Wildfires now the dominant contributor of unhealthy levels of air pollution for pregnant women in the US
Air pollution has significantly declined over the last decades in the US, but the share of prenatal exposure to pollution caused by wildfires has more than doubled, potentially offsetting hard-won improvements
Frontiers
Wildfires are well-known health risks. Now, in a new Frontiers in Environmental Health study, researchers in the US have – for the first time – evaluated how prenatal PM2.5 exposure has changed over space and time on a national scale, and what role wildfires play in shaping these trends.
“We show that prenatal exposure to PM2.5 in the US declined substantially between 2003 and 2019, reflecting major improvements in air quality,” said first author Menglu Liang, an assistant clinical professor at the University of Maryland. “At the same time, the contribution of wildfire smoke to prenatal PM2.5 exposure has more than doubled. By 2019, wildfire smoke had become the dominant contributor to high-pollution days experienced during pregnancy in the US.”
Rising wildfire pollution
The team ran two computer models to simulate air pollution. The first run included human and natural sources of PM2.5 pollution; including cars, industry output, and fires. In the second run all fire-related pollution was removed. The difference was the estimate for how much PM2.5 pollution was due to wildfires. The researchers also determined on how many days unborn babies were exposed to PM2.5 pollution exceeding 35 micrograms of particles per cubic meter (exceedance days). Levels beyond this value are considered unhealthy by the US Environmental Protection Agency.
The study included around 64.5 million pregnancies between 2003 and 2019. Demographic data, such as mothers’ ethnicities, whether they lived in urban or rural areas, and county income levels were also included. The data covered 2,842 counties in 48 states.
Over the study period, daily prenatal PM2.5 exposure from all sources decreased by around 34% on average. However, when PM2.5 pollution caused by wildfires was excluded, the decline was steeper – around 37% – suggesting that wildfires offset some gains. Wildfires’ contribution to total prenatal PM2.5 pollution more than doubled, from around 3.7% to 8.2%.
By the end of the study period, approximately 65% of prenatal PM2.5 exceedance days could be attributed to air pollution caused by smoke from wildfires – a striking fraction, said the team. This, however, does not mean that gains made through legislation and other efforts to limit pollution were in vain.
“Those gains have genuinely protected pregnant people and unborn babies, and without them exposures today would be considerably worse,” said Liang. “What our findings show is that wildfire smoke is offsetting a growing share of these hard-won improvements.”
Unequal distribution
PM2.5 pollution from fires is potentially more harmful than an equal amount of PM2.5 from other sources, a growing body of research suggests. These effects are particularly dangerous to pregnant people and are linked to adverse birth outcomes like low birth weight and preterm births.
“These outcomes matter well beyond the delivery room and harms experienced before birth can echo for decades,” said Liang. “That makes prenatal exposure a particular public health concern.”
Some regions were disproportionately affected by fire-caused PM2.5 pollution. In the north-west, wildfire contributions to PM2.5 surged from 5.1% to 13.3%. In the western states, wildfires’ share to PM2.5 pollution rose from approximately 3.7% to 9.7%.
“Wildfire pollution burdens were highest for low-income, rural, American Indian and Alaska Native communities, and areas with shortages of maternity care services,” said Liang.
Recent trends
Since the study period ended, several major wildfire seasons have occurred, including the 2020 fires in the western states and the 2023 Canadian wildfire smoke events affecting much of the eastern US. While the study cannot predict if air pollution due to wildfires has continued to rise since 2019, it is plausible that the proportion of prenatal exceedance days attributable to wildfire smoke has continued to increase, said the team. To confirm this, however, analyses of more recent data are necessary.
There are some limitations to the study, the authors pointed out. For example, they were not able to monitor if people had moved across counties or regions during their pregnancies, or if individuals protected themselves from smoke using masks, air filters, or temporary relocation.
“The central contribution of this work is a shift in perspective. Prior research has mapped very well where wildfire smoke occurs, but public health resources are allocated to populations, not to acres,” concluded Liang. “With these results, we can begin to answer who is exposed and where they live. We need this information to target interventions and allocate resources fairly.”
Journal
Frontiers in Environmental Health
Method of Research
Data/statistical analysis
Subject of Research
Not applicable
Article Title
Wildfire Contributions to Prenatal PM₂.₅ Exposure in the Contiguous United States, 2003–2019
Article Publication Date
7-Aug-2026
