Short-term exposure to PM_2.5 constituents, extreme temperature events and stroke mortality

Fine particulate matter (PM_2.5) pollution and extreme temperature events (ETEs) are main environmental threats to human health. Elevated stroke mortality has been growingly linked to PM_2.5 mass exposure, while its relationship with PM_2.5 constituents was extensively unstudied across the globe. Additionally, no prior assessments have investigated the interactive effects of PM_2.5 constituents and ETEs on stroke mortality.

Province-wide records of 320,372 stroke deaths collected in eastern China during 2016-2019 were analyzed using an individual-level time-stratified case-crossover design. Daily gridded estimates of PM_2.5 mass and its major constituents (i.e., black carbon [BC], organic matter [OM], ammonium [NH₄⁺], sulfate [SO₄^2-], and nitrate [NO₃^-]) were assigned to stroke cases on case days and control days at the residential address. We assessed 12 ETEs defined by multiple combinations of air temperature thresholds (2.5-10th percentiles for cold spell, 90-97.5th percentiles for heat wave) and durations (2-4 days). Conditional logistic regression model was applied to investigate associations of short-term exposure to PM_2.5 constituents and ETEs with stroke mortality. Odds ratio and its 95% confidence interval (CI) were assessed for an interquartile range (IQR) increase in each PM_2.5 constituent and on ETEs days compared with non-ETEs days. Additive interactive effects were quantitatively evaluated via relative excess odds due to interaction (REOI), attributable proportion due to interaction (AP), and synergy index (SI).

Elevated overall stroke mortality was significantly related to PM_2.5 constituents, with the largest odds observed for NO₃^- (1.04, 95% CI: 1.03-1.04, IQR = 11.25 μg/m³), followed by OM (1.03, 1.03-1.04, IQR = 7.97 μg/m³), NH₄⁺ (1.03, 1.02-1.04, IQR = 6.66 μg/m³), BC (1.03, 1.02-1.03, IQR = 1.41 μg/m³), and SO₄^2- (1.03, 1.02-1.03, IQR = 6.67 μg/m³). Overall, higher risks of stroke mortality were identified in analyses using more rigorous thresholds and lengthened durations of ETEs definitions, ranging from 1.19 (1.17-1.21) to 1.55 (1.51-1.60) for heat wave, and 1.03 (1.02-1.05) to 1.11 (1.08-1.15) for cold spell, respectively. We observed consistent evidence for the synergistic effects of heat wave and PM_2.5 constituents on both ischemic and hemorrhagic stroke mortality, where compound exposures to heat wave and secondary inorganic aerosols (i.e., NO₃^-, SO₄^2-, and NH₄⁺) posed greater increases in risk (0.23< REOI <0.81, 0.16< AP <0.39, and 2.63< SI <8.19).

Short-term exposure to both PM_2.5 constituents and ETEs were associated with heightened stroke mortality, and heat wave may interact synergistically with PM_2.5 constituents to trigger stroke deaths.