The toxic cocktail of heat and air pollution in a changing world

New data reveals synergistic impact of heatwaves and air pollution on mortality. The good news is that these two challenges can be addressed through unified strategies.

Hot weather and air pollution pose a significant health and environment risk on their own. Heatwaves can be deadly, and air pollution is only second to high blood pressure in terms of global mortality every year. But, when these two are combined, they become even more dangerous. An increasing amount of data is showing that heat and air pollution can be synergistic and amplify the health impact for those co-exposed. This is especially relevant in a world with a changing climate, where heat records are quickly being broken, heatwaves are becoming more frequent, and the widespread impacts of heat exposure lead to risks for people and environment. Seeing the dual impact of heat and air pollution adds weight to integrated solutions that are able to simultaneously tackle both heat and air pollution exposure, are a key way forward in protecting health and the environment from these synergistic impacts.

Summer is becoming an increasingly challenging, and dangerous, time for humans. Year-on-year, figures for mortality related to summer weather conditions, namely heat, show high and increasing levels of mortality. For example, in Europe in 2022, an estimated 61000 people died due to heat [1]. In 2023, this number was estimated at just under 48000, the second highest in the study period from 2015 onward, only beaten by 2022 [2]. A significant increase in heat-related morbidity and mortality has been seen in the European region since 2000, with a 30% increase in heat-related mortality, and an increase in mortality seen in 94% of nearly 1000 European regions monitored [3]. While multiple factors contribute to the pathway between heat exposure and vulnerability, and morbidity and mortality, research is showing that the combined risk of heat and air pollution exposure can be greater than the risk of each.

Multiple studies have looked at outcome for the exposure to heat, as well as exposure to air pollution, both being linked to morbidity and mortality in their own right. However, new research into the joint effect of both heat and air pollution together indicates that cumulative effect of both risk factors could be greater than the sum of risks between the two, with variable effects at different levels of exposure to air pollution. A case study in Greece, published this year, found that when mortality increases were compared between high temperature days of low and high air pollution, much more temperature-related mortality was seen on days of high air pollution [4]. The effect was seen both when looking at PM10 and ozone. The increased heat-related mortality on high air pollution days was especially evident for respiratory deaths. Another study in the USA looked at increased risk of deaths related to PM2.5 on high temperatures days, with results showing a 21% increase in all-cause mortality when both PM2.5 pollution and temperature were high [5]. This is in comparison to a single-exposure mortality risk of 6% on high temperature days, and 5% on high PM2.5 days, illustrating the increased cumulative risk for co-exposure. Looking at cause-specific mortality, this study also indicated stronger effects for respiratory mortality.

Wider systematic review and analysis supports these regional results, with two large-scale analyses across hundreds of cities in multiple countries finding effect modification of mortality in high temperatures when air pollution levels increased. An analysis of 482 locations in 24 countries showed consistent evidence that for all considered air pollutants (PM10, PM2.5, O3, NO2), there was increased cardiovascular and respiratory mortality risk at high temperatures [6]. This was more so for respiratory mortality than cardiovascular. A wider analysis of 620 cities in 36 countries, analysing over 22 million deaths, found above expected mortality rates for PM10, PM2.5, O3 and NO2 when temperature was high [7]. In addition, the analysis also found that at days with the highest temperature, the association between pollution levels and mortality risk was higher than for lower levels, therefore, not linear [7]. A 2020 systematic review of the literature found synergistic effects of high temperature and pollution on mortality, including for cardiovascular and respiratory mortality, and the strongest evidence being found for high temperature exposure when paired with PM2.5, and also ozone. The studies are in general time-series studies of short-term effects [8].

While it is unclear exactly how high air pollution and high temperature create synergistic risk, multiple mechanisms have been proposed. Physiological responses to high temperatures may increase cardiorespiratory effort, leading to greater intake and susceptibility to pollutants, and less ability to detoxify [6,7]. Various mechanisms related to inflammation have been proposed, processes which are potential dangerous on their own, but may also increase pollutant absorption, as well as coagulation and dehydration increasing blood concentration [6,7]. Bio- and photochemical pathways have also been proposed, such as the association with high temperatures and sunlight leading to ozone formation and respiratory effects [6]. Social pathways may also explain some association, such as people spending more time outdoors in warmer weather, and exposure related to response to wildfire events, leading to more exposure to air pollutants [6,9].

