Two new studies link exposure to wildfire smoke in women during pregnancy to risks to newborns’ health.

By Kimberly Yang

Climate change is the primary cause behind more frequent and severe extreme weather in recent years; intensifying wildfires, heatwaves, and cyclones are all hallmarks of the tipping scale of global warming. In the past couple of years, extreme weather events have occurred worldwide. Europe experienced its hottest summer in 2024, the same year that Japan recorded heatstroke-inducing temperatures. The Los Angeles wildfires in January of 2025 were among the worst ever recorded.

This alarming shift carries harmful implications for human health as well, particularly for climate-vulnerable groups. Increased heatwaves, droughts, air pollution, and heat stress have accompanied the rise in wildfire occurrences in recent years—all of which can negatively impact health. This concerning trend threatens to deeply affect prenatal health and subsequently birth outcomes. A new study, published in Environmental Science and Technology in June 2025, led by Roxana Khalili, postdoctoral researcher at the University of Southern California has found that exposure to wildfire smoke and heat stress before and during pregnancy is linked to a higher risk of adverse birth outcomes.

The study found that the relationship between heat stress exposure, otherwise known as wet bulb globe temperature, and low birth weight was not as straightforward.

“We did see something that was the opposite of what we expected,” Khalili said. “An increase in exposure to wet bulb globe temperature appeared to have a protective effect for low birth weight in the third trimester.”

She explains that this counterintuitive pattern may be driven by what she calls “exposure aversion” behavior—pregnant individuals may be more likely to take measures to limit their exposure, such as staying inside if higher temperatures or wildfire smoke pollution levels occur.

Her team used data from an ongoing study that recruits low-income and Hispanic mothers in Los Angeles County, especially in climate-vulnerable and environmentally burdened neighborhoods. By using this cohort, the study aims to home in on these disparities and examine how climate stress is affecting marginalized populations.

The study also compiled data on residential history, meteorological measures, heat index, wildfire occurrences, and air pollution to measure heat stress and wildfire smoke exposure. Data for low birth weight (under 2,500 grams) were obtained from medical records.

Khalili’s research indicates that the time before pregnancy, otherwise known as the  preconception period, and the first trimester of pregnancy are especially crucial and sensitive windows of exposure. Increases in the daily maximum heat index during the preconception period were linked to higher odds of having a small-for-gestational-age baby—an infant smaller than the expected size for its developmental stage. Similarly, these odds also increased with more exposure to active wildfire and polluted air days during the first trimester. Days with moderate levels of wildfire smoke, known as medium-density smoke days, in the first trimester were associated with greater odds of low birth weight as well.

The study’s findings conflicted with another 2024 California study, which found that exposure to fine particulate matter in the air due to wildfires was associated with higher odds of adverse birth outcomes across all trimesters.

“It was really kind of consistent across the entire time,” said Amy M. Padula, an environmental researcher at the University of California, San Francisco, who conducted the conflicting study, said. “There’s no safe time for wildfire smoke during a pregnancy,”

She adds that both findings contribute to a growing body of evidence linking climate change and maternal health, despite the two studies’ differing results. This may be due to discrepancies in research methods, as varying strategies in measuring wildfire smoke exposure could have played a role. For example, Padula’s study examined exposure by week instead of trimester.

Khalili’s findings have profound implications in addressing climate-related health disparities. Living in a heat-vulnerable neighborhood nearly doubled the odds of having a smaller baby in the study. The most climate-vulnerable neighborhoods experienced a higher risk of a small-for-gestational-age baby, which was linked to heat stress exposure prior to conception.

Padula noted that such disparities are rooted in both socioeconomic and systemic inequities. “We’re always concerned about who’s getting the most exposed, who has the ability to protect themselves against this, who’s the most vulnerable,” she said. “Not just because they may be pregnant, but because of other social and structural factors.”

For example, she notes that areas of lower socioeconomic status often have cooling systems that bring in outside air and have poor air filtration, which has been linked to more adverse birth outcomes.

This highlights the need to address climate-related issues in a way that recognizes how interconnected environment and social factors, rather than homing in on any one single exposure to an environmental stressor.?

However, this study has some limitations. The small sample size for some birth outcomes, such as low birth weight, may have limited the study’s ability to detect certain associations. Additionally, the study did not account for where participants worked or how much time they spent outdoors, which could have caused errors for those who spent much of their day outside.

“We are also working on a future study right now that does use personal monitoring data,” Khalili said. This will help account for unmeasured factors from this study, including time spent working or otherwise outside of the home. She adds, “Using personal monitoring data, we’ll be able to better understand the patterns of exposure measurement error.”

Looking to the future, Khalili also mentions the possibility of conducting a follow-up study about long-term impacts on health later in life for these low birth weight babies. She hopes that this study and future research building off of it will inform public health programs and policy that can help better protect at-risk mothers as climate stressors intensify.

This study investigates and sheds light on the environmental health disparities that may be exacerbated by the worsening state of climate change. Despite its limitations, it highlights a pressing issue that concerns how our current climate challenges and how our lives right now will shape the health of generations to come.