New USC Study Reveals Impact of Wildfire Smoke and Heat Stress on Birth Outcomes

In a striking new study published in Environmental Science & Technology, researchers have unveiled the profound impacts of wildfire smoke and heat stress on birth outcomes, particularly in women residing in climate-vulnerable neighborhoods. Spearheaded by USC postdoctoral researcher Roxana Khalili, PhD, this investigation delves into how environmental stressors in the periconceptional period — the crucial window right before and during early pregnancy — can markedly alter fetal development and increase the risk of adverse outcomes.

The research confronts an urgent and emerging public health challenge, as climate change intensifies the frequency and severity of wildfires and heatwaves, disproportionately affecting populations with limited resources. Unlike prior work that has mainly focused on air pollution exposure during pregnancy, this study pioneers in examining exposures specifically during the month preceding conception and the first trimester, underscoring an often-overlooked critical timeframe. Khalili emphasizes, “Our understanding of wildfire smoke’s effects around conception has been minimal, yet this period may represent a vulnerable window for fetal morbidity.”

Drawing on comprehensive data from the Maternal and Developmental Risks from Environmental and Social Stressors (MADRES) cohort — a longitudinal study of 713 pregnant women in Los Angeles from 2016 to 2020 — the team integrated advanced environmental exposure assessments. They utilized CalFIRE databases to map wildfires’ scale, placement, and duration, combined with the NOAA hazard mapping system to estimate wildfire smoke density accurately. Employing sophisticated atmospheric modeling, they calculated ground-level particulate concentrations, focusing on fine particle pollution such as black carbon, soot, and biomass combustion byproducts, and linked these exposures to the precise residential locations of participants recorded daily.

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Heat stress, an equally critical but less explored climate hazard during pregnancy, was quantified using multifactorial meteorological data capturing temperature, humidity, and wind speed. Crucially, the researchers incorporated solar radiation metrics accounting for sun angle and cloud cover to reflect the thermal burden experienced during direct sunlight exposure more realistically. This nuanced approach allowed for a more precise estimation of the actual physiological stress pregnant women endure in variable environmental conditions.

The team also examined the intersectionality of environmental hazards with neighborhood-level vulnerabilities by integrating geospatial data from the California Urban Heat Island Index and the US Climate Vulnerability Index. These composite indices represent multidimensional stressors, including intensified urban heat, wildfire exposure risk, and socioeconomic factors such as income disparity, housing quality, and access to healthcare. This integration enabled a layered understanding of how cumulative burdens can amplify the deleterious effects of climate exposures.

The results were compelling: increased exposure to wildfire smoke and excessive heat both during the month before conception and in the early stages of pregnancy were strongly linked to a higher probability of infants being small for gestational age (SGA). Infants classified as SGA fall below the 10th percentile in weight relative to their gestational age, a condition that predisposes them to severe neonatal complications like hypoxia, as well as long-term developmental challenges encompassing metabolic disorders, cognitive delays, and neurobehavioral deficits.

Moreover, the study revealed a confluence of smoke exposure with low birth weight outcomes, defined as birth weights under five pounds and eight ounces, particularly for women exposed to moderate wildfire smoke density in the first trimester. This dual finding highlights wildfire smoke as a potent environmental teratogen with implications spanning from immediate postnatal health through adulthood.

Significantly, the investigation brought to light that residing in a climate-vulnerable neighborhood magnified the adverse impacts of heat stress before conception on SGA risk. In fact, the effect size for heat stress nearly doubled among women in these high-risk areas, suggesting that social and infrastructural inequities exacerbate biological vulnerabilities to climate stressors. This connection amplifies calls for targeted interventions addressing not only individual behaviors but also systemic disparities.

Further biomarkers of fetal growth disturbance emerged through analyses of Fenton Z-scores, a standardized measure indicating deviations of an infant’s size from norms for their gestational age. The data demonstrated a clear trend: prolonged wildfire exposure days during pregnancy correlated with significant declines in these scores, signaling impaired fetal development consistent with patterns observed in chronic environmental stress models.

Given the escalating wildfire seasons in California, exacerbated by hotter, drier conditions driven by climate change, these findings sound the alarm for urgent public health strategies. Simple personal mitigations, such as remaining indoors or utilizing air filtration and cooling devices, although beneficial, do not suffice to counteract community-level vulnerabilities. Khalili stresses the necessity for systemic approaches that bolster neighborhood resilience — including infrastructure upgrades, improved healthcare access, and socio-economic support — to buffer the compounded risks faced by pregnant women.

Rima Habre, director of USC’s CLIMA Center and co-investigator, underscores the imperative of intersectional research. “The interplay of environmental hazards with social determinants creates a multiplier effect on health risks,” Habre remarks. “Future investigations must adopt holistic frameworks to fully capture these cumulative burdens and inform effective, equity-centered interventions.”

The multidisciplinary team involved environmental scientists, epidemiologists, obstetricians, and public health experts, drawing on collaborations between USC’s Keck School of Medicine, Sonoma Technology Inc., Eisner Health, and Emory University. Their integrative methodological approach, underscored by meta-analytic techniques, robust geospatial analytics, and advanced atmospheric modeling, sets a new benchmark for examining the health consequences of climate-induced environmental stressors on reproductive outcomes.

As climate change forecasts predict further intensification of wildfire frequency and duration, this study not only advances scientific understanding but also serves as a call to action. Policymakers and healthcare providers must urgently devise and implement protective measures that acknowledge and address the multifaceted nature of climate vulnerability, particularly for populations at the intersection of environmental and social marginalization.

Ultimately, the work led by Khalili and Habre illuminates the silent, insidious toll of wilderness fires and extreme heat on the youngest and most fragile—our next generation. Through deep scientific inquiry and community-sensitive responses, a path forward emerges to safeguard maternal and infant health in an era of escalating climatic adversity.

Subject of Research: Effects of wildfire smoke and heat stress exposure during preconception and early pregnancy on birth outcomes.

Article Title: Adverse Birth Outcomes Associated with Heat Stress and Wildfire Smoke Exposure During Preconception and Pregnancy

News Publication Date: 18-Jun-2025

Web References:
https://doi.org/10.1021/acs.est.4c10194

References:
Khalili, R. et al., Environmental Science & Technology, 2025.

Keywords: Pregnancy, Heat, Extreme weather events, Climate change, Human reproduction, Gestational age, Smoke, Soot, Black carbon, Pollution, Wildfires

Tags: adverse outcomes from environmental exposureclimate change and public healthenvironmental science and maternal healthenvironmental stressors and fetal developmentheat stress during pregnancylongitudinal study on pregnancy and environmentmaternal health and climate vulnerabilitypericonceptional period impactsUSC study on wildfire effectsvulnerable populations and pregnancy riskswildfire smoke and birth outcomeswildfire smoke exposure risks