While these results show analysis of now historic numbers, they stand to change as our climate changes. Due to climate change, shifting weather patterns are meaning more heat and more heat exposure. Some population groups are more vulnerable to impact from these changes than others, such as younger people, older people and those of lower socioeconomic status. Coincidentally, these populations that are more vulnerable to heat are also more vulnerable to air pollution, but it isn’t only vulnerability that is intertwined, it is also risk, as climate change can also lead to increased air pollution through heat, ozone formation and wildfires [10], as well as possibly through wider changes, such as deforestation and desertification. Special mention should be made of cities, which already see increased levels of air pollution, as well as increased heat trapping and exposure due to urban heat islands [10]. In a changing climate, and considering the synergistic effects of heat and air pollution, on top of trends of increasing urbanisation both in Europe and globally, there is a significant risk for increasing mortality into the future, which is complimented by wider trends in ageing and chronic disease prevalence [10].

Luckily, these joint problems have joint solutions. Air pollution and climate change have many shared sources, such as industry, heating, agriculture and transport, but therefore, also have shared solutions. Solutions that control emissions of CO2 often also control air pollutant emissions. Sustainable agricultural practices can lower carbon dioxide as well as ammonia emissions. Sustainable mobility solutions can reduce reliance on fossil fuel-based transport, as well as the air pollutants that come with private motor vehicle use, such as PM and NO2. Renewable and sustainable energy systems emit less CO2 and air pollutants, and energy efficiency solutions can reduce the need for electricity generation. Nature-based solutions, including infrastructure interventions such as urban greening, offer buffering of extreme weather and air pollution, especially relevant to people living in urban areas including at-risk populations. Importantly, actions that combat vulnerability and inequity in populations and communities also protect against multiple types of environmental stressors including air pollution, climate change and heat.

Heat and air pollution pose significant health risks individually, but the cumulative risk of co-exposure to both high temperature and high levels of air pollution synergistically add to an even greater risk. Evidence shows that this holds true across multiple pollutant types and multiple locations. In the face of a changing climate, with commonalities between sources of carbon emissions and air pollutants, there is reason for concern and action. However, these commonalities also mean common solutions, and there are a number of ways that integrated planning and solutions can help to reduce the risk from both environmental extremes as well as air pollution. Sustainability-focused system changes, nature-based solutions, and actions to reduce risk and vulnerability across inequal populations provides answers to how to address these multi-pronged crises that threaten health and environment, and often provide cost-benefits far outweighing the cost of implementation. One final study found that in some contexts, the costs of meeting the Paris Agreement of limiting global warming to 1.5°C could be offset just from the health co-benefits of reducing air pollution impact [11]. This study is now 6 years old, and since then we have even more insight into the risks of both climate change and air pollution. When a fuller picture of impacts, interactions and common solutions is looked at, it becomes logical that action to fiercely reduce emissions is the only acceptable way forward.

References:
[1] Ballester, J. Heat-related mortality in Europe during the summer of 2022. Nature Medicine. 2023
[2] Gallo, E. et al. Heat-related mortality in Europe during 2023 and the role of adaptation in protecting health. Nature Medicine. 2024
[3] Copernicus. Extreme weather and human health. ESOTC 2023: Climate Policy and Action. 2023
[4] Zafeiratou, S. et al. Modification of heat-related effects on mortality by air pollution concentration, at small-area level, in the Attica prefecture, Greece. Environmental Health. 2024
[5] Rahman, M. et al. The Effects of Coexposure to Extremes of Heat and Particulate Air Pollution on Mortality in California. American Journal of Respiratory and Critical Care Medicine. 2022
[6] Rai, M. at al. Heat-related cardiorespiratory mortality: Effect modification by air pollution across 482 cities from 24 countries. Environmental International. 2023
[7] Stafoggia, M. et al. Joint effect of heat and air pollution on mortality in 620 cities of 36 countries. Environment International. 2023
[8] Anenberg, S. et al. Synergistic health effects of air pollution, temperature, and pollen exposure. Environmental Health. 2020
[9] Kinney, P. et al. Heatwaves and Air Pollution: a Deadly Combination. American Journal of Respiratory and Critical Care Medicine. 2022
[10] European Environment Agency (EEA). Combined effects of air pollution and heat exposure in Europe: time for action. European Environmental Agency. 2023
[11] Markandya, A. et al. Health co-benefits from air pollution and mitigation costs of the Paris Agreement: a modelling study. The Lancet Planetary Health. 2018