Air pollution, particularly particulate matter (PM2.5), poses a significant threat to global public health. Wildfires, a major source of PM2.5, have intensified in recent years, raising concerns about their impact on mortality. While previous studies have investigated the effects of total PM2.5, research specifically on the health consequences of wildfire-related PM2.5 remains limited, particularly in regions with high wildfire activity like Brazil. Existing evidence often focuses on single cities or regions during specific burning seasons. There is a critical need for comprehensive, nationwide studies to understand the effects of wildfire-related PM2.5 on cause-specific mortality across different demographics and geographic locations. This study fills this gap by analyzing a nationwide mortality dataset in Brazil, a country with extensive wildfire activity. The study is designed to quantify the mortality burden linked to wildfire-related PM2.5, considering variations across causes of death, age groups, sex, and regions. The findings are expected to inform public health policies and interventions to mitigate the harmful effects of wildfire smoke.

Recent epidemiological studies have indicated a stronger association between wildfire-related PM2.5 and adverse health outcomes compared to PM2.5 from other sources. For example, Aguilera et al. (2021) found higher increases in respiratory hospitalizations associated with wildfire-specific PM2.5 than with non-wildfire PM2.5. Studies also highlight the increased toxicity of biomass particles, particularly on children's respiratory health. Toxicological studies have further demonstrated differences in the toxicological properties of aerosols from various sources, with biomass particles exhibiting greater toxicity than those from fossil fuels. Several studies in Brazil have investigated the premature deaths attributable to air pollution from fire emissions but have limitations like focusing on total particulate matter and not cause-specific mortality or wildfire-specific PM2.5. This study addresses these knowledge gaps using a nationwide dataset and focusing on cause-specific mortality to provide a more comprehensive understanding of the health effects of wildfire-related PM2.5 in Brazil.

This study used a nationwide mortality dataset from the Brazil Mortality Information System (SIM) covering the period from 2000 to 2016. The data included information on municipality, age, sex, death date, and cause of death (coded according to ICD-10). Daily death counts were aggregated into 510 immediate regions in Brazil. Population data were obtained from the Brazilian Institute of Geography and Statistics (IBGE). Meteorological data (temperature and dew point) were sourced from the European Centre for Medium-Range Weather Forecasts Reanalysis, v5 (ERA5). Daily wildfire-related PM2.5 concentrations were estimated as the difference between GEOS-Chem model simulations with and without fire emissions. A two-stage time-series approach was employed. First, region-specific associations between daily wildfire-related PM2.5 and cause-specific mortality were analyzed using a quasi-Poisson generalized linear model, adjusting for meteorological factors, day of week, and public holidays. A cross-basis function was used to model the lagged effects of PM2.5 on mortality. In the second stage, region-specific estimates were pooled using meta-analysis. Attributable mortality fractions, rates, and deaths were calculated using established methods. Sensitivity analyses were conducted to assess the robustness of the results by varying the maximum lag time, degrees of freedom in the cross-basis function, and meteorological variables' degrees of freedom. The dlnm and mvmeta R packages were used for data analysis.

The study found a significant linear association between wildfire-related PM2.5 and cause-specific mortality. A 10 µg/m³ increase in daily mean wildfire-related PM2.5 was associated with a 3.1 (95% CI: 2.4–3.9) increase in all-cause mortality, 2.6 (1.5–3.8) increase in cardiovascular mortality, and 7.7 (5.9–9.5) increase in respiratory mortality. The effects were acute, with some displacement on lag days 2–6. Cumulative relative risks over 0–14 days showed significant increases in mortality for all causes and subgroups. Adults aged ≥60 years and females exhibited greater sensitivity to the acute impacts of wildfire-related PM2.5 exposure. Geographic variations were also significant, with the Southeast region showing the highest effect estimates. Attributable mortality fractions were 0.70% (0.41–0.98) for all-cause, 0.63% (0.14–1.10) for cardiovascular, and 1.73% (0.98–2.45) for respiratory mortality. An estimated 130,273 all-cause deaths, 32,961 cardiovascular deaths, and 33,807 respiratory deaths were attributable to acute wildfire-related PM2.5 exposure. Attributable mortality rates were substantially higher in those aged 60 years or older compared to younger age groups. Sensitivity analyses confirmed the robustness of the findings.

This study provides robust epidemiological evidence of the acute health effects of wildfire-related PM2.5 exposure on mortality in Brazil. The findings highlight the substantial mortality burden attributable to wildfire smoke, particularly in vulnerable populations (older adults and females) and geographic regions (Southeast). The strong association between wildfire-related PM2.5 and respiratory mortality aligns with previous research. The findings for cardiovascular mortality are notable, as they represent the first large-scale study to quantify this association in Brazil. The geographic variation observed may reflect differences in wildfire frequency, population density, and socioeconomic factors. This study emphasizes the importance of targeted public health interventions and policies to mitigate the effects of wildfire smoke. Measures such as improved air quality monitoring, public health advisories, and personal protective measures are crucial for reducing the mortality burden.

This large-scale nationwide study provides robust evidence of the significant mortality burden attributable to short-term exposure to wildfire-related PM2.5 in Brazil. The findings highlight the vulnerability of older adults and females, as well as geographic disparities. The study underscores the need for comprehensive public health interventions and policies aimed at mitigating the health risks associated with wildfire smoke. Future research could explore the combined effects of wildfire-related PM2.5 and other pollutants, and investigate long-term health impacts.

This study has several limitations. First, only wildfire-related PM2.5 was considered, and the joint effects with other pollutants were not assessed. Second, using region-average exposure estimates might underestimate the actual individual exposure levels. Finally, the study could not differentiate between wildfires and deliberately set fires. These limitations should be considered when interpreting the findings.