Regional sensitivity to ambient heat and hyperthermia-related emergency department visits in the continental United States

Background: Predictions of more frequent, more intense, and longer-lasting heat waves for most of the United States produce an urgent need for public health plans to reduce heat-related mortality and morbidity. In order to be effective, these plans must be based upon a thorough understanding of all factors affecting risk. Objective: We characterized demographic and geographic variations in risk of hyperthermia-related emergency department (ED) visits across climate regions within the United States. Methods: We extracted 11,270 hyperthermia-related ED visits from Truven Health MarketScan® Research dataset for 141 metropolitan statistical areas (MSAs) for years 2000-2010. We constructed different measures of ambient heat using hourly and daily meteorological observations from the National Climatic Data Center. We then merged the health and meteorological data by day and MSA. We used a time-stratified case crossover approach to estimate the risk of hyperthermia-ED visit for these different temperature metrics. We estimated separate risks for each MSA and used random effects meta-analysis to combine those risks by latitude categories and U.S. climate regions. Results: For each of the three different measures that we examined (maximum temperature, alternate heat index, SSC), we find that exposure to heat on the day are the strongest predictor of ED visit. We find evidence that an extended period of heat wave increased the risk of an ED visit above same-day heat exposure. We also find that daily air pollution measures, ozone and PM2.5 levels did not appear to increase the risk for a hyperthermia-related ED visit. From a random effects meta-analysis on city-specific risk estimates for hyperthermia-related ED visits, we estimate higher risks in the Central and the North Central parts of the country compared to the South and Southwest. Conclusions: These results suggest that effective heat alerts must be timely, vary by climate region, and include triggers for unusual heat days and periods of extended heat.