Temporal trends in short-term PM _2.5 exposure and adverse birth outcomes in Japan: nationwide modeling study across four decades

Abstract Maternal exposure to PM 2.5 during pregnancy is associated with elevated risks of adverse birth outcomes, including preterm birth (PTB), low birth weight (LBW), and small-for-gestational-age (SGA). Nonetheless, comprehensive investigations into temporal trends in this association remain scarce, primarily because long-term datasets integrating detailed exposure information with extensive birth records have been lacking. This study investigated temporal trends in the associations between short-term PM 2.5 exposure and adverse birth outcomes (PTB, LBW, and SGA) in Japan over a 44 year period, and assessed subgroup-specific vulnerabilities. We analyzed nationwide vital statistics of singleton live births in Japan from 1980 to 2023, linked with daily municipality-level PM 2.5 estimates from the Modern-Era Retrospective Analysis for Research and Applications, Version 2. A two-stage analysis with 5 year stratification was conducted to estimate relative risks per 10 µ g m −3 increase in the 3 d moving average of PM 2.5 concentrations. In the first stage, municipality-specific associations were assessed using a time-stratified case-crossover design. In the second stage, municipality-specific estimates were pooled using a random-effects meta-analysis. Temporal trends of the association were assessed using linear trend analysis. Stratification analyses by subgroups were performed to explore potential vulnerabilities. The analysis included PTB ( n = 2143 950), LBW ( n = 3362 124), and SGA ( n = 3429 611) across 1747 municipalities. Short-term PM 2.5 exposure was associated with elevated risks of all outcomes in early study periods, but risks for LBW and SGA declined over time ( p -trend < 0.001 and p = 0.033, respectively). There was no evidence of temporal change for PTB. Subgroup analyses indicated stronger associations in winter for PTB and in spring for LBW and SGA, with higher risks observed in northern Japan. Over the past four decades, short-term PM 2.5 exposure-related LBW and SGA risks have decreased in Japan, whereas PTB risk has remained stable. The findings highlight the need for geographically and seasonally tailored interventions to efficiently protect vulnerable populations.