Wildfire smoke tied to higher risk of preterm birth

Breathing wildfire smoke while pregnant may raise the risk of preterm birth, especially in the western United States, according to research findings published this week in The Lancet journal.  

Researchers analyzed more than 20,000 births nationwide from 2006 to 2020 and found that exposure to wildfire-related fine-particle pollution during those pregnancies was linked to higher odds of preterm delivery in the West. 

“Wildfires are becoming more frequent and intense, and our findings suggest that smoke exposure during pregnancy may have real consequences for maternal and infant health,” said co-author Dr. Catherine Karr, a UW Medicine pediatrician.  

Karr was one of 42 authors nationwide who studied the smoke exposure statistics. Lead authors included Allison Sherris, a postdoctoral scholar in the Karr lab at the University of Washington’s Department of Environmental and Occupational Health Sciences.  

Researchers examined the intensity, frequency and timing of maternal exposure to fine-particulate matter (PM2.5) from wildfire smoke during pregnancy and assessed its association with preterm birth. Among their key findings: 

• For every small increase in average wildfire-related PM2.5 exposure during pregnancy, the risk of preterm birth rose by about 7%. Exposure to both high-intensity smoke days and longer periods of consecutive days with high-intensity smoke was tied to higher risk. 
• Timing mattered. Exposure in the second trimester exposure to low- and moderate-level smoke days, and late-pregnancy exposure to high-concentration smoke days, showed the strongest associations with preterm birth.  

These findings might suggest that the tiny smoke particles interfere with placenta development, Karr said.  

“The second trimester is a period in pregnancy which sees the richest and most intense growth of the placenta,” she said. “This is such an important part of fetal health and growth and development.” 

The particles are so tiny, she added, that they can be inhaled and make their way into a woman’s bloodstream and find their way into the placenta or fetus. 

Preterm birth, defined as delivery before 37 weeks of gestation, is a leading cause of infant illness and death, the authors noted. Identifying environmental risk factors such as wildfire smoke can help public health officials and clinicians protect vulnerable populations. 

“Preterm birth is a really important public health problem in the United States, and it's generally been increasing over time,” Karr said. “It affects about 10% of pregnancies in the U.S.” 

“I am concerned about preterm birth because children who are born preterm are more likely to have long lasting health consequences,” she said. This can range from negatively affect a child’s respiratory system and neurodevelopmental health “that may manifest as asthma, autism or learning disabilities. These conditions can carry lifelong consequences,” she added. 

The researchers emphasize the need for public health guidance tailored to pregnant individuals during wildfire season, including access to clean indoor air, early warnings about smoke events, and strategies to reduce exposure. 

“It’s helpful to be aware of the air quality index or AQI, which is a really handy way of knowing whether the air quality in your area is safe or you need to take precautions,” Karr said.  

The U.S. Environmental Protection Agency publishes current measures of outdoor air quality on AirNow.gov, which has a corresponding smartphone app. Pregnant people should try to stay indoors, with filtered air, when the site’s color-coded warnings flag poor air quality as yellow, orange and worse.  Pregnant people who must venture outside should wear a well-fitting N95 respirator and use the recirculate-air function while traveling in a car, Karr said. 

“Preterm birth has strong implications for future child health. Better understanding of environmental conditions and how to reduce their impacts, such as wildfire smoke, will pay off as a healthier population into the future,” she added. 

This work is part of the National Institutes of Health-funded Environmental Influences on Child Health Outcomes Program, known as ECHO. ECHO aggregates data and researchers from study sites nationwide to better understand how environmental exposures shape child health and development. 

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