Air Quality and Heatwaves
Response to the WMO report: investigating the link between heatwaves and air pollution
Anyone with even a passing interest in the environment, or for that matter anyone who has picked up a newspaper in the past couple of years, will be aware that the climate crisis is making itself felt more and more by the day. From the flooding that devastated the Libyan city of Derna, to the wildfires that have raged across Lahaina Island in Hawaii and the droughts that plagued Sri Lanka earlier in 2023, the news headlines tell a story of us stumbling from one climate disaster to another.
Even the UK, a relatively temperate country thought immune to large fluctuations in weather, has suffered many meteorological challenges over the past decade. The heatwaves of 2022 were considerably disruptive for the UK, with the mercury surpassing 40C: temperatures of 40.2C were recorded at Heathrow, officially the highest temperature ever recorded in the UK. Alongside this heatwave came a whole host of health, societal, and infrastructural challenges; not only did rail lines buckle, tarmac melt, and vegetation burn, but recent data from the ONS has shown the hot weather claimed over 4,500 lives in England alone.
As a direct result of this, the mortality and toll on humanity associated with heatwaves is becoming a more prominent theme in activism and policymaking. However, what appears to receive less attention are the more indirect health effects of heatwaves.
The World Meteorological Organisation Report
One recent report that has seemingly redressed this issue was published by the World Meteorological Organisation (WMO) at the start of September.
Entitled the WMO “Air Quality and Climate Burden Bulletin”, this was the third in a series of publications where authors compile the latest climate research to highlight the strong relationship between climate change and air quality. In particular, it assesses how the levels and geographic distribution of traditional pollutants – including nitrogen oxides, ground-level ozone, and particulate matter (PM) – are changing in the face of a warming climate. What makes the work so valuable is that the researchers have investigated not only the future trends of heatwaves and air pollution, but also how it will impact our health and societies more generally.
This issue discussed four heatwave-related phenomena that have the potential to impact air quality, which we have summarised below.
Increasing Particulate Matter Concentrations
The term ‘particulate matter’ does not refer just to one pollutant but is instead an umbrella term used to describe a range of chemicals varying in size, shape, and composition. These airborne particles are normally stratified according to their size. Chemicals smaller than 2.5 microns (PM2.5) are more likely to travel deeper and deposit on the surface of the lungs, making them more likely to cause tissue damage and inflammation and in turn more serious long term health impacts.
According to the WMO report, the incidence and severity of what’s known as ‘PM2.5 anomalies’, i.e. periods of unusually high PM2.5 levels, can be greatly affected by wildfires and desert storms, both of which are exacerbated by heatwave activity. Whilst the occurrence of wildfires and desert storms were generally lower in 2022 than in 2021, there were still fire-driven anomalies over the Amazon basin, Alaska, South Africa, and the north-western US, and high dust intrusions in Europe, China, and the eastern US.
With respect to the impact of wildfires, extreme heat experienced during a heatwave creates favourable conditions for fires to propagate. The burning of dry, combustible vegetation during these hot periods is known to greatly increase the PM2.5 concentrations in the air, with NASA showing that one heatwave in September 2022 correlated with unusually high wildfire activity over the north-western US. This led to the Environmental Protection Agency (EPA) issuing air quality warnings due to pollutant concentrations being well above identified safe limits across the region during that period.
Similarly, atmospheric circulation and meteorological conditions from dust storms encourage the transport of dust from the deserts of North Africa to Europe. The dust clouds have high concentrations of PM2.5, which is then dispersed in areas across Europe. During 2022, desert dust and PM2.5 intrusions were common in Europe, and when the circulation pattern fosters this kind of intrusion, hot air enters alongside. It would thus seem a positive feedback loop is possible, whereby the temperature increases due to hot, dry air intruding from desert regions, contributing to the desertification of Europe. This would further increase the concentrations of localised dust and high PM2.5 concentrations.
Exposure to PM2.5, even over a short time period, can present a serious health risk. For PM2.5, short-term exposures (~24-hours) is associated with increased admission to hospital for heart- and lung-related illness (particularly for those with pre-existing conditions), asthma attacks, and the exacerbation of bronchitis symptoms. In fact, compared to the most common ambient air pollutants, PM2.5 presents the greatest health burden amongst the population. This is likely to be exacerbated by heatwaves as extreme heat is already known to impact cardiovascular health and worsen chronic respiratory disease, the coincidence of both this and high PM2.5 concentrations will contribute to increased mortality rates.
Effects of Ground-Level Ozone
Ozone has garnered a lot of media and scientific attention over the past few decades, mostly due to its protective, anti-UV properties that shield us from the sun’s rays. However, fewer know about its more harmful qualities, particularly when present at ground level. When inhaled by humans, ground-level ozone causes a variety of health problems, from chest pains, coughing, and throat irritation to bronchitis and lung inflammation.
There is also a strong link between the incidence of heatwaves and ground-level ozone concentrations. Ground-level ozone is usually produced through complex photochemical (light-induced) reactions involving nitrogen oxides and volatile organic compounds (VOCs), which can be produced naturally by plants or anthropogenically through the burning of fossil fuels. Therefore, it is much more likely to form under already polluted conditions.
