Long-term mortality burden trends attributed to black carbon and PM₂.₅ from wildfire emissions across the continental USA from 2000 to 2020: a deep learning modelling study

Abstract

Summary Background Long-term improvements in air quality and public health in the continental USA were disrupted over the past decade by increased fire emissions that potentially offset the decrease in anthropogenic emissions. This study aims to estimate trends in black carbon and PM₂.₅ concentrations and their attributable mortality burden across the USA. Methods In this study, we derived daily concentrations of PM₂.₅ and its highly toxic black carbon component at a 1-km resolution in the USA from 2000 to 2020 via deep learning that integrated big data from satellites, models, and surface observations. We estimated the annual PM₂.₅-attributable and black carbon-attributable mortality burden at each 1-km² grid using concentration–response functions collected from a national cohort study and a meta-analysis study, respectively. We investigated the spatiotemporal linear-regressed trends in PM₂.₅ and black carbon pollution and their associated premature deaths from 2000 to 2020, and the impact of wildfires on air quality and public health. Findings Our results showed that PM₂.₅ and black carbon estimates are reliable, with sample-based cross-validated coefficients of determination of 0·82 and 0·80, respectively, for daily estimates (0·97 and 0·95 for monthly estimates). Both PM₂.₅ and black carbon in the USA showed significantly decreasing trends overall during 2000 to 2020 (22% decrease for PM₂.₅ and 11% decrease for black carbon), leading to a reduction of around 4200 premature deaths per year (95% CI 2960–5050). However, since 2010, the decreasing trends of fine particles and premature deaths have reversed to increase in the western USA (55% increase in PM₂.₅, 86% increase in black carbon, and increase of 670 premature deaths [460–810]), while remaining mostly unchanged in the eastern USA. The western USA showed large interannual fluctuations that were attributable to the increasing incidence of wildfires. Furthermore, the black carbon-to-PM₂.₅ mass ratio increased annually by 2·4% across the USA, mainly due to increasing wildfire emissions in the western USA and more rapid reductions of other components in the eastern USA, suggesting a potential increase in the relative toxicity of PM₂.₅. 100% of populated areas in the USA have experienced at least one day of PM₂.₅ pollution exceeding the daily air quality guideline level of 15 μg/m³ during 2000–2020, with 99% experiencing at least 7 days and 85% experiencing at least 30 days. The recent widespread wildfires have greatly increased the daily exposure risks in the western USA, and have also impacted the midwestern USA due to the long-range transport of smoke. Interpretation Wildfires have become increasingly intensive and frequent in the western USA, resulting in a significant increase in smoke-related emissions in populated areas. This increase is likely to have contributed to a decline in air quality and an increase in attributable mortality. Reducing fire risk via effective policies besides mitigation of climate warming, such as wildfire prevention and management, forest restoration, and new revenue generation, could substantially improve air quality and public health in the coming decades.