Wildfire smoke had become the dominant source of the worst air pollution days experienced during American pregnancies by the end of the last decade, according to a national analysis published in Frontiers in Environmental Health. By 2019, roughly 65% of days when prenatal fine particle exposure crossed into unhealthy territory were attributable to wildfire smoke.
The finding rests on an unusually large dataset. Researchers examined about 64.5 million pregnancies between 2003 and 2019 across 2,842 counties in 48 states, linking county-level pollution estimates to demographic data including maternal ethnicity, urban or rural residence, and county income level.
For families, the practical meaning is a shift in when to pay attention rather than a new danger. Overall air quality during pregnancy improved substantially across the study period. What changed is the shape of the exposure: fewer bad days from traffic and industry, and a growing share of the remaining bad days concentrated into intense smoke episodes that arrive with little warning and can affect regions far from any fire.
The Modeling Method That Isolated the Wildfire Share
The researchers ran two parallel simulations of prenatal fine particulate exposure. The first included human and natural sources, including cars, industry output, and fires. The second removed all fire-related pollution. The difference between them produced the wildfire-attributable share.
They also counted exceedance days, defined as days when fine particulate matter exceeded 35 micrograms per cubic meter. Levels beyond that value are considered unhealthy by the U.S. Environmental Protection Agency.
The results show two trends moving in opposite directions. Average daily prenatal exposure from all sources fell about 34% over the study period. /With wildfire smoke removed from the calculation, the decline was steeper, about 37%, indicating that fires offset part of the gains achieved through emissions controls. Meanwhile, the wildfire share of total prenatal exposure more than doubled, rising from roughly 3.7% to 8.2%.
Menglu Liang, an assistant clinical professor at the University of Maryland and the study's first author, said the emissions reductions themselves worked. "Those gains have genuinely protected pregnant people and unborn babies, and without them exposures today would be considerably worse," Liang said in the study announcement, adding that wildfire smoke is offsetting a growing share of those improvements.
The Boundary Between Exposure Data and Health Outcomes
This is an exposure study, not an outcomes study. The researchers quantified how much wildfire smoke pregnant people breathed and where those people lived. They did not measure birth weight, gestational age at delivery, or any other pregnancy outcome in this analysis.
That distinction should shape how readers interpret the headline number. A growing body of separate research suggests that fine particles from fires may be more harmful than an equal amount of fine particulate matter from other sources, and prenatal exposure to fine particulate matter has been linked to adverse birth outcomes including low birth weight and preterm birth. But this paper does not itself demonstrate that the rising wildfire share has produced more adverse births.
The authors identify other limitations directly. They could not track whether individuals moved between counties or regions during pregnancy, and they had no data on whether people used masks, ran air filtration, or temporarily relocated during smoke events. All three would reduce actual exposure below the modeled estimate.
The study period also ends in 2019, before the 2020 Western fire season and the 2023 Canadian smoke episodes that reached much of the eastern United States. The authors say it is plausible that the wildfire share of prenatal exceedance days has continued climbing since then, but that confirming it requires analysis of more recent data.
Where the Burden Falls Most Heavily
Some regions were disproportionately affected. In the Northwest, the wildfire contribution to prenatal fine particle exposure surged from 5.1% to 13.3%. In the Western states, it rose from about 3.7% to 9.7%.
Liang said the burdens were highest for low-income and rural communities, for American Indian and Alaska Native communities, and for areas with shortages of maternity care services.
Liang framed the contribution of the work as a change in perspective. Prior research mapped where wildfire smoke occurs, but public health resources are allocated to populations rather than to acres. "With these results, we can begin to answer who is exposed and where they live," Liang said, adding that the information is what allows interventions and resources to be targeted fairly.
Smoke does not stay local, which is why the finding matters for households in the Midwest and Northeast as much as for Denver, Phoenix, or Seattle. Recent seasons have carried Canadian and Western smoke across those regions for days at a time, as MedicalDaily documented during this summer's air quality alerts.
Practical Steps for Pregnant People This Season
Nothing in this study changes prenatal care guidance. It does strengthen the case for treating smoke days the way pregnant people already treat extreme heat days.
The most useful habit is checking the local air quality index each morning during fire season and treating readings above the unhealthy for sensitive groups threshold as a signal to move exercise indoors and limit time outside. Running a portable air cleaner with a HEPA filter in the bedroom, keeping windows closed during smoke events, and using the recirculation setting while driving all reduce indoor particle levels meaningfully. A well-fitted N95 offers protection during unavoidable outdoor time, though it is not a substitute for staying inside. MedicalDaily has covered the broader health toll of this year's fire season in more detail.
Pregnant people who develop shortness of breath, chest tightness, persistent cough, or reduced fetal movement during or after a smoke episode should contact their obstetric provider rather than waiting for a scheduled visit. Those without insurance can access prenatal care through federally qualified health centers, and most state Medicaid programs cover pregnancy-related visits.
MedicalDaily has previously covered research linking wildfire smoke exposure to preterm birth risk. The new analysis addresses a different question: not whether smoke harms pregnancies, but how much of the harmful air pregnant Americans breathe comes from fire.
Key Questions Answered
What did the study actually find? That by 2019, about 65% of days when prenatal fine particle exposure exceeded 35 micrograms per cubic meter were attributable to wildfire smoke, and that the wildfire share of total prenatal exposure more than doubled between 2003 and 2019.
Does this mean air quality during pregnancy has gotten worse? No. Average daily prenatal exposure fell about 34% over the study period. The concern is that wildfire smoke offset part of that improvement and now accounts for most of the remaining severe pollution days.
Did the study measure harm to babies? No. It measured exposure only. Other research has linked prenatal fine particle exposure to preterm birth and low birth weight, but this analysis did not examine birth outcomes.
Is this only a Western states problem? The Northwest and West showed the largest increases, but smoke travels. Recent fire seasons have pushed Western and Canadian smoke across the Midwest and Northeast for extended periods.
What should a pregnant person do on a smoke day? Check the local air quality index, keep windows closed, move exercise indoors, run a HEPA air cleaner if available, and use recirculated air in the car. A fitted N95 helps during unavoidable outdoor time.
When should someone call their doctor? Shortness of breath, chest tightness, a cough that persists after the smoke clears, or any change in fetal movement warrant a call to the obstetric provider rather than waiting.
Do the findings apply to today's fire seasons? The data end in 2019. The authors say it is plausible the wildfire share has kept rising, but confirming that requires analysis of more recent data.
