Objective To investigate the impact of heat wave exposure on 120 emergency visits in Hefei, China, to identify susceptible populations and lag patterns of effects, and to provide evidence for formulating health protection strategies against heat waves and optimizing pre-hospital emergency medical resource allocation.

Methods Daily meteorological, air pollutant, and ambulance call data were collected for Hefei from 2015 to 2024. Twelve heat wave definitions were constructed by combining temperature thresholds with duration. A quasi-Poisson regression model combined with a distributed lag non-linear model was employed to examine the effects of heat waves on 120 emergency visits. The optimal heat wave definition was selected based on the quasi-Akaike information criterion. For ambulance calls, stratified analyses were conducted by sex, age, and cause of call.

Results A total of 972 155 ambulance calls were recorded in Hefei from 2015 to 2024, of which 56.26% were for males, 65.43% were for individuals aged 15-64 years, and 36.51% were for accidental injuries. Compared with non-heat wave days, all heat wave types were significantly associated with an increased risk of emergency visits, with effects diminishing as lag time increased. At the same duration, the cumulative risk of emergency visits exhibited a fluctuating pattern of an initial decrease followed by an increase with rising temperature thresholds. For example, at a duration of 2 days, the cumulative relative risk (RR) (95% confidence interval CI) decreased from 1.091 (1.066, 1.117) at the 90th percentile threshold to 1.088 (1.058, 1.118) at the 95th percentile, then rose to 1.092 (1.049, 1.136) at the 97.5th percentile. At the same temperature threshold, the effect declined with extended duration. Taking the 90th percentile threshold as an example, the cumulative RRs for durations of 2, 3, and 4 days were 1.091 (95%CI: 1.066-1.117), 1.088 (95%CI: 1.063-1.113), and 1.079 (95%CI: 1.054-1.104), respectively. Stratified analyses revealed no sex differences in heat wave-related emergency visits risk. Among age groups, individuals aged ≥65 years were more susceptible to heat waves, with a RR of 1.159 (95%CI: 1.107-1.213). Cause-specific analyses showed that the risk was most pronounced for circulatory diseases (RR=1.133, 95%CI: 1.066-1.205), whereas the risks for respiratory diseases (RR=1.063, 95%CI: 0.969-1.165) and accidental injuries (RR=1.028, 95%CI: 0.984-1.074) did not reach statistical significance. Between-group comparisons with circulatory diseases as the reference indicated that the risk of heat wave-related emergency visits for circulatory diseases was significantly higher than that for accidental injuries (P < 0.05), but not significantly different from that for respiratory diseases.

Conclusion Heat waves in Hefei significantly increase residents' risk of 120 emergency visits, with the elderly and individuals with circulatory diseases being more vulnerable. Targeted protective measures for these susceptible populations should be strengthened.