Heatwaves and Air Pollution: a Deadly Combination

Climate change is one of the most pressing health challenges of our time, and the rapid pace of change means we will face growing health risks in the coming decades. Although gradual trends are important for some factors, such as a rise in sea level, human impacts are especially driven by extremes of weather, including heat waves, intense rain events, coastal storms, flooding, draft, and others (1). These, in turn, can influence a series of downstream impacts, including increases in wildfire frequency and extent, which have adverse health impacts. Indeed, trends of warmer, drier conditions in California have led to a fivefold increase in the area burned by wildfires in California between 1972 and 2018 (2).

In addition to their immediate impacts on loss of life and property, wildfires release vast quantities of particulate pollution that travel long distances, potentially creating prolonged periods of exposure and adverse health outcomes for populations over wide areas. With the success of clean air programs addressing anthropogenic pollution sources, wildfire is becoming the dominant source of high concentrations of fine particulate matter (PM_2.5) pollution in California. This is because of the increasing frequency and severity of wildfires that now extend far beyond the summer season in California. Because wildfire emissions cannot be controlled easily via regulations, and fires often coincide with periods of warm temperatures, an issue of special concern is whether the combination of exposure to high PM_2.5 and temperature would create adverse health impacts that are additive or even synergistic.

Individually, both PM_2.5 and temperature have been associated with a wide range of adverse impacts, including respiratory and cardiac conditions, as well as premature mortality (3–8). Wildfire smoke contains a complex mix of combustion particles, including those related to the burning of homes, cars, and other products, and may present unique health challenges (9). In this issue of the Journal (pp. 1117–1127), the results of a new study by Rahman and colleagues provide substantial evidence that combined extremes of PM_2.5 and temperature in California over recent years have had superadditive effects on premature deaths (10).

The study included data on daily all-cause, cardiovascular, and respiratory deaths for all of California from 2014 to 2019. These were analyzed using a case-crossover design in relation to four strata of exposure to daily temperature and PM_2.5 concentrations: days with only extreme PM_2.5, days with only extreme temperature, days with extremes for both PM_2.5 and temperature, and days with no extremes. Four cutoffs for extreme days were analyzed on the basis of the 90th, 95th, 97th, and 99th percentiles of the respective exposure distributions. Results showed substantially higher mortality effects of the combined exposure than for the individual exposures, higher than the sum of the individual exposures. Results were more pronounced, though noisier, for respiratory and cardiovascular mortality than for total mortality and minimum versus maximum daily temperature.

What might be responsible for the observed synergistic mortality impacts of extremes of temperature and PM_2.5? Combined extreme events may involve a different compositional mix of PM_2.5 than other days, perhaps driven by wildfire smoke and chemical transformations facilitated by high temperatures. Another possibility is that population exposure to smoke and/or temperature change because of alterations in time activity patterns during fire events. Joint exposures to other pollutants such as ozone during combined events could also play a role. It has been noted that high temperatures are associated with an elevation in ozone, thus potentially triggering greater interactions of biological pathways to adversely affect the cardiorespiratory system. Because of the growing severity of wildfires that include the loss of buildings and homes and their contents, multiple chemical markers have been found in wildfire PM, including multiple phthalates, thus potentially enhancing the toxicity of wildfire smoke over large areas where smoke is transported (9).

With accelerating climate change and the emergence of wildfires as a significant source of high air pollution episodes, it will be increasingly important to understand and mitigate the human health effects of exposure to degrees of air pollution and temperature. Future research should explore further whether, how, and to what extent wildfire PM_2.5 has a distinct toxicity profile as compared with nonwildfire PM_2.5 and how those factors are influenced by temperature. Relatedly, we need to understand the mechanisms by which wildfire smoke interacts with and affects the respiratory and cardiovascular systems at higher temperatures. Toxicologic and epidemiologic studies will be needed to both understand mechanistic pathways and potential therapeutic interventions, as well as to quantify population impacts and extend findings to intermediate morbidity outcomes on the pathway to mortality. Because the extreme events studied here were relatively rare events, there is also potential for developing forecast systems that could provide advance warning to patients and care providers of anticipated extreme events. More information will also be needed on effective measures to protect individuals from exposure to extreme climate events, including building air cleaning and conditioning systems. The urgency for further research on these topics will continue to intensify together with further changes in our climate.