Characterizing Heat Health Outcomes and Informing Public Health Interventions with Medical Claims Data in California

Heat is now the deadliest weather hazard in the United States. California faces this hazard in two forms, acute heat waves affecting millions and chronic exposure, which affects its large agricultural workforce. The state directs climate resilience resources using tools such as composite vulnerability indices and competitive grant programs. Whether these instruments reach the communities actually experiencing heat-related illness has not been systematically tested against health outcomes. We map heat-related Medicaid claims across California ZIP codes from 2011 to 2019 to evaluate who bears the health burden of heat, how well existing vulnerability indices capture it, and whether state grant funding reaches the highest-burden communities. We show that ZIP codes with the highest claim rates have lower median incomes, more farmworkers, and more mobile homes than the state average. Heat-related claim rates rise 24.4% per 1 °C in majority-cropland ZIP codes, compared with 20.6% per 1 °C in majority built-up areas. Of three vulnerability indices tested, only the CDC heat and health index, which itself incorporates emergency room data, correlates strongly with observed claim rates. Our analysis suggests that State Extreme Heat and Community Resilience Program funding broadly tracks county-level claim counts, but several high-burden counties, including Kern, Fresno, and Imperial, are substantially underfunded. We conclude that using medical claims data in conjunction with indices could lead to a more effective allocation of funding to communities experiencing heat risk in California than considering indices alone.