BACKGROUND: Recent record-breaking hot temperatures in Alaska have raised concerns about the potential human health implications of heat expo-sure among this unacclimated population.OBJECTIVES: We estimated cardiorespiratory morbidity associated with days above summer (June-August) heat index (HI, apparent temperature) thresholds in three major population centers (Anchorage, Fairbanks, and the Matanuska-Susitna Valley) for the years 2015-2019.METHODS: We implemented time-stratified case-crossover analyses of emergency department (ED) visits for International Classification of Diseases, 10th Revision codes indicative of heat illness and major cardiorespiratory diagnostic codes using data from the Alaska Health Facilities Data Reporting Program. Using conditional logistic regression models, we tested maximum hourly HI temperature thresholds between 21.1 & DEG;C (70 & DEG;F) and 30 & DEG;C (86 & DEG;F) for a single day, 2 consecutive days, and the absolute number of previous consecutive days above the threshold, adjusting for the daily average concentration of particulate matter & LE;2.5 lg.RESULTS: There were increased odds of ED visits for heat illness above a HI threshold as low as 21.1 & DEG;C (70 & DEG;F) [odds ratio (OR) =13.84; 95% confi-dence interval (CI): 4.05, 47.29], and this increased risk continued for up to 4 d (OR = 2.43; 95% CI: 1.15, 5.10). Asthma and pneumonia were the only respiratory outcomes positively associated with the HI: ED visits for both were highest the day after a heat event (Asthma: HI >27 & DEG;C (80 & DEG;F) OR = 1.18; 95% CI: 1.00, 1.39; Pneumonia: HI >28 & DEG;C (82 & DEG;F) OR = 1.40; 95% CI: 1.06, 1.84). There was a decreased odds of bronchitis-related ED visits when the HI was above thresholds of 21.1-28 & DEG;C (70-82 & DEG;F) across all lag days. We found stronger effects for ischemia and myocardial infarc-tion (MI) than for respiratory outcomes. Multiple days of warm weather were associated with an increased risk of health impacts. For each additional preceding day above a HI of 22 & DEG;C (72 & DEG;F), the odds of ED visits related to ischemia increased 6% (95% CI: 1%, 12%); for each additional preceding day above a HI of 21.1 & DEG;C (70 & DEG;F), the odds of ED visits related to MI increased 7% (95% CI: 1%, 14%).DISCUSSION: This study demonstrates the importance of planning for extreme heat events and developing local guidance for heat warnings, even in areas with historically mild summertime climates. https://doi.org/10.1289/EHP11363
