Wildfire smoke is now the primary pollutant affecting pregnancy, study shows

A new study examining pollution levels across a 16-year span found that while general pollution levels from sources such as cars and industry have significantly declined, recent increases in wildfire smoke air pollution have offset those gains and could be contributing to negative outcomes for pregnant women and babies such as low birth weight and preterm births.  

Published by Frontiers in Environmental Health, the study included data from about 64.5 million pregnancies between 2003 and 2019, covering more than 2,800 counties in 48 states. Even in regions that are not strongly associated with wildfire, such as the southeast region of the U.S., wildfire pollution rates grew substantially during that time period. The study did not account for any movement between counties or regions during pregnancy, or whether pregnant people used any protection against smoke such as masks, air filters or temporary relocation.

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Wildfire pollution is included in the same type of pollution as exhaust from cars or industrial outputs, but the study authors looked at two models of pollutants, one of which removed all fire-related pollution to determine how much of it was correlated with wildfire. Over the study period, about 65% of the days that recorded pollution levels above the National Ambient Air Quality Standard could be attributed to wildfire smoke.

Menglu Liang, leading author of the study and assistant clinical professor of biostatistics at the University of Maryland, said in a news release that efforts to reduce other forms of pollution have not been in vain.

“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.”

The study time period does not include some of the most recent, most explosive wildfire seasons in the United States and Canada, such as 2020, 2024 and this year. By mid-July this year, there were more than 800 wildfires burning across Canada, leading to hazardous levels of wildfire smoke across parts of the Midwest and northeast, and in late July, a rash of wildfires across Oregon created hazardous air over much of the northwest. 

Recent reports show Oregon is on track to have its worst fire season on record, with nearly 1.8 million acres already burned and at least two months left of the peak season. The previous record was set in 2024, with 1.9 million acres burned.

The study found that some regions were disproportionately affected by the smoke, including in the Northwest, where rates of pollution from wildfire more than doubled over that period. The study noted that the burdens were highest in low-income, rural areas with significant populations of American Indian and Alaska Native communities, and areas with maternity care shortages.

 Recent studies have correlated wildfire smoke exposure with an increased risk of preterm birth, low birth weight and other negative outcomes such as stillbirth and some congenital anomalies. Scientists believe the body’s inflammatory responses to wildfire smoke can be harmful to the placenta, which is central to fetal survival, and some of the pollutants could cross the placental barrier and affect fetal growth and development.

“These outcomes matter well beyond the delivery room and harms experienced before birth can echo for decades,” Liang said in the release. “That makes prenatal exposure a particular public health concern.”

Stateline reporter Kelcie Moseley-Morris can be reached at [email protected].