Wildfires now the dominant contributor to unhealthy levels of air pollution for pregnant women in the US

Wildfires are well-known health risks. Now, in a new Frontiers in Environmental Health study, researchers in the U.S. have—for the first time—evaluated how prenatal PM_2.5 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 PM_2.5 in the U.S. 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 PM_2.5 exposure has more than doubled. By 2019, wildfire smoke had become the dominant contributor to high-pollution days experienced during pregnancy in the U.S.”

Rising wildfire pollution

The team ran two computer models to simulate air pollution. The first run included human and natural sources of PM_2.5 pollution, such as cars, industrial output and fires. In the second run, all fire-related pollution was removed. The difference was the estimate for how much PM_2.5 pollution was due to wildfires.

The researchers also determined on how many days unborn babies were exposed to PM_2.5 pollution exceeding 35 micrograms of particles per cubic meter (exceedance days). Levels beyond this value are considered unhealthy by the U.S. 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 PM_2.5 exposure from all sources decreased by around 34% on average. However, when PM_2.5 pollution caused by wildfires was excluded, the decline was steeper—around 37%—suggesting that wildfires offset some gains. Wildfires’ contribution to total prenatal PM_2.5 pollution more than doubled, from around 3.7% to 8.2%.

By the end of the study period, approximately 65% of prenatal PM_2.5 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

PM_2.5 pollution from fires is potentially more harmful than an equal amount of PM_2.5 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 PM_2.5 pollution. In the Northwest, wildfire contributions to PM_2.5 surged from 5.1% to 13.3%. In the Western states, wildfires’ share of PM_2.5 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 U.S.

While the study cannot predict whether 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 whether people had moved across counties or regions during their pregnancies or whether 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.”

Who’s behind this story?

Sadie Harley

BSc Life Sciences & Ecology. Microbiology lab background with pharmaceutical news experience in oil, gas, and renewable industries.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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