Updated: 22 May 2020

• Human exposure to ambient ozone and extended heat conditions can exacerbate cardiopulmonary disease and respiratory illnesses. Whether the combined exposure to heat and ozone could also aggravate COVID-19 symptoms and prognosis is a subject under evaluation, but has not yet been fully established.
• People with prior long-term exposure to high levels of air pollution might face the combined effect of increased vulnerability to heat stress and increased vulnerability to more severe COVID-19 symptoms.
• Recent observations of air pollution (e.g. nitrogen dioxide (NO2), airborne particulate matter (PM)) show a downward trend in primary pollutants due to decreased transport and economic activities as a result of COVID-19 control measures. How these recent changes in air quality will affect extreme heat, ambient ozone levels and heat-related health impacts has not yet been established.

Populations in cities/regions with higher air pollution levels may be more vulnerable to both heat stress and more severe COVID-19 symptoms.

• High levels of ambient ozone and extreme heat can put an additional burden on health, health systems and emergency services (McGregor et al., 2015) that are already under pressure due to the COVID-19 pandemic. In preparing for the combined risks/burden of extreme heat and COVID-19, health services need to be aware of the possible interactions with air pollution.
• Every effort should be made to keep levels of ozone and particulate matter as low as possible during heatwaves (Matthies et al., 2008), especially during the COVID19 crisis.
• National and international meteorological and environmental organizations can play a proactive role in warning relevant government departments of the location and timing of likely episodes of high air pollution and heatwaves.
• Decision-making authorities and bodies can review the daily ozone concentrations during pandemics, at least when there is evidence that this benefits health outcomes (Learn more: see Air pollution due to ozone: health impacts and effects of climate change)

Exploring the link between air pollution, heat exposure and COVID-19 is multifaceted. It requires a consideration of the possible associations between: long-term exposure to air pollution and the vulnerability to heat and COVID-19; ozone and heat exposure, and the vulnerability to more severe COVID-19 symptoms; and changes in air pollution and possible changes on surface temperature/heat, ambient ozone and heat-related health effects due to implementation of public health and social measures.

Impact of long-term air pollution exposure on vulnerability to both heat and COVID19: Long-term exposure to air pollution increases the risk of numerous diseases, including cardiovascular and respiratory disease (Cohen et al., 2017; WHO, 2018). These pre-existing conditions are not only linked to increased vulnerability to extreme heat (McGregor et al., 2015; Oudin Åström et al., 2015; WHO, 2011; Kenny et al., 2010; Gosling et al., 2009), but recent findings suggest that they could also be risk factors for more severe symptoms in COVID-19 infection (Yang et al., 2020; Jain and Yuan, 2020). Moreover, a recent study identified significant associations between long-term exposure to PM2.5 (particles with a diameter ≤2.5 µm) and ozone and elevated risk of acute respiratory distress syndrome (ARDS) among older adults in the United States (Rhee et al., 2019).

In response to the COVID-19 outbreak, several studies found that prior long-term exposure to air pollution is associated with increased vulnerability to fatal COVID-19 infection; this association has been reported in Europe (Ogen, 2020; Conticini et al., 2020; Travaglio et al., 2020) and the United States (Wu et al., 2020). It is not yet established whether these correlations represent causal effects of air pollution or if they possibly result from confounding factors such as urban density or certain population characteristics. Additionally, it is unclear if the recent lockdown-induced downward air pollution trend (see below) may counteract the possible effects of long-term exposure. Therefore, people with long-term exposure to high levels of air pollution might face the combined effect of increased vulnerability to extreme heat as well as increased vulnerability to more severe COVID-19 symptoms.

Impact of combined exposure to extreme heat and photochemical smog on COVID-19 symptoms/prognosis: Heat exposure can worsen underlying health conditions and is associated with increased (cardiovascular and respiratory) mortality (WMO, 2019; McGregor et al., 2015; Bunker et al., 2016; Gosling et al., 2009; Matthies et al., 2008). Furthermore, higher ambient temperatures, solar intensity and stable conditions (e.g. weak winds) during heat episodes can contribute to the formation of photochemical smog, including ambient ozone (a highly oxidative pollutant). The increased use of airconditioning can result in higher ozone levels as well (Abel et al., 2018). This means that periods of extreme heat can coincide with ozone episodes (Zhang et al., 2019; Varotsos et al., 2019; Kinney, 2018; U.S. EPA, 2013; U.S. EPA, 2006; Aw and Kleeman, 2003).

