Smart meter-derived air conditioning penetration rate modifies the association between ambient temperature and emergency department visits.

Background: Air conditioning data are typically available only at coarse spatial resolutions. We examined modifications of heat-morbidity relationships using finer spatial resolution – specifically, ZIP code-level air conditioning penetration rates derived from household-level smart meter electricity records in southern California (2015-2019). Methods: We first estimated ZIP code-specific relative risk (RR) between heat-related emergency department visits and 4-day lagged temperatures at the 95 th to 50 th temperature quantile. Meta-regression was used to estimate effect modification, as measured by the ratio of RR. Results: Air conditioning penetration rates ranged from 23% to 96% (median=79%). Among non-coastal regions, a 10% increase in air conditioning penetration was associated with lower heat and emergency department visit risk: ratio of RR=0.79 (95% CI: 0.68, 0.91) for dry bulb temperature and 0.80 (95% CI 0.70, 0.92) for wet bulb temperature. Air conditioning penetration did not modify heat-morbidity associations in coastal regions. Conclusion: Our results indicate that neighborhoods with higher air conditioning penetration rates were associated with lower heat-related health risks in non-coastal regions.