Long-term exposure to wildland fire smoke PM2.5 and mortality in the contiguous United States.

Despite the substantial evidence on the health effects of short-term exposure to ambient fine particles (PM_2.5), including increasing studies focusing on those from wildland fire smoke, the impacts of long-term wildland fire smoke PM_2.5 exposure remain unclear. We investigated the association between long-term exposure to wildland fire smoke PM_2.5 and non-accidental mortality and mortality from a wide range of specific causes in all 3,108 counties in the contiguous U.S., 2007-2020. Controlling for non-smoke PM_2.5, air temperature, and unmeasured spatial and temporal confounders, we found a non-linear association between 12-month moving average concentration of smoke PM_2.5 and monthly non-accidental mortality rate. Relative to a month with the long-term smoke PM_2.5 exposure below 0.1 μg/m³, non-accidental mortality increased by 0.16-0.63 and 2.11 deaths per 100,000 people per month when the 12-month moving average of PM_2.5 concentration was of 0.1-5 and 5+ μg/m³, respectively. Cardiovascular, ischemic heart disease, digestive, endocrine, diabetes, mental, and chronic kidney disease mortality were all found to be associated with long-term wildland fire smoke PM_2.5 exposure. Smoke PM_2.5 contributed to approximately 11,415 non-accidental deaths/year (95% CI: 6,754, 16,075) in the contiguous U.S. Higher smoke PM_2.5-related increases in mortality rates were found for people aged 65 above. Positive interaction effects with extreme heat (monthly number of days with daily mean air temperature higher than the county's 90^th percentile warm season air temperature) were also observed. Our study identified the detrimental effects of long-term exposure to wildland fire smoke PM_2.5 on a wide range of mortality outcomes, underscoring the need for public health actions and communications that span the health risks of both short- and long-term exposure.