The causal links between ambient heat events and all-cause emergency department visits across Chinese mainland

While ambient heat events were an established driver of acute health effects and increased emergency department (ED) visits worldwide, the broader population-level ramifications of recurrent heatwaves—especially their regional and demographic variations—were still poorly characterized, thereby hindering the development of equitable adaptation strategies. Based on data from the National Health Commission (NHC) of China and the National Environmental and Meteorological Database during 2010–2022, we conducted a nationwide causal inference analysis of ambient heat events and all-cause ED visits across 3085 county-level units, including counties and districts, within 31 provincial-level administrative regions in China. Moreover, we also assessed the dose–response relationship and examined effect modification by demographic, socioeconomic, and other environmental factors. We observed that heatwave occurrence, as well as the number and duration of heatwaves were each positively associated with the number of annual ED visits, with relative risks (RRs) of 1.071 (95% CI: 1.050, 1.091), 1.060 (1.049, 1.071), and 1.020 (1.017, 1.024), respectively. A clear dose–response relationship was identified with counties experiencing ≥4 heatwaves per year having a 28.2% (18.0%, 39.6%) greater rate of ED visits compared with those with no heatwaves. Moreover, females and those in low-economic regions experienced a 9.6% (6.4%, 12.9%) and 11.7% (8.7%, 14.7%) higher risk than their counterparts. The effects were more pronounced in regions with higher PM 2.5 concentrations and lower green coverage, particularly across Central, South, and East China. These findings supported heat–health warning systems and ED capacity planning, with adaptation resources prioritized for these high-risk regions and communities.