IntroductionIn the context of climate change, the increasing frequency of extreme temperature events and air pollution incidents pose significant threats to the sustainability of rapidly expanding urban areas1. Extreme heat events are becoming a common summer occurrence, exposing urban populations worldwide unevenly and leading to increased health risks and significant economic losses2,3,4,5. High-temperature environments are often closely associated with severe air pollution. Ozone has emerged as the primary pollutant in urban areas during the summer, with 12% of the population in 261 cities worldwide exposed to high levels of ozone pollution in 2020 (80–160 µg/m3), particularly in China and India6. The proportion of urban areas in the global ozone-attributable mortality burden continues to rise7.Extreme heat events and ozone pollution are frequently simultaneous within environments characterized by intense solar radiation, reduced wind speeds, and low precipitation levels8,9,10. The increased severity and frequent exposure to high temperatures and ozone pollution present significant health risks to the global population, especially as their combined effects are more harmful than individual events, disproportionately affecting vulnerable populations such as the elderly, young children, and low-income groups11,12,13,14,15,16. Research indicates that elevated environmental temperatures can alter an individual’s physiological response and sensitivity to ozone pollution, with co-exposure to high temperatures and ozone levels potentially resulting in synergistic effects17,18. Schnell and Prather19 systematically demonstrated that climate change contributes to the simultaneous occurrence of particulates, ozone, and extreme temperatures in eastern North America, where extreme events often cluster in time and space, presenting the highest pollution levels and hottest temperatures, thus posing significant potential health risks. In several regions of Europe, it has been observed that the daily excess mortality rates associated with compound heat-ozone events are 1.8 to 3.5 times higher than those resulting from individual heatwaves or ozone pollution events20. A study of 250 counties in China showed that exposure to concurrent heatwaves and high O3 pollution was associated with higher all-cause mortality risk, with annual average excess deaths attributed to concurrent events reaching 6,249 from 2017 to 2020, 5.7 times higher than from 2013 to 201615.The concurrent occurrence of extreme heat and ozone pollution constitutes one of the most influential compound climate events worldwide19,21. Considering the spatial distribution changes of extremely high temperature and ozone pollution events and their adverse health impacts on residents, it is essential to investigate the risks and disparities associated with exposure to these compound events. This analysis facilitates the precise identification of vulnerable areas and delivers specific information for shaping future public health policies that aim to mitigate and adapt to the risks posed by climate change. Research on high temperatures and ozone pollution in China from 2013 to 2020 has found that the simultaneous occurrence of these extreme events is intensifying across the country22. Particularly, the North China Plain has become the region most severely threatened by the compound extreme events of high temperatures and ozone pollution in China23. In the future, climate change is expected to exacerbate global exposure to compound extreme events of heatwaves and ozone pollution, with low-income countries facing the most adverse effects24. However, the patterns of compound exposure to urban heatwaves and high ozone pollution remain to be elucidated, especially in densely populated East Asia, and previous studies have lacked quantitative analysis of the spatiotemporal changes in the compound exposure of these two events.Here, we conducted high-resolution longitudinal estimates of the urban population in China exposed to compound extreme heat-ozone conditions from the summer of 2003 to 2020. To achieve this, we coordinated the estimates of daily maximum temperature and ozone pollution at a high resolution (1 km spatial resolution) in China with annual urban area and population data. For each pixel, extreme temperature and ozone pollution days were defined using absolute thresholds, and the number of days of compound events during the summer was calculated, as well as the exposure, annual compound event days multiplied by the total population. Subsequently, we estimated the exposure levels and annual growth rates nationwide and at the city level from 2003 to 2020. At the city level, we quantified the contributions of compound events and population growth to exposure to better understand the spatial and temporal patterns of differences in exposure to compound events. Meanwhile, we also compared the changes in exposure differences across urban areas.ResultsTemporal trend of compound events exposureChina extreme heat and ozone pollution compound event days increased by 180%, from 489 thousand days in 2003 to 1.37 million days in 2020, and an annual increase of 61 thousand days yr−1 (Fig. 1a). The urban population exposure to extreme heat and ozone pollution increased by 67% in 18 years, from 3.8 billion person-days in 2003 to 6.3 billion person-days in 2020, growing by 0.2 billion person-days yr−1 (Fig. 1d). Both the compound events exposure and days showed a significant growth trend (P 35 °C) and high concentration ozone (>100 µg/m³), b extreme heat event days, c high concentration ozone event days, d population exposure to compound heat and ozone, e population exposure to extreme heat, and f population exposure to high concentration ozone.Full size imageSpatial feature of compound event exposureWe calculated total compound exposure for each city (SI Appendix, Table S1) and estimated the annual increase in exposure from 2003 to 2020 (SI Appendix, Table S2). From 2003 to 2020, the number of days and exposure to compound events of high temperatures and ozone pollution in Chinese cities increased rapidly, with the growth trend exhibiting significant spatial heterogeneity (Fig. 2). The number of compound event days and exposure in Chinese cities in 2020 significantly increased compared to 2003, especially in eastern cities. Cities in the North China Plain, the Yangtze River Basin, and the southeastern coast have shown marked increases in compound event days and exposure levels. In 2020, these regions comprised most of the top 30 cities with the highest compound exposure and fastest exposure growth, indicating strong spatial clustering. In 2020, the 15 cities with populations over 5 million collectively added over 500 days of extreme heat-ozone events annually, with Chongqing and Chengdu increasing by nearly 3000 days. At the same time, just the top 15 cities contributed 50% of the national urban annual rate increase in compound event exposure. From 2003 to 2020, 38% (141) of cities in China had statistically significant exposure trajectories (P