Combined effects of heatwave and ozone on specific causes of emergency ambulance dispatches in Nanjing, China

Elevated ambient temperatures and ozone (O 3 ) pollution are each independently associated with substantial public health risks. Nevertheless, their combined effects particularly the interaction patterns in relation to emergency ambulance dispatches, remain inadequately explored. This study aimed to investigate the association between emergency ambulance dispatches (EADs) and compound heatwave-ozone events under various definitions, and to quantify interaction effects between heatwave and O 3 . We performed a time series analysis using 444,517 non-accidental EADs records in 11 counties in Nanjing, China (2018–2022). Daily air pollution and meteorological grid data (spatial resolution: 1 km×1 km) were used to estimate county-specific exposures. Heatwave was defined according to a combination of multiple temperature thresholds (90 and 95th percentiles) and duration (2–4 days). 30 compound events were defined based on six heatwave definitions, and five compound-event scenarios. A quasi-Poisson regression model with distributed lags was applied to estimate the association between the compound events and EADs. The interactive effects of heatwave and O 3 pollution events were assessed using the relative excess risk (RERI). Significant increases in all-cause EADs were observed when heatwave occurred within seven days after ozone pollution, the cumulative relative risks ranged from 1.09 (95% confidence interval [CI]: 1.03, 1.15) to 1.15 (95% CI: 1.03, 1.29). Notably, circulatory EADs increased significantly when a separate heatwave event (defined as the 95th percentile) followed an O 3 pollution event within three days, the highest cumulative relative risk was 1.10 (95% CI: 1.01, 1.21). In contrast, respiratory EADs showed a stronger response when heatwave event (defined as the 90th percentile threshold) followed an ozone pollution event within seven days, the highest cumulative relative risk was 1.53 (95% CI: 1.24, 1.89). Moreover, female individuals showed higher susceptibility to the compound events. Cumulative relative risk varied substantially across age groups under different scenarios. The study presents a refined sequential compound-event framework for assessing the acute health risks based on EAD records and underscores the need for targeted public health interventions to mitigate risks under climate change.