Decades of progress cleaning up America’s air are quietly being undone by wildfire smoke, and the latest research on wildfire smoke pregnancy exposure shows pregnant people are on the front line. A sweeping new national study covering 64.5 million pregnancies found that wildfire pollution has more than doubled as a share of the fine particle pollution that expectant mothers breathe, becoming the dominant source of the most dangerous air quality days in the country.
Published in the journal Frontiers in Environmental Health, the analysis is the first to map how prenatal exposure to fine particulate matter (PM2.5) has changed across the United States over time and what role wildfires play in shaping those trends. Here is what the researchers found, why it matters for fetal development, and what pregnant people can actually do during smoke season.
What the National Study Found
A team led by Menglu Liang, an assistant clinical professor at the University of Maryland, combined federal air quality data with two computer models: one simulating pollution from all sources, and one with wildfire pollution removed entirely. The comparison, spanning 2003 to 2019 across more than 2,800 counties in 48 states, revealed a tale of two trends.
1. Overall pollution fell, but wildfires ate the gains. Daily prenatal PM2.5 exposure from all sources dropped about 34 percent over the study period. Strip wildfires out of the picture, and the decline was steeper, around 37 percent. In other words, wildfire smoke measurably slowed the progress that cleaner cars and factories delivered.
2. Wildfire smoke more than doubled its share. Wildfires went from contributing roughly 3.7 percent of prenatal PM2.5 exposure to 8.2 percent, and by 2019 they were the dominant contributor to high-pollution days during pregnancy in the U.S.
3. Bad air days are now mostly smoke days. Over the study period, about 65 percent of the days that exceeded the EPA’s healthy air threshold (35 micrograms of fine particles per cubic meter) could be attributed to wildfire smoke.
4. The problem is national, not just a Western issue. Pollution from wildfires grew substantially even in regions not usually associated with fires, including the Southeast. The sharpest increases hit the Northwest, where wildfire pollution rates more than doubled.
“Those gains have genuinely protected pregnant people and unborn babies, and without them, exposures today would be considerably worse,” Liang said. “What our findings show is that wildfire smoke is offsetting a growing share of these hard-won improvements.”
Why PM2.5 Matters So Much During Pregnancy
PM2.5 particles are about 30 times smaller than the width of a human hair, small enough to slip past the body’s natural defenses, lodge deep in lung tissue, and cross into the bloodstream. During pregnancy, that systemic exposure reaches a developing fetus through the placenta; one study even found that air pollution particles get into the fetal placenta directly. Researchers have linked prenatal PM2.5 exposure to low birth weight, preterm birth, autism, and asthma, according to the American Pregnancy Association.
What makes wildfire smoke especially concerning is its chemistry. Studies suggest smoke particles can be more toxic than an equal mass of urban pollution, and they travel vast distances, blanketing communities far from any flame. As IL previously reported, climate change is intensifying risks for pregnant women across multiple fronts, and lengthening fire seasons keep adding fuel to this particular threat.
The Evidence Goes Beyond One Study
The Frontiers analysis joins a rapidly growing body of research linking wildfire smoke to specific pregnancy outcomes.
Preterm birth
A Stanford University cohort study of more than 2.5 million California births found that 86 percent of pregnant individuals were exposed to at least one day of wildfire smoke. Each additional day of exposure during pregnancy raised the odds of a spontaneous preterm birth by about 0.3 percent, with smoke between 14 and 20 weeks of gestation linked to medically indicated preterm delivery, according to the study published in Environmental Epidemiology.
Preterm labor complications
Researchers at the University of California, Irvine, analyzed more than 627,000 births during California’s intense 2017 to 2018 fire seasons. Higher exposure to wildfire PM2.5 and combustion byproducts, particularly in the second trimester, was significantly associated with both preterm birth and preterm premature rupture of membranes. The team estimates that roughly 7 percent of preterm births in the study population may be attributable to elevated wildfire smoke exposure.
Birth defects
A study of pregnancies in California’s San Joaquin Valley found that higher wildfire smoke exposure early in pregnancy was associated with increased risks of certain structural anomalies, including cleft lip and a heart condition called coarctation of the aorta, according to research in the Journal of Exposure Science & Environmental Epidemiology.
