms in the past few years due to drought. The highest dust concentrations in Australia are observed in inland regions close to the Lake Eyre basin, and regions occupied by dryland agriculture (Leys et al. 2018).

Figure 31 Timeseries of 1-hour dust, measured as total suspended particulates, after filtering out data for fog and smoke

In 2019–20, total dust emissions in Australia reached an estimated 40 tonnes per hectare (Figure 32a). Peak dust emissions occur in the summer months when the ground is dry and has less vegetation coverage than in winter (Figure 32d).

Airborne soil dust comes in a range of particle sizes, from less than 2.5 µm to more than hundreds of micrometres in diameter. The larger particles deposit onto the ground quite quickly and do not travel very far (Figure 32b). However, larger particles can cause visibility issues, disrupt transport networks and increase the need for more cleaning of surfaces such as solar panels.

Smaller particles remain airborne for longer and can travel hundreds or thousands of kilometres, sometimes causing increased particle concentrations in cities far from the dust source. Smaller particles are more likely to be washed out of the atmosphere by rains (Figure 32c), which in 2019–20 deposited dust over New South Wales, Tasmania and Victoria.

Figure 32 Annual modelled (a) airborne emission, and (b) dry and (c) wet deposition of dust particles <2 to 60 μm in diameter, for 12 months (July 2019 to June 2020); (d) bar chart, with a logarithmic vertical scale, showing the monthly variation in emissions and deposition, summed across Australia

Airborne dust has health effects. Dust can cause respiratory difficulties, and mineral dust contains metals such as iron that cause lung inflammation.

For example, a 2019 study found that high airborne dust concentrations are likely to have contributed to increased allergies, and ear and lung infections in Indigenous children living in 66 remote Western Australian communities (Shepherd et al. 2019). The study suggested that lungs that had been previously exposed to continuous long-term dust had a reduced capacity to expel pathogens. During the study, the incidence of hospital visits plateaued with moderate dust measurements, and children became more sensitive to subsequent exposures when dust measurements were not necessarily high. Dust suppression through vegetation programs would benefit the health of these communities.