Wildfire smoke and heat stress affect birth outcomes in vulnerable communities

Wildfire smoke and heat stress affect birth outcomes in vulnerable communities

Exposure to wildfires and heat blasts can negatively impact birth outcomes for women, particularly in climate-challenged neighborhoods, according to a recent study by USC postdoctoral researcher Roxana Khalili, PhD. The study just published inEnvironmental Science and Technologyis one of the first to show that living in areas more vulnerable to the deleterious effects of climate change exposures can significantly alter the effects of heat stress on adverse birth outcomes, even among women exposed to these conditions in the month before pregnancy.

We already know that poor air quality is linked to adverse health outcomes and that pregnant women and fetuses are particularly vulnerable. Our knowledge about the specific effects of wildfire smoke during pregnancy has been limited. We know even less about the impact of these dangers immediately or before conception. “

Roxana Khalili, PhD, researcher, Department of Population and Public Health Sciences, Keck School of USC and lead author of the study

Khalili worked with Rima Habre, director of the USC Climate Center, to examine the associations between exposure to wildfire smoke and heat stress on infant health. They used data from the Madres (Maternal and Developmental Risks from Environmental and Social Stressors) cohort, an ongoing study of pregnant women in Los Angeles.

“Where you live affects your health,” Khalili said. "This is also the timing of your exposure during or immediately before pregnancy. We wanted to better understand how these risks may differ for women who lived in neighborhoods that experience more climate-related stressors and have fewer resources to cope with them."

Measurement of exposure

The research team examined 713 births among Madres participants between 2016 and 2020. They used data from the Calf (California Department of Forestry and Fire Protection) to determine the location, size, and duration of each wildland fire in Southern California during the study period. They used the NOAA Hazard Mapping system to calculate smoke density from each fire and used sophisticated modeling methods to calculate ground-level smoke concentrations.

Heat stress is another risk factor for pregnant women and fetuses, in part because pregnancy hinders a woman's ability to regulate body temperature. To measure heat stress exposures in study participants, Khalili and her colleagues used meteorological data documenting daily temperature, relative humidity and wind speed. They also considered a separate measurement that takes into account sun angle and cloud cover to measure heat stress in direct sunlight, which provides a better measure of how hot a given day feels outdoors.

Next, the team pinpointed those LA neighborhoods most vulnerable to climate risks using mapping data from the California Urban Heat Island Index and the U.S. Climate Vulnerability Index, two geospatial tools that analyze and map layers of data. These tools allow researchers to conduct deeper, more nuanced analyzes that better assess the cumulative effects of factors ranging from excessive heat and smoke fires to socioeconomic conditions.

Where there is smoke...

Researchers found that greater exposure to wildfire smoke and excessive heat in the month before conception and the first trimester of pregnancy was associated with a greater likelihood of having a small baby boy (SGA). A child is considered SGA if their birth weight is below the 10th percentile of their expected weight based on their gestational age. SGA places a child at higher risk for health problems, from immediate complications at birth such as low oxygen levels to long-term challenges including potential metabolic, cognitive and neurodevelopmental disorders. The team also found an association between pregnant women exposed to moderate levels of smoke in the first trimester and having a low birth weight infant or an infant weighing less than five pounds, eight ounces. As with SGA, low birth weight is associated with a number of potential health problems in both the short and long term.

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

The study also suggested that living in a climate-sensitive neighborhood could increase the likelihood of a SGA birth, particularly for women exposed to heat during pregnancy. For women living in the most climate-controlled neighborhoods, the study showed how heat stress nearly doubled during the bias on the likelihood of an SGA birth.

Khalili and her colleagues also found that Fenton Z-scores, which measure how much a child's height deviates from average for their age, decreased significantly with more wildfire days experienced during pregnancy. These findings are particularly relevant given the prolonged fire events that Los Angeles experienced in January 2025 and that many cities across the United States are currently facing.

“Understanding what happens to women during wildfires and excessively hot days can help us identify protective measures, develop leadership and plan interventions,” Khalili said.

Given how frequent and long California seasons are expected to become, such interventions will be urgently needed. However, these must go beyond simple actions an individual can take – such as indoors, air filters or air conditioning – to understand and address what makes a community more resilient to these impacts.

Habre, who developed and oversaw the project, agrees that the study is breaking out new properties and that more holistic, actionable research is also needed.

“As the recent fires in Los Angeles demonstrated, infrastructure, socioeconomic and health vulnerabilities can combine with excessive climate and environmental factors to magnify health risks,” Habre said. “Only by considering the cumulative impacts of community exposures can we begin, now and into the future, to truly quantify the health risk of climate hazards and target interventions to strengthen community resilience.”

About the study

Additional authors of the study include Yisi Liu, Yan Xu, Karl O'Sharkey, Tingyu Yang, Nathan Pavlovic, Crystal McClure and Fred Lurmann of Sonoma Technology, Inc., Petaluma, CA; Brendan Grubbs and Layla al Marayati from the Department of Obstetrics and Gynecology, Keck School of Medicine of USC; Deborah Lerner and Nathana Lurvey of Eisner Health; and Carmen J. Marsit from the Rollins School of Public Health, Emory University, Atlanta, GA.

This study was supported by the National Institute of Environmental Health Sciences, National Institutes of Health (NIH) [P50MD015705, P50ES026086, R01ES027409, P30ES007048], the Environmental Protection Agency [83615801], the National Heart Lung and Blood Institute (NIH) [P20HL176204], and the USC Provost Fund.

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