Both wildfires and extreme heat have intensified over the past two decades — and this summer, I found myself traveling to two epidemiology conferences to talk about what that means for people most vulnerable to both.
I’m a PhD candidate in epidemiology at UC San Francisco, where I work at PRHE as a Graduate Student Researcher. We study how environmental exposures affect reproductive health, particularly during pregnancy. Right now wildfire smoke and extreme heat are two of the most pressing exposures.
When heat and smoke hit at the same time
In June, I attended the Society for Epidemiologic Research Annual Meeting in Phoenix, where I presented our research on the reproductive health effects of 
joint exposure to wildfire smoke and extreme heat during pregnancy. Nearly all of California’s largest and most destructive fires have burned in the last 25 years, increasingly at the wildland-urban interface, where fires consume homes and their contents in addition to vegetation. Fires now produce close to half of the state’s annual PM2.5 emissions and have eroded much of the progress California has made on air quality over decades. Meanwhile, extreme heat is the leading weather-related cause of death in the U.S., and some parts of California could see heatwaves become dramatically more frequent in the coming decades.
What makes this especially alarming is that these two hazards increasingly occur at the same time. During the 2020 fire season, most of California’s land area and roughly half of its residents faced high-intensity wildfire smoke and extreme heat simultaneously. And when the two occur together, research suggests the harm is greater than their separate effects would predict.
What this means for pregnancy
Preterm birth – delivery before 37 weeks – is one of the strongest predictors of perinatal death. More than one in ten U.S. births are preterm, a rate that continues to rise, and the condition costs the healthcare system upward of $25 billion annually. Both wildfire smoke and extreme heat have plausible pathways to early delivery: fine particles drive oxidative stress and inflammation, and interfere with placenta function while extreme heat can strain the cardiovascular system through dehydration and changes in blood flow. Pregnancy already places additional demands on these systems, potentially increasing vulnerability to environmental stressors.
Despite how often these two hazards now occur together, only a handful of studies have looked at their combined effects on preterm birth. Our research examines exactly that question, following a statewide cohort of California births from 2006-2020 to understand when during pregnancy exposure may be most harmful, whether the two hazards together are associated with greater risk than either exposure alone, and whether that harm falls unevenly across sociodemographic groups. We’re also looking at what might buffer some communities from the worst effects, with things like access to green space and community-level air conditioning.
A different kind of reproductive risk
The second conference took me to Munich in August, for the International Society for Environmental Epidemiology annual meeting, where nearly 2,000 
researchers from more than 70 countries gathered, and extreme heat and wildfire smoke again dominated the agenda.
There my presentation took the research in a different direction: wildfire smoke and semen quality. Male factor infertility contributes to a substantial and growing share of infertility, and researchers are searching for environmental causes that might be preventable. Previous studies have found that semen samples collected during smoke events tend to have lower sperm concentrations and total counts, and reduced motility, meaning the sperm that are present swim less efficiently. One study linked wildfire smoke exposure to elevated odds of azoospermia, a condition in which no sperm are present in the ejaculate. Animal studies point in the same direction, but the human evidence has remained thin, which is exactly what drew us to the question.
What we found and where we’re headed
To build a stronger evidence base, we assembled a study at a San Francisco fertility clinic, with nearly 10,400 men and more than 28,000 semen samples collected between 2015-2023. To our knowledge, this is the largest study to date examining the effects of wildfire smoke on semen quality. For each sample, we estimated daily wildfire smoke exposure at the patient’s home ZIP code over the 74 days it takes to produce sperm from scratch. What we found was consistent: higher wildfire smoke exposure was associated with reductions in semen volume, sperm concentration, total sperm count, and total motile count or the total number of actively moving sperm. And when we looked at the same men across multiple samples, their semen quality was consistently lower during more smoke-exposed periods compared to cleaner ones.
Next, we’re extending this work to women undergoing in vitro fertilization (IVF), tracking wildfire smoke exposure across stages of treatment, from ovarian stimulation and egg retrieval through fertilization, embryo development, and transfer.
Taken together, these projects share a sobering premise: extreme heat and wildfire smoke are both getting worse, they’re increasingly hitting vulnerable communities at the same time, and their effects are showing up in areas of reproductive health that remain understudied. As is so often the case with environmental hazards, the burden of exposure falls most heavily on those least able to avoid it. Understanding who is most affected — and what might protect them — will become increasingly important as wildfires and extreme heat become a more common part of life.
About the author
Aimee Lansdale, MS is a Graduate Student Researcher at PRHE. She is a PhD candidate in the Epidemiology and Translational Science Program at UC San Francisco focused on examining the effects of environmental exposures during pregnancy on birth outcomes. Aimee is also affiliated with the ECO-Health Center and BUSPH-HSPH CAFE Research Coordinating Center.
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