Extreme heat and wildfire smoke, a harmful combination for health

At the beginning of March 2024, the Société de protection des forêts contre le feu (SOPFEU) issued its first fire risk bulletin of the year for three regions of Quebec, one month earlier than usual. There has been much less precipitation in the form of snow over the winter and this snow will disappear much earlier in the spring, increasing the risk of wildfires. Remember that in 2023, there were huge forest fires in Quebec and elsewhere in Canada, with an area burned greater than the sum of the last 20 years. Smoke from these wildfires in the north of the country migrated south, giving several major Canadian cities some of the worst air quality in the world.

Extreme heat events and wildfires have increased in recent years, and are expected to continue to increase in the coming years with global warming. Higher temperatures and a drier climate intensify the risk of large wildfires. Forest fires and heatwaves are increasingly intense, long-lasting, and affecting larger areas. This means that populations are increasingly exposed to episodes of extreme heat and, at the same time, to smoke from wildfires.

Exposure to these two phenomena increases the risk of multiple adverse health effects, including cardiac and pulmonary complications. Extreme heat can lead to dehydration and dilation of blood vessels, heat stress, and pressure on the thermoregulation system, thus putting a strain on the cardiovascular and pulmonary systems (see our article). In 2019, it was estimated that globally, 360,000 deaths are attributable to high temperatures each year. Inhaling fine particles contained in forest fire smoke can produce inflammation and oxidative stress, which causes cell breakdown and increases the risk of cardiopulmonary disease (see our article on the subject). It has been estimated that between 340,000 and 680,000 deaths worldwide are attributable to fine particles (PM_2.5) every year.

Since high temperatures and wildfire smoke have similar physiological effects and regularly occur at the same time, it is possible that they act synergistically and exacerbate adverse health effects. A few studies have examined the combined effects of extreme heat and ambient pollution, but only recently has a group of American researchers specifically studied the combined effects of heat and smoke from wildfires.

In this study, researchers compared hospitalization data for 67% of Californians with high-resolution smoke images taken from satellites and weather data. Between 2006 and 2019, the number of hospitalizations for cardiorespiratory disorders recorded during a day increased significantly depending on the amount of smoke from wildfires present in the air (see Figure 1 below). The number of hospitalizations increased particularly when heatwaves and smog episodes caused by forest fires occurred at the same time. The sum of hospitalizations during days of extreme heat (without smoke) and those recorded during days of smoke from wildfires (without heatwaves) is significantly lower than the number of hospitalizations on days when heatwaves and smog episodes caused by wildfires occur at the same time.

Figure 1. Associations between the risk of hospitalization for a cardiorespiratory disorder and extreme heat only, wildfire smoke only, and concomitant exposure to extreme heat and wildfire smoke. Adapted from Chen et al., 2024.

This synergistic effect between exposure to extreme heat and wildfire smoke is concerning. Since exposure to 15 µg/m³ of PM_2.5 on a day of extreme heat is equivalent in risk of hospitalization to exposure to 35 µg/m³ of PM_2.5 on a day without heat, government agencies will probably need to adjust their standards and make them stricter on days of double exposure.

In the California study, the synergistic effects of heat and smoke from wildfires were higher in communities with lower levels of education, lower incomes, denser populations, where ethnic minorities are more represented, and where there are fewer trees. In these communities, reduced access to health care, lack of means to protect themselves from heat, and high prevalence of comorbidities may explain why they are more severely affected by the synergistic effects of heat and smoke.

Extreme heat and wildfire smoke are two harmful exposures that are increasingly occurring simultaneously in the context of global warming. As the number of days of combined exposure increases, the synergistic effects between extreme heat and wildfire smoke will become more important for accurate estimation of the health burden and should be incorporated into the development of public warning systems.