Child behavior and development
An exploratory study drew blood from women in their first trimester and mapped wildfire smoke exposure to their home addresses. The prenatal proteome showed enrichment in proteins involved in inflammation, neurodevelopment, and tissue development, which in turn predicted more exuberant and less fearful behavior when the children were 3 to 5 years old, according to the findings in Environmental Research.
Who Faces the Highest Risk
The burden of wildfire smoke exposure is not evenly distributed.
- Rural, low-income communities bore the highest exposure burdens nationally, according to the Frontiers in Environmental Health analysis.
- American Indian and Alaska Native communities were disproportionately represented in the hardest-hit regions.
- Asian and Black patients showed stronger associations between smoke exposure and preterm birth in the UC Irvine data.
- People with lower pre-pregnancy BMI and those facing high everyday pollution or extreme heat saw amplified risks.
- Pregnancies beginning in warmer months coincided with more intense and prolonged fire seasons.
- People with limited access to air conditioning or filtration face higher indoor exposure because they cannot retreat to a cleaner-air space during multi-day smoke events.
Risk also compounds. The combination of wildfire smoke and everyday air pollution was particularly notable in the UCI data, and extreme heat layered on top of smoke further elevated the danger. Since wildfire seasons are growing longer and more intense as the planet warms, the threat wildfires pose in a changing climate is set to keep expanding into new regions and new months of the year.
What Pregnant People Can Do During Smoke Season
You cannot control a wildfire plume, but you can meaningfully cut your exposure. Public health agencies and our own testing reviews point to a layered approach:
1. Watch the AQI, not the sky. The Air Quality Index translates smoke levels into action categories. When it enters unhealthy ranges, treat it as a cue to move your day indoors. Outside the U.S., global trackers like IQAir and the World Air Quality Index project cover monitors in nearly every country.
2. Build one clean-air room. The EPA’s clean room approach combines a sealed room, recirculated air, and a properly sized HEPA purifier. Our deep dive into science-backed plans for safer indoor air shows how to size filtration, seal infiltration gaps, and avoid indoor particle sources like frying and candles. Choosing the right hardware matters too: which air purifiers actually protect your home from wildfire smoke breaks down what to look for.
3. Time outdoor activity carefully. If you must go outside, go when pollution is lowest, keep exertion light, and wear a well-fitting N95 respirator.
4. Loop in your prenatal care team. Clinicians can adjust schedules and guidance when smoke episodes coincide with high-risk periods of pregnancy.
FAQ: Common Wildfire Smoke Pregnancy Questions
Can the smell of smoke harm my unborn baby?
Odor alone signals the presence of combustion gases, but the bigger concern is the invisible PM2.5 traveling with it. If you can smell smoke, fine particle levels are likely elevated enough to warrant protective action, especially during pregnancy.
What air quality index is considered bad during pregnancy?
Many obstetric guidance sources treat AQI values above 100 (unhealthy for sensitive groups) as the threshold for extra caution during pregnancy, and above 150 as a reason to stay indoors with filtration running. In the U.S., check the daily AQI at AirNow.gov. For data outside the U.S., IQAir’s world air quality map reports live readings from more than 40,000 monitoring stations across 143 countries, and the global WAQI map aggregates official government monitors worldwide. Both report using the U.S. AQI scale or PM2.5 concentrations in micrograms per cubic meter, so the same caution thresholds apply.
Is brief smoke exposure, like a bonfire, dangerous while pregnant?
Short, low-level exposures are far less concerning than multi-day wildfire episodes, which research links to measurable risk. Still, avoiding prolonged time directly in heavy smoke during pregnancy is sensible, particularly in the first and second trimesters when the studies found the strongest effects.
Does wildfire smoke exposure guarantee a problem with my pregnancy?
No. The studies show population-level increases in risk, not certainty for any individual pregnancy. Millions of people were exposed to smoke in these cohorts, and the vast majority of babies were born healthy. The goal of the research is prevention, not alarm.
