Aerosol pH Associated with Preterm Birth Risk: Unmasking the Hidden Role of Atmospheric Acidity in Maternal-Fetal Health.

Preterm birth (PTB) remains a global health challenge, with atmospheric particulate matter (PM) exposure implicated as a modifiable risk factor. As major chemical components of PM, sulfate, nitrate, and ammonium (SNA) have been linked to PTB. However, discrepancies between epidemiological and toxicological evidence hinder understanding of SNA's health effects, therefore requiring a potential modulating factor to explain its biological impact. In this study, we propose aerosol acidity as a key modulator of PM bioactivity. In a Beijing birth cohort (2,520 singleton pregnancies, November 1, 2022, and October 31, 2023), we assessed trimester-specific exposure to SNA and aerosol acidity. In addition, distributed lag models (DLMs) were applied to identify critical exposure windows and estimate odds ratios (ORs) for PTB risk. Our findings demonstrated significant trimester-specific effects of aerosol acidity on PTB risk. A 1-unit decrease in aerosol pH during the second-trimester exposure significantly increased PTB risk (adjusted OR = 1.54, 95% CI: 1.02-2.07). Weekly-specific analysis identified the 21st to 27th gestational weeks as the most susceptible window for aerosol acidity exposure, with elevated PTB risk observed in the late second trimester. These identified critical windows coincided with peak placental vascular remodeling. By bridging epidemiological-toxicological discrepancies, our findings establish aerosol acidity as a novel, actionable target for PTB prevention. We advocate integrating aerosol acidity metrics into air quality assessment and prioritizing SNA emission controls-the dominant acidity drivers in northern China-to protect vulnerable pregnancy windows.