Risk of preterm birth associated with urban dry island effect and its interaction with temperature and ozone exposure in Chinese cities

<p indent="0mm">Recent studies have increasingly pointed to the potential risks that high temperature and humidity pose to pregnant women, particularly in increasing the likelihood of preterm birth. However, within urban areas, where humidity tends to be lower, the impact of the Urban Dry Island (UDI) effect on preterm birth remains underexplored. This study aims to investigate whether UDI exposure at the individual level offers any protective benefits against preterm birth and whether this protective effect persists even under conditions of high urban temperature and ozone exposure. This research is based on data from the China Birth Cohort Study (CBCS), encompassing 103040 birth records up until December 2020. By integrating residential address data with gridded relative humidity data, the study individualizes the assessment of UDI exposure. The UDI index, a measure of dryness in urban environments, was calculated for each participant based on their specific location. To examine the relationship between UDI exposure and the risk of preterm birth, the study employed a generalized linear mixed-effects model. This model allowed for the adjustment of potential confounding factors, including maternal characteristics such as age, pre-pregnancy BMI, education level, ethnicity, second-hand smoke exposure, air purifier use, and environmental factors such as proximity to major roads, noise disturbance, residential NDVI level, and average temperature during pregnancy. The findings of this study reveal a consistent and significant protective effect of UDI exposure against preterm birth within the 21-day period leading up to delivery. Specifically, for every quartile decrease in the UDI index lagged by one week, the risk of preterm birth decreased by 10% (OR=0.90, 95% CI: 0.85, 0.95). This suggests that lower humidity in urban areas may help reduce the likelihood of preterm birth. Additionally, interaction analyses indicated that UDI may partially mitigate the increased risk of preterm birth associated with high temperature and high ozone exposure. Further stratified analyses showed variations in the protective effect of UDI across different subgroups. Notably, the protective effect of UDI was weaker among pregnant women who were over <sc>30 years</sc> old, had lower socioeconomic status, or lived in areas with higher levels of greenery (as indicated by NDVI). These findings suggest that while the UDI effect may generally offer a protective benefit against preterm birth, certain populations within urban environments may be more vulnerable and receive less benefit from this effect. In conclusion, the Urban Dry Island effect appears to have a protective role against preterm birth, especially during the critical weeks leading up to delivery. However, the strength of this protective effect varies among different demographic groups. The weaker protection observed among older, socioeconomically disadvantaged, and greener neighborhood residents highlights these groups as potentially susceptible subpopulations within urban mothers. This suggests the importance of developing targeted interventions for these high-risk groups to better protect them against the adverse outcomes associated with urban environmental exposures during pregnancy. The study’s findings underscore the complex interplay between urban environmental factors and maternal health, providing valuable insights for public health strategies aimed at reducing preterm birth rates in urban settings.