Wildfire smoke exposure during pregnancy linked to higher risk of preterm birth

About 10% of American babies are born prematurely. Birth before 37 weeks can lead to a cascade of health risks, both immediate and long-term, making prevention a vital tool for improving public health over generations.

In recent years, researchers have identified a potential link between wildfire smoke—one of the fastest-growing sources of air pollution in the United States—and preterm birth, but no study has been big or broad enough to draw definitive conclusions. A new study led by the University of Washington makes an important contribution, analyzing data from more than 20,000 births to find that pregnant people who are exposed to wildfire smoke are more likely to give birth prematurely.

“Preventing preterm birth really pays off with lasting benefits for future health,” said lead author Allison Sherris, a UW postdoctoral researcher in environmental and occupational health sciences. “It’s also something of a mystery. We don’t always understand why babies are born preterm, but we know that air pollution contributes to preterm births, and it makes sense that wildfire smoke would as well. This study underscores that wildfire smoke is inseparable from maternal and infant health.”

In the study, published in The Lancet Planetary Health, researchers used data from the Environmental influences on Child Health Outcomes (ECHO) program, a federal research project focused on how a wide range of environmental factors affect children’s health. The sample included 20,034 births from 2006–2020 across the contiguous United States.

Researchers estimated participants’ average daily exposure to fine particulate matter (PM_2.5) generated by wildfire smoke, and the total number of days they were exposed to any amount of smoke. They estimated the intensity of smoke exposure by how frequently participants were exposed to wildfire PM_2.5 levels above certain thresholds.

They found that pregnant people exposed to more intense wildfire smoke were more likely to give birth prematurely. In mid-pregnancy, exposure to any smoke was associated with an elevated risk of preterm birth, with that risk peaking around the 21st week of gestation. In late pregnancy, elevated risk was most closely associated with exposure to high concentrations of wildfire PM_2.5, above 10 micrograms per cubic meter.

“The second trimester is a period of pregnancy with the richest and most intense growth of the placenta, which itself is such an important part of fetal health, growth and development,” said co-author Dr. Catherine Karr, a UW professor of environmental and occupational health sciences and of pediatrics in the UW School of Medicine. “So it may be that the wildfire smoke particles are really interfering with placental health. Some of them are so tiny that after inhalation they can actually get into the bloodstream and get delivered directly into the placenta or fetus.”

The link was strongest and most precise in the Western U.S., where people were exposed to the highest concentrations of wildfire, PM_2.5, and the greatest number of high-intensity smoke days. Here, the odds of preterm birth increased with each additional microgram per cubic meter of average wildfire PM_2.5.

It’s possible those results were more precise simply because the West experiences more wildfire smoke on average, making the exposure model perform better, Sherris said. But there may be other factors behind the regional differences.

The composition of wildfire smoke is different across the country. In the West, smoke tends to come from fires nearby, while in places like the Midwest, smoke has typically drifted in from faraway fires. Smoke’s toxicity changes as it ages and reacts with sunlight and airborne chemicals, which could have affected the results. Researchers also noted that external factors like co-occurring heat or housing quality may have effects that aren’t fully understood.

Researchers hope that future studies will examine the exact mechanisms by which wildfire smoke might trigger preterm birth. But in the meantime, Sherris said, evidence for a link is now strong enough to take action.

“There are a couple of avenues for change,” Sherris said. “First, people already get a lot of public health messaging and information throughout pregnancy, so there’s an opportunity to work with clinicians to provide tools for pregnant people to protect themselves during smoke events. Public health agencies’ messaging about wildfire smoke could also be tailored to pregnant people and highlight them as a vulnerable group.”