Spatial Heterogeneity in Synergistic Effects of Extreme Heat and NO 2 Exposures on Cardiorespiratory Hospitalizations in California.

Abstract

Background: Extreme heat and high air pollution levels may co-occur and act synergistically. The synergistic health effects of extreme heat and nitrogen dioxide (NO 2 ) , and the fine-scale spatial heterogeneity in such joint effects, remain unclear. We aimed to investigate the synergistic effect of extreme heat and NO 2 exposures on cardiorespiratory hospitalizations in California and explore its spatial heterogeneity.

Methods: Daily ZIP Code Tabulation Area (ZCTA)-level hospitalization data, 2000-2019, were obtained from the California Department of Health Care Access and Information. Extreme heat and NO 2 exposures were defined as days when the heat index and NO 2 concentration exceeded a ZCTA-specific threshold, respectively. We first investigated the state-level synergistic effects with a case-crossover analysis and then estimated the ZCTA-specific synergistic effects using a within-community matched design combined with a spatial Bayesian hierarchical model. Finally, we explored how community characteristics modify these effects using meta-regressions.

Results: We found mild synergistic effects of extreme heat and NO 2 exposures on cardiorespiratory hospitalizations at the state level, with a relative excess risk due to interaction (RERI) of 0.005 (95% confidence interval = -0.002, 0.011). Yet, great spatial heterogeneity was observed, with ZCTA-specific RERIs ranging from -1.08 to 2.22. Higher RERIs were observed in communities with lower socioeconomic status, higher population density, reduced green space, greater proportions of racial and ethnic minority residents, and higher historical temperatures.

Conclusions: Our findings underscore the need for adaptation policies that integrate compound exposures to heat and air pollution and inform the development of targeted intervention strategies to protect vulnerable communities.