Exposure to wildfire smoke and heat linked to worse birth outcomes in climate-vulnerable areas

Exposure to wildfire smoke and heat linked to worse birth outcomes in climate-vulnerable areas | Image Credit: © David A Litman - stock.adobe.com.

A new study from the Keck School of Medicine at the University of Southern California has found that exposure to wildfire smoke and heat stress before and during early pregnancy is associated with an increased risk of adverse birth outcomes. These risks are magnified for women living in climate-vulnerable neighborhoods, even if exposure occurred in the month prior to conception.^1,2

The findings, published in Environmental Science & Technology, stem from the MADRES cohort, an ongoing study of pregnant women in Los Angeles. The research team, led by postdoctoral scholar Roxana Khalili, PhD, analyzed 713 births between 2016 and 2020 to assess how environmental factors—specifically wildfire smoke and heat—might affect infant health.

“We already know that poor air quality is associated with adverse health outcomes and that pregnant women and fetuses are especially vulnerable,” said Khalili. “Our knowledge about the specific effects of wildfire smoke during pregnancy has been limited. We know even less about the impact of these hazards right around or before conception.”

To evaluate exposure, the researchers used wildfire data from CalFIRE, NOAA satellite-based smoke density maps, and advanced modeling techniques to estimate daily smoke-related particle pollution. Heat stress exposure was measured using temperature, humidity, and meteorological data, with additional consideration of sun exposure and wind speed to calculate how hot a day actually felt outdoors.

The study found that exposure to both wildfire smoke and excessive heat in the month before conception and the first trimester was significantly associated with increased odds of delivering a small-for-gestational-age (SGA) infant. An infant is classified as SGA if their birth weight is below the 10th percentile for their gestational age.

“Overall, we found consistent associations between increased exposure to wildfire smoke and heat stress during preconception and the first trimester with the odds of having an SGA infant,” Khalili said.

The researchers also noted that women exposed to moderate levels of smoke during the first trimester had an increased risk of delivering a low-birth-weight baby, defined as weighing less than 2,500 grams (5 lbs, 8 oz). Both SGA and low birth weight are known risk factors for short- and long-term health complications in infants.

In addition to environmental exposure, the study examined the role of neighborhood climate vulnerability using data from the U.S. Climate Vulnerability Index and California’s Urban Heat Island Index. These tools help quantify environmental, infrastructure, and social stressors that may affect a community’s ability to adapt to climate-related hazards.

The study found that climate vulnerability significantly modified the impact of heat stress. For women living in the most climate-vulnerable areas, the effect of preconception heat exposure on SGA births nearly doubled. Khalili emphasized that “where you live makes a difference in your health. So does the timing of your exposure during or immediately before pregnancy.”

The researchers also observed lower Fenton growth z-scores—a measure of how an infant’s size compares to population norms—for infants exposed to more wildfire days during pregnancy. This trend was especially noticeable during the second trimester and with specific smoke indicators like high particulate concentrations and light-density smoke days.

“Understanding what’s happening to women during wildfires and excessively hot days could help us identify protective measures, develop guidance, and plan interventions,” said Khalili.

Rima Habre, ScD, who developed and oversaw the project, said the findings emphasize the need for broader solutions that go beyond individual behavior changes like staying indoors or using air purifiers.

“As the recent Los Angeles fires have demonstrated, infrastructure, socioeconomic, and health vulnerabilities can combine with excessive climate and environmental factors to magnify health risks,” said Habre. “It is only by looking at the cumulative impacts of burdens communities are facing, now and into the future, that we can start to truly quantify health risks of climate hazards and target interventions to strengthen community resilience.”

The researchers call for further studies that use personal monitoring data to better identify critical windows of exposure and strategies to mitigate risk for pregnant women living in climate-vulnerable settings.

References:

1. Keck School of Medicine of USC. New USC study links wildfire smoke exposure and heat stress to adverse birth outcomes. Eurekalert. June 18, 2025. Accessed June 20, 2025. https://www.eurekalert.org/news-releases/1088115
2. Khalili R, Liu Y, Xu Y, et al. Adverse Birth Outcomes Associated with Heat Stress and Wildfire Smoke Exposure During Preconception and Pregnancy. Environmental Science & Technology. Published online June 18, 2025. doi:https://doi.org/10.1021/acs.est.4c10194