Wildfire activity in the United States has increased substantially in recent decades. Smoke fine particulate matter (PM2.5), a primary wildfire emission, can remain in the air for months after a wildfire begins, yet large-scale evidence of its health effects remains limited.
Methods:
We obtained hospitalization records for the residents of 15 states between 2006 and 2016 from the State Inpatient Databases. We used existing daily smoke PM2.5 estimations at 10-km2 grid cells across the contiguous United States and aggregated them to ZIP codes to match the spatial resolution of hospitalization records. We extended the traditional case–crossover design, a self-controlled design originally developed for studying acute effects, to examine associations between 3-month average exposure to smoke PM2.5 and hospitalization risks for a comprehensive range of cardiovascular (ischemic heart disease, cerebrovascular disease, heart failure, arrhythmia, hypertension, and other cardiovascular diseases) and respiratory diseases (acute respiratory infections, pneumonia, chronic obstructive pulmonary disease, asthma, and other respiratory diseases).
Results:
We found that 3-month exposure to smoke PM2.5 was associated or marginally associated with increased hospitalization risks for most cardiorespiratory diseases. Hypertension showed the greatest susceptibility, with the highest hospitalization risk associated with 0.1 µg/m3 increase in 3-month smoke PM2.5 exposure (relative risk: 1.0051; 95% confidence interval = 1.0035, 1.0067). Results for single-month lagged exposures suggested that estimated effects persisted up to 3 months after exposure. Subgroup analyses estimated larger effects in neighborhoods with higher deprivation level or more vegetation, as well as among ever-smokers.
Conclusions:
Our findings provided unique insights into medium-term cardiorespiratory effects of smoke PM2.5, which can persist for months, even after a wildfire has ended.