A new study led by the University of California, Irvine, found that exposure to fine particulate matter, 2.5 micrometers or less in diameter, known as PM_2.5 and related elements, such as black carbon and nitrates, during pregnancy significantly increase the odds of spontaneous preterm birth, a major threat to maternal and fetal health.

Corresponding author, Jun Wu, Ph.D., professor of environmental and occupational health at UC Irvine Joe C. Wen School of Population & Public Health, collaborated with researchers at Kaiser Permanente in Southern California and other institutions to publish their findings in JAMA Network Open.

"Though the causes of spontaneous preterm birth are complex and not fully understood, our study identifies air pollution as a contributing factor, highlighting the need for targeted interventions," said Wu. "Our research adds to extensive evidence that communities facing socioeconomic challenges, limited green space, and heightened environmental stressors, such as wildfire smoke and extreme heat, are particularly vulnerable to maternal health impacts."

This population-based cohort study analysed data from more than 400,000 singleton live births delivered between 2008 and 2018 within the Kaiser Permanente healthcare system in Southern California.

Researchers examined the associations between spontaneous preterm birth and exposure to PM_2.5 as well as five of its constituents: sulfate, nitrate, ammonium, organic matter, and black carbon. The analysis was further refined by considering other effect modifiers such as green space, wildfire-related exposure, and maximum daily temperature during pregnancy.

The study found that roughly 19,300 cases of spontaneous preterm birth (4.73% of total births) occurred among the study sample. Exposure to PM_2.5 and its constituents, particularly black carbon, nitrate, and sulfate, was significantly associated with an increased risk of spontaneous preterm birth, especially in the second trimester.

Every interquartile range increase in PM_2.5 exposure during pregnancy was associated with 15% higher odds of spontaneous preterm birth. Individuals impacted by social determinants of health like lower educational attainment or income, those living in areas with limited green space, or those exposed to more wildfire smoke or extreme heat were at significantly higher risk for spontaneous preterm birth associated with PM_2.5 exposure.

"Our research demonstrates a strong link between air pollution and spontaneous preterm birth, but more importantly, it underscores the disproportionate impact on vulnerable communities," said the first author, Anqi Jiao, a doctoral student in the Environmental Health Sciences program at Wen Public Health. "This evidence provides a clear directive for policymakers and public health officials to focus on reducing environmental risks for those who are most affected."

The study's findings point to the critical need for targeted public health interventions to address air quality, especially for pregnant individuals in lower-income or environmentally disadvantaged communities. By identifying key periods of vulnerability, such as the second trimester of pregnancy, and specific pollutants, this research offers actionable insights for reducing the incidence of preterm birth through environmental policy and health care practices.