Under the hot and stagnant conditions experienced during heatwaves, the precursor pollution needed to form ground-level ozone builds up. Hotter, sunnier environments also result in increased rates of atmospheric chemical reactivity and boosts the emission of VOCs from plants, further contributing to ozone formation, as illustrated in Figure 1. In fact, during the July 2022 heatwave in Europe, hundreds of air quality monitoring sites exceeded the World Health Orginisation’s ozone air quality healthy level for an 8-hour period for the first time. The health impacts of such exposure are yet to be seen.
Figure 1: Atmospheric chemical activity during a heatwave event (Source: WMO Report)
It appears evident that in the new environmental context presented by a warming climate and the increased occurrence of heatwaves, a focus on ground-level ozone may be required to counterbalance the deterioration in air quality heatwaves may bring.
How Agriculture and Air Quality Overlap
Global agricultural output and crop production have increased extensively over the past 60 years, approximately quadrupling. Whilst this has alleviated hunger throughout much of the world and brought millions out of food poverty, this process has consequently brought about rampant pollution in our environment. Not only has a growth in agriculture impacted land use and the contamination of water sources, but it has also been a large contributor to the diminishing air quality around the world.
However, what fewer consider are the impacts in the opposite direction – air pollution, it turns out, is able to impact agricultural output. Again, these effects are largely as a result of ozone and particulate matter (PM):
In many of the world’s largest agricultural regions, ozone is often found at high levels. At high enough concentrations, ground-level ozone can cause damage to plants in the vicinity, including food crops, through well-elucidated mechanisms. Generally, agricultural processes are through fossil fuel burning, biomass burning, and the release of methane. The WMO report, referencing a 2018 study from an international team of scientists, revealed that global crop losses as result of high ozone concentrations average 4.4%-12.4% of staple food crops, reaching 15%-30% in key areas of India and China.
Agricultural processes also lead to the formation of particulate matter, including ammonium nitrate particulates generated through the use of mineral and natural fertilisers, as well as land clearing and burning. By impacting leaf functioning, plant metabolic processes, and physically blocking light from reaching the leaves, high PM concentrations could theoretically diminish crop yields, yet research within this area is limited.
Overall, this demonstrates that traditional pollutants, which are already presenting an extensive health burden and threat to our health, could simultaneously impact our food security. As the emission of each of these pollutants is enhanced during heatwaves and high-temperature events, this research is beginning to emphasise the multiple ways which heatwaves disrupt our health, wellbeing, and livelihood.
Atmospheric Nitrogen Deposition
The deposition of nitrogen-containing (N) compounds, namely nitrogen oxides, ammonia, and organic N compounds, are known to have a fertilising effect on plants and are essential for their growth. However, the WMO report found recent research demonstrating that N deposition in high enough quantities can actually negatively impact ecosystem health, reducing the effectiveness of ecosystem services such as biodiversity, clean drinking water, food, and carbon storage.
Such high levels of N deposition are likely to occur during periods of high wildlife activity. In multiple studies referenced, forest fires across the continental US throughout 2022 greatly increased the rate of N deposition downwind of the affected areas. In California, there was an increase in N deposition of ~78% between the summers of 2021-22, with models predicting this could have negative effects on the survival and growth of different plant species.
Thus, with the incidence of forest fires only set to grow as a result of more frequent and severe heatwaves, we need to be aware of the wider ecosystem damage that they can cause. These subsequently impact the services provided by ecosystems, further showing the interlinked threats heatwaves have on our way of life, including our ability to access safe food and clean drinking water.
Shade the UK Recommendations
Whilst this report is likely to worry many about the future of our climate and health, we should note that each of these challenges have solutions. Through the joint effort of policymakers, urban planners, activists, and various stakeholders, we can develop lived spaces that are more robust during heatwaves and the air pollution they can cause. The report itself was keen to point this out; using an example from São Paulo, Brazil, research shows that greening the urban environment can have positive impacts on both the Urban Heat Island (UHI) effect, exacerbated during heatwaves, and air quality.
In light of this report, we at Shade the UK recommend that the UK government commit to certain policy positions that would allow us to tackle the worst effects of our warming climate. These include:
Develop stronger contingency planning for wildfires. This will allow emergency services to combat fires more effectively and limit the air pollution-associated health risks that comes with them.
Deliver air pollution warnings during heatwaves. As the population already receives heat warnings during extreme weather events, the public should be provided with information on limiting air pollution exposure during heatwaves.
Commit to improving air quality overall, particularly in urban areas that both have higher levels of pollution and see more intense heatwaves as a result of the Urban Heat Island (UHI) effect.
Expand urban green spaces. This will both limit the UHI effect and may contribute to bringing down pollution in cities, as well as the general benefits urban green spaces can bring.
Ensure existing air quality policy, strategies, and action planning consider the risk of heatwaves and their specific impacts associated with public exposure and health issues. Commit to improving understanding of the ties between air quality, wellbeing, and heatwaves, and equip the population with the resources to minimise health risks associated with these issues.
Written by Oliver Longstaff
