Effect modification of greenness on the associations between air pollutants and out-of-hospital cardiac arrest: Evidence from a multi-city individual-level case-crossover study in China

Objectives The aim of this study was to examine the effect modification by greenness on the associations between short-term exposure to air pollutants and the risk of out-of-hospital cardiac arrest (OHCA). Study design An individual-level time-stratified case-crossover study. Methods We conducted a multi-city individual-level time-stratified case-crossover study including 62,915 OHCA cases in China, from 2020 to 2023. Short-term exposures to air pollutants were assigned based on the onset date and location for each case. Greenness (normalized difference vegetation index, NDVI, and enhanced vegetation index, EVI) was evaluated at case locations using annual maximum values. Effect modification by greenness was assessed using conditional logistic regression with linear interaction terms between air pollutants and greenness. Results Short-term exposure to air pollutants exhibited significant associations with OHCA, and greenness modified these associations. For 0.1-unit increase in NDVI 250m , the percent changes in the odds of OHCA per 10 μg/m 3 increase in pollutants were −1.62% (95% CI: −2.84% to −0.39%, P interaction = 0.010) for PM 2.5 , −1.13% (95% CI: −2.01% to −0.24%, P interaction = 0.013) for PM 10 , and -0.74% (95% CI: −1.34% to −0.13%, P interaction = 0.018) for O 3 . For 0.1-unit increase in EVI 250m , the corresponding percent change was −2.47% (95% CI: −4.77% to −0.11%, P interaction = 0.040) per 0.1 mg/m 3 increase in CO. These interactions were consistently reproduced for PM 2.5 and PM 10 at coarser resolutions (500-m and 1000-m) and across alternative lag periods. Subgroup analyses observed significant greenness modifying effects among older (≥65 years) and cardiac etiology cases, and no significant heterogeneity was detected across age, etiology or any other examined subgroups. Conclusion Short-term exposure to air pollutants was associated with OHCA, and greenness modified the associations of OHCA with specific air pollutants, including PM 2.5 , PM 10 , O 3 , and CO. These findings highlight green space as an important contextual factor for understanding population susceptibility to ambient air pollutants, emphasizing the need to consider green infrastructure when assessing or addressing air pollution-related health impacts.