The impact of short-term exposure to ambient ozone on the susceptibility to COVID-19 infection (via the modulation of lung host defences (U.S. EPA, 2013; Zhang et al., 2019)) is not yet established. However, short-term exposure to ambient ozone does cause adverse respiratory symptoms, inflammation of the airways, the aggravation of underlying respiratory conditions (e.g. asthma and Chronic Obstructive Pulmonary Disease (COPD)), and increased respiratory mortality. Adverse impacts have also been observed for cardiovascular effects, including cardiovascular mortality (Zhang et al., 2019; U.S. EPA, 2013; U.S. EPA, 2006). Moreover, there could be a possible synergistic effect between heat stress and ambient ozone – several studies have found a larger heat-related mortality effect on days with higher levels of ambient ozone (Krug et al., 2019; Scortichini et al., 2018; Chen et al., 2018; Analitis et al., 2014; Katsouyanni and Analitis, 2009). As both photochemical smog and heat episodes could exacerbate cardiovascular and respiratory conditions, the severity of COVID-19 symptoms along with the prognosis of COVID-19 patients might consequently be aggravated during heatwaves too.

Impact of lockdown measures on air pollution and, consequently, extreme heat, ambient ozone and heat-related health risks: Worldwide, COVID-19 lockdown measures have significantly reduced industrial/commercial activities as well as the volume of traffic. Consequently, recent air pollution levels (e.g. NO2, PM) show a downward trend as, for example, observed in Northern Italy and other European regions (CAMS, 2020; ESA, 2020a), China (ESA, 2020b; NASA, 2020a) and Northern India (NASA, 2020b). However, ambient ozone is a secondary pollutant and its formation and breakdown involves complex processes that are influenced by many factors1 (Zhang et al., 2019; Marr and Harley, 2002; Vardoulakis and Heaviside, 2012). A recent modelling study (Huang et al., 2020), for example, found that in some Chinese regions ambient ozone levels could have increased due to lockdown-induced reductions in the emission of nitrogen oxides (NOX). As lockdown measures continue, changing air pollution levels are expected to continue as well. How these recent changes in air quality may affect the health risks of extreme heat has not yet been established:

• Impact of decreasing air pollution on surface temperature and heat: Decreasing levels of particulate matter (aerosols) can reduce the reflection of incoming solar radiation directly, but also indirectly via aerosol–cloud interactions. Modelling studies indicate that future reductions in aerosols are linked to more severe temperature/heat extremes (Zhao et al., 2019; Samset, 2018; Wang et al., 2016; Horton et al., 2016). Whether, and to what extent, the lockdown-induced decline in air pollution will affect extreme heat in the near future still needs to be established.
• Impact of decreasing levels of air pollution on heat–health-related impacts during heatwaves: High levels of air pollutants have been linked to increased morbidity/mortality during heat (Krug et al., 2019; Scortichini et al., 2018; Chen et al., 2018; Analitis et al., 2014; Katsouyanni and Analitis, 2009; Ren et al., 2008). Yet, it is unclear how the lockdown-induced changes in air pollution levels will affect the health impacts of heat episodes. For example, as higher levels of PM10 (particles with a diameter ≤10 µm) have been reported to increase the mortality effect of prolonged heat (Analitis et al., 2014), the lockdown-induced declines in PM10 might lower the heatwave effect on mortality. The impact of lockdown measures on ambient ozone formation and breakdown during heat episodes is not yet established and could be very location specific.

¹ Lower levels of certain air pollutants (nitrogen oxides (NOX), volatile organic compounds (VOC), and carbon monoxide (CO)) could result in reduced ozone formation, while a decrease in nitric oxide (NO) might reduce ozone breakdown. A reduction in aerosols may possibly favour ozone formation via increased incoming solar radiation (Zhang et al., 2019; Marr and Harley, 2002; Vardoulakis and Heaviside, 2012).