Wildfire exposure and health outcomes: An umbrella review of systematic reviews and meta-analyses

Introduction: Wildfires are a growing concern due to their significant impact on wildlife, air quality, and health, and are increasing under climate change. Although several systematic reviews have explored the relationship between wildfire smoke and human health outcomes, a comprehensive overview of the overall epidemiological evidence remains needed. Thus, this umbrella review aimed to comprehensively synthesize the overall epidemiological evidence on the human health effects of wildfire smoke. Methods: This umbrella review followed PRISMA guidelines and was registered on PROSPERO (ID: CRD42024529782). We systematically searched PubMed, Scopus, and Web of Science for systematic reviews published up to March 25, 2024, using terms related to wildfires and health outcomes. The risk of bias was assessed using the AMSTAR-2 tool. Results: A total of 27 reviews were included in the analysis: 9 systematic reviews with meta-analyses and 18 systematic reviews without meta-analyses, published between 2010 and 2024. A total of 115 different health outcomes were examined, with respiratory morbidity being the most frequently reported. Other key health outcomes reported included mortality, cardiovascular conditions, mental health, and adverse birth outcomes. The review identified consistent associations between wildfire smoke exposure and all-cause mortality, respiratory morbidity, and mental health conditions such as PTSD, depression, and anxiety. However, findings related to cause-specific mortality, cardiovascular outcomes, and other health effects were less consistent. The quality assessment revealed a high proportion of systematic reviews and meta-analyses had critically low quality, coupled with inadequate reporting and risk assessment practices. Conclusion: Wildfires present significant health risks, impacting the respiratory system, birth outcomes, and mental health. Future research should utilize longitudinal and mechanistic studies to elucidate long-term effects and biological pathways of wildfire smoke, improve exposure assessment methods with advanced technologies, and review studies should adhere to established environmental health research review guidelines to enhance methodological rigor and global understanding.