Association between ambient air pollution a week prior to delivery and preterm birth using a nationwide study in Sweden.

Background: Air pollution exposure has been linked with increased risk of preterm birth, which is one of the leading causes of infant mortality. Limited studies have attempted to explore these associations in low-polluted areas. In this study, we aimed to assess the association between short-term exposure to ambient air pollution and preterm birth in Sweden.

Method: In this population-based study we included preterm births between 2014 and 2019 from the Swedish Pregnancy Register. We applied a spatiotemporal model to estimate daily levels of particulate matter <2.5 μm (PM), PM < 10 μm (PM), nitrogen dioxide (NO and ozone (O) at the residential address of each participant. We applied a time-stratified case-crossover design with conditional logistic regression analysis to estimate odds ratios (OR) of preterm birth per 10 μg/m (PM, NO, O) and 5 μg/m (PM) increase in air pollution exposure at 0-6-day lag. Two-pollutant models were applied to evaluate the independent association of each exposure on preterm birth. We also stratified by maternal characteristics to identify potential effect modifiers.

Results: 28,216 (4.5%) preterm births were included. An increase in O exposure was associated with increased odds of preterm birth [OR = 1.06 per 10 μg/m (95% CI, 1.02; 1.10]. PM and PM were not significantly associated with preterm birth, and NO displayed a negative nonlinear association with preterm birth. We did not observe any notable effect modification, but we found suggestive larger associations between O and preterm birth when stratifying by male sex, spontaneous delivery, and spring season.

Conclusions: Increased O exposure one week before delivery was associated with an increased risk of preterm birth in Sweden, a country with levels of air pollution below the current World Health Organization air quality guidelines. Increases in O levels with climate change make these findings especially concerning.