1 School of Architecture, Southeast University, 2 Sipailou, Nanjing, 210096, China; Jiangsu Province Engineering Research Center of Urban Heat and Pollution Control, Southeast University, 2 Sipailou, Nanjing, 210096, China.
2 School of Architecture, Southeast University, 2 Sipailou, Nanjing, 210096, China; Energy and Environment Group, Department of Building, Civil and Environmental Engineering, Concordia University, Montreal, H3G 1M8, Canada.
3 School of Architecture, Southeast University, 2 Sipailou, Nanjing, 210096, China; Global Centre for Clean Air Research, Department of Civil and Environmental Engineering, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford Surrey, GU2 7XH, United Kingdom; Institute for Sustainability, University of Surrey, Guildford Surrey, GU2 7XH, United Kingdom.
4 School of Architecture, Southeast University, 2 Sipailou, Nanjing, 210096, China; Jiangsu Province Engineering Research Center of Urban Heat and Pollution Control, Southeast University, 2 Sipailou, Nanjing, 210096, China. Electronic address: shijie_cao@seu.edu.cn.

Climate change is reshaping indoor-outdoor pollution dynamics, yet current building standards rarely reflect this shift. In rapidly urbanizing regions with high ambient PM_2.5 levels, building airtightness offers an underutilized strategy to mitigate indoor exposure and associated health burdens. An analysis of 36 major Chinese cities across five climate zones shows that improving building airtightness lowers indoor PM_2.5 and reduces COPD-related mortality. We identify an airtightness effectiveness transition zone that broadly aligns with the summer monsoon boundary, where weakened summer pressure gradients can offset up to half of the expected infiltration gains. At best practice levels, indoor PM_2.5 declines by 10.6 % and COPD deaths by 6.2 %, with the largest benefits in cold regions. Western cities display disproportionately high COPD mortality despite lower exposures, highlighting structural inequities in vulnerability. These results support climate-specific airtightness targets paired with ventilation and filtration, retrofit prioritisation for leaky and vulnerable housing, and the use of joint energy health metrics to guide funding, offering a potentially scalable pathway to healthier and more energy-efficient buildings in rapidly urbanizing regions.