Heat alerts and emergency department utilization: evidence from San Diego County, California

Background Extreme heat poses a significant risk to human health, underscoring the importance of early warning systems in mitigating such impacts. However, evidence regarding their effectiveness remains limited, particularly for emergency department (ED) visits. This study aimed to assess the associations between heat advisory and warning issuance and performance and ED visits in San Diego County, California, and to further examine differences by cause and demographic characteristics. Methods Daily adult ED visit data for the warm season (May–October) were linked to historical heat alerts and high-resolution gridded temperature data. ED visits were modeled using Poisson regression with ZIP code fixed effects, controlling for temperature, seasonality, and temporal factors. Main analyses evaluated associations between alert issuance and ED visits across 2012–2023, while secondary analyses examined differences by alert performance, classified as true positive, false positive, false negative, or true negative based on temperature-based criteria in effect during 2012–2016. Results Heat advisories were associated with a 2.1% increase in all-cause ED visits (95% CI: 0.4, 3.9), as well as larger increases in neurological (13.5%; 95% CI: 1.3, 27.1) and heat-related visits (21.8%; 95% CI: 1.6, 46.1), while warnings were exclusively associated with increased heat-related ED visits (35.8%; 95% CI: 3.5, 78.3). In subgroup analyses, associations with all-cause ED visits were more pronounced among females and older adults, with warnings associated with a 12.7% increase among those aged 65 years and older (95% CI: 6.8, 18.8). During 2012–2016, heat alerts exhibited high specificity but limited sensitivity, and their performance had minimal overall influence on ED visits. Results were robust across alternative model specifications. Conclusion Heat advisories and warnings were associated with limited changes in ED utilization, primarily confined to heat-related conditions and to females and older adults.