Association of surrounding greenness with preterm birth and the potential mediating role of ambient air pollution

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Abstract
Growing evidence has linked environmental exposures to preterm birth (PTB), yet the interplay between greenness, air pollution, and PTB remains understudied in middle-income settings. This study aimed to evaluate the association between surrounding greenness and the odds of PTB and to examine whether ambient air pollution mediates this association. We analyzed a national retrospective cohort of 3.98 million singleton live births from 31 provinces of Iran between 2013 and 2018. Prenatal exposure to surrounding greenness was characterized by normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI) within 500–3000 m buffers around geocoded delivery hospital locations. Multivariable logistic regression models were used to assess the associations of greenness with overall PTB and its subtypes, adjusting for demographic, reproductive, and environmental covariates. Causal mediation analysis was applied to quantify mediation by fine particulate matter (PM2.5) and nitrogen dioxide (NO2), and analyses were stratified by maternal age, education, and residential area. Among 3,976,387 singleton live births included in the analysis, 256,612 (6.45%) were PTB. Higher prenatal greenness, assessed by NDVI and EVI across buffers, was consistently associated with lower odds of PTB and its subtypes. Compared with the lowest NDVI-3000 m tertile, the highest tertile was associated with approximately 14–26% lower odds of overall PTB, moderate PTB, and very PTB. Mediation analyses indicated that PM2.5 and NO2 together explained about 15–25% of the greenness–PTB association, with smaller mediated proportions for PTB subtypes. Findings were broadly consistent when using EVI-based metrics, and mediation effects of PM2.5 and NO2 were more pronounced among rural-dwelling mothers. Greenness exposure was associated with lower odds of PTB, and this association was partially mediated by reduced ambient PM2.5 and NO2 pollution. These results underscored the importance of green space as a modifiable environmental determinant to mitigate PTB burden in middle-income countries.

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