Background Intraseasonal variation in acute health effects of extreme heat remains insufficiently investigated. Emergency ambulance calls (EACs) may offer timely insights into the population's health during such extreme heat events.

Objective To analyze intraseasonal variation in the association between extreme heat and hourly EACs during summer in Dezhou City, Shandong Province, China.

Methods We collected data on all-cause hourly EACs in Dezhou City from 2021 to 2022 and assigned hourly temperature and humidity data (with a spatial resolution of 0.0625° × 0.0625°) to call addresses. Summer in this study was defined as from June to September each year, with June to July considered as early summer and August to September as late summer. Extreme heat was defined as the 99th percentile of the temperature range during the summer. We employed a time-stratified case-crossover design using conditional logistic regression integrating distributed-lag nonlinear models to compare the association between extreme heat and the risk of hourly EACs in both early and late summer periods.

Results A total of 80389 EACs were recorded in Dezhou City during the study period. The analysis revealed a U-shaped association between hourly ambient temperature and EACs during summer, with the most significant effect observed at lag 0-30 h. Using the optimal temperature of 20.0°C as a reference, the cumulative odds ratio (OR) (lag 0-120 h) for extreme heat was 1.55 (95%CI: 1.40, 1.71) throughout summer. The cumulative effect of extreme heat was higher in late summer (OR=2.38, 95%CI: 1.91, 2.97) than in early summer (OR=1.37, 95%CI: 1.22, 1.54) (P<0.0001). Additionally, individuals aged 60 years and above had a higher risk throughout summer (OR=1.98, 95%CI: 1.70, 2.30) compared to those under 60 years (OR=1.23, 95%CI: 1.06, 1.42) (P<0.0001).

Conclusion Intraseasonal variation is observed in the strength of association between extreme heat and hourly EACs during summer in Dezhou City. The higher risk observed in late summer than in early summer indicates that repeated exposures to heat may escalate health risks, and older adults are more vulnerable.