Multi-pollutant assessment of health impacts of 2021 summer wildfires in eastern and central Mediterranean Basin

Ever-increasing wildfires in scale and duration have resulted in enormous human and material losses, and adverse health outcomes due to short- and long-term exposure to diverse air pollutants emitted from fires. Historically, the Mediterranean Basin, characterized by hot and dry summers, has been particularly affected by wildfires, and the situation is deteriorating as climate change worsens and the regional populations grow rapidly. To assess the health impacts due to short-term exposure to air pollution caused by the 2021 summer wildfires in eastern and central Mediterranean Basin, we demonstrate a multi-pollutant approach based on the Weather Research and Forecasting online-coupled Chemistry (WRF-Chem) model. The WRF-Chem model was used to simulate concentrations of major air pollutants such as fine particulate matter (PM_2.5), SO₂, NO₂, and O₃, in a fire and no-fire scenario. Elevated short-term exposure of the population to air pollutants were associated with excess all-cause mortality using relative risks (RRs) for individual pollutants based on previously published meta-analyses.

Our estimates indicate that the additional short-term exposure to O₃, which is predicted to increase due to the wildfires, resulted in the highest number of excess deaths of 608 (95% CI: 456-771) over the entire region of investigation during the wildfire season between mid-July to early October 2021. This is followed by 270 (95% CI: 177- 370) excess deaths due to elevated PM_2.5 exposure, rendering the health effect of increased O₃ from wildfires larger than the effect of increased PM_2.5. This is shown to be largely reasoned by the spatially more widespread impact of wildfires on O₃. In contrast, the excess mortality caused by NO₂ and SO₂ emitted from wildfires is estimated low. This may be ascribed to the different sources of air pollutants, with NO₂ a marker of traffic, while SO₂ originating primarily from emissions from fossil fuel combustion, e.g., from power plants. Our study concludes with a discussion on uncertainties associated with the multi-pollutant health impact assessment and suggests a critical scrutiny of estimates based thereupon.