Ultrafine particles and stroke hospitalizations: New insights of exposure-outcome threshold, daily peaks, and comorbidities.

Stroke is a leading cause of death and morbidity worldwide. Prior research has found that exposure to air pollution increases the risk of stroke, but few studies have focused on the ultrafine particles (UFPs)-stroke relationship. We investigated the association between UFPs and stroke hospitalizations and disparities by stroke subtypes, demographics, and seasonality. In this time-stratified case-crossover study, stroke hospitalizations (mainly ischemic stroke and hemorrhagic stroke) were identified using principal diagnosis from New York State hospital discharge data. UFPs and covariates, including co-pollutants and meteorological factors, were derived from a widely validated GEOS-Chem/APM model. Excess risk per each interquartile range (ERIQR) increase was estimated. We found significantly positive associations between UFP concentrations and overall strokes, driven primarily by ischemic stroke (IS), with the strongest association at the multi-day lag 0-6 (ERIQR: 4.4%, 95% CI: 1.3%, 7.6%). Higher risks were observed among females (ERIQR: 8.4%, 95% CI: 3.2%, 13.9%), Non-Hispanic (ERIQR: 4.2%, 95% CI: 1.0%, 7.5%), Black (ERIQR: 6.5%, 95% CI: 2.0%, 11.2%), adults aged 60 and older (ERIQR: 4.8%, 95% CI: 1.3%, 8.4%), and during the winter season (ERIQR: 10.5%, 95% CI: 0.2%, 21.8%). IS admissions exhibited two daily peaks at approximately 6 am and 8 pm. The risk of IS increased when the UFP concentrations reached around 6000 particles/cm3. A higher burden of comorbidities, especially hypertension, was found among patients admitted on high-UFP days. In conclusion, elevated UFP concentrations were associated with increased IS hospitalizations and comorbidity burden, with large disparities across demographic groups and seasons.