The Relationship Between Extreme Heat and Heat-Related Illness Hospital Visits in Maricopa County, Arizona
Eric Wang
03/09/2026
Extreme heat events have become more frequent and more intense around the world. In Arizona in 2025, the total number of heat-related illness emergency department visits was 4770 individuals, and 977 individuals died from heat-related illness. Previous studies on heat, hospitalizations, and deaths consistently found that excess heat increased the risk of heat-related hospitalization. In this study, I assess whether the relationship between higher temperatures and heat-related illness hospitalizations is linear or if it increases more rapidly with more extreme heat. I obtained daily temperature and heat-related hospital visit data from the Maricopa County Heat Reports between 2020 and 2026. I fit quadratic and linear time series regressions, with the daily percentage of heat-related illness hospital visits among total hospital visits as the outcome and the daily maximum temperature as the covariate. I observed a statistically significant quadratic relationship between maximum temperature and heat-related illness hospital visits. At 110 degrees Fahrenheit (F), the quadratic time series regression predicts that 0.588% of total hospital visits will be for heat-related illnesses (95% confidence interval [CI]: 0.580% to 0.597%), and at 119 degrees F, it predicts that 1.209% of total hospital visits will be for heat-related illnesses (95% CI: 1.184% to 1.233%. The issue of extreme heat will persist due to global warming. Thus, we need to take informed actions to mitigate the negative health effects of exposure to extreme heat.