Association between a radical urbanization index and lung disease risk in China: longitudinal evidence from the CHARLS

BACKGROUND: Rapid urbanization has been a defining feature of China's development, yet its implications for respiratory health and the role of environmental conditions remain incompletely understood. We examined the association between urbanization intensity, measured by a radical urbanization index reflecting accelerated land expansion, population concentration, and economic growth, and the risk of chronic lung disease among Chinese adults. METHODS: We analyzed longitudinal data from 7,405 participants in the China Health and Retirement Longitudinal Study from 2011 to 2020. Incident chronic lung disease was identified based on self-reported physician diagnoses during follow-up. Urbanization exposure was assessed at baseline and linked to residential location. Kaplan-Meier methods and log-rank tests were used to compare cumulative incidence across urbanization quartiles. Cox proportional hazards models and restricted cubic splines were applied to estimate hazard ratios and dose-response relationships, with sequential adjustment for demographic characteristics, health-related factors, and baseline environmental indicators. Mediation analyses were conducted to explore whether selected environmental factors may statistically mediate the association between urbanization and lung disease using a two-model framework for time-to-event outcomes. RESULTS: During follow-up, 1,318 participants developed incident chronic lung disease. Cumulative incidence differed significantly across urbanization quartiles. Higher urbanization intensity was associated with an increased risk of lung disease in crude and partially adjusted models, but the association was attenuated and no longer statistically significant after additional adjustment for environmental indicators. Mediation analyses suggested that several air pollutants showed statistically significant indirect effects linking urbanization to lung disease. Temperature, dryness, and nitrous oxide exhibited indirect effects opposite to the total association, suggesting inconsistent mediation, whereas other environmental indicators did not show evidence of mediation. CONCLUSIONS: Higher urbanization intensity was associated with a higher risk of chronic lung disease in crude and partially adjusted analyses, although this association was attenuated after accounting for multiple environmental indicators. The findings suggest that environmental conditions may represent potential pathways linking urbanization patterns to respiratory health. These results should be interpreted cautiously given the observational study design.