Wildfire Smoke Is Now a Bigger Prenatal Threat Than Human Sources of Air Pollution

In 1965, a presidential advisory panel issued a report warning that pervasive chemical and carbon pollution was harming environmental health and changing the climate. Five years later, President Richard Nixon signed the Clean Air Act. 

Since that 1970 law and its amendments were passed, combined emissions of a half dozen harmful air pollutants from vehicles and industrial sources have decreased by nearly 80 percent. 

But mounting evidence shows that pollution from wildfire smoke is eclipsing those air-quality gains. The unchecked burning of fossil fuels is creating warmer, drier conditions that make wildfires more intense and destructive, and blanket communities in toxic smoke that is more harmful than pollution from other sources. 

Now, a new study shows that smoke from wildfires is also reversing the health benefits of reducing air pollution for society’s most vulnerable population: babies in the womb.

Increasing wildfire activity is “systematically offsetting prenatal air quality gains” from reducing emissions from vehicles, refineries, factories and other human sources, researchers from the University of Maryland, College Park, report in the study, published Thursday in Frontiers in Environmental Health. 

Low-income, rural and Indigenous populations bore the disproportionate burden of wildfire’s contribution to poor air quality, the authors found.

The major contribution of the study is its focus on a “very specific population that is quite at risk for adverse health outcomes,” said senior author Michel Boudreaux, associate professor of health policy and management. “We were looking at infants that were born and what their exposure was like during the prenatal period.”

Prenatal exposure to wildfire smoke significantly increases the likelihood that babies will be born too early or too small, raising their risk of dying early or developing chronic respiratory conditions and other diseases later in life. 

The University of Maryland team studied how wildfire smoke and human sources of air pollution changed across the contiguous United States between 2003 and 2019, looking at air pollution data from the Environmental Protection Agency and live birth records. (The EPA datasets do not include years beyond 2019.)

To understand how much wildfire smoke contributed to unhealthy air exposures for unborn babies, the team used computer models to estimate exposures to fine particle pollution, or PM2.5, from both human sources, such as cars and industry, and natural sources, like wildfires. The team modeled air quality without emissions from fires to calculate how much of that total pollution came from wildfires. 

The researchers obtained data for nearly 65 million babies born over 16 years and determined how many days they were exposed in the womb to the EPA threshold for unhealthy air.

Average daily prenatal exposure to air pollution from human sources dropped by 37 percent during that time, the analysis found, but contributions from wildfires doubled, offsetting those gains. 

Although the study does not integrate data on preterm birth, birthweight or other outcomes, it is a very good exposure-characterization study, said Ilona Jaspers, an inhalation toxicologist and professor of pediatrics at the University of North Carolina, Chapel Hill, who was not involved in the study.

“What this study does do well is confirm and expand the well-established finding that ambient PM2.5 in the United States is increasingly wildfire-dominated, offsetting Clean Air Act gains, but applies a birth-weighted lens to it,” she said.

Wildfire smoke exposures are likely to have vast public health consequences, Jaspers said. In previous studies, Jaspers and her colleagues used prescription insurance claim data to show that exposure during specific perinatal periods can lead to childhood respiratory diseases.

The research doesn’t look at how much time the mother was spending outside, if she wore an N95 mask or if she had an air-filtration system in her home, which Boudreaux acknowledges as a limitation of the study. But data sources like Purple Air that track wildfire smoke are detecting a large amount of pollution in the form of a PM2.5 inside people’s homes, he said.

The new study highlighted regions and populations that were disproportionately affected by wildfire smoke. In the West and Northwest, wildfires’ contributions to air pollution more than doubled over the study period. And the communities where wildfires are responsible for the largest reversals in air quality—low-income, rural and American Indian and Alaska Native populations—often have the least access to neonatal care.

Since exposure to wildfire smoke early in life can cause life-long health problems, reducing exposure in the prenatal period will yield long-term benefits, Boudreaux said.

The last six years have seen several catastrophic fires in the West and Canada, Boudreaux said. If the team were to extend the study through 2026, he said, they’d likely see wildfires make an even higher contribution to unhealthy prenatal exposures. But that’s just speculation, he cautioned.

What is clear, he said, is that wildfire smoke now accounts for an increasing share of “acute, massive air pollution days” that endanger babies’ health. “What that tells me is that the next frontier is how do we help people manage exposure to wildfire smoke, which is a different kind of exposure than ambient pollution from other sources.”

That means providing communities with resources like clean-air shelters, air filters and better access to healthcare when they are exposed, he said. Ultimately, he said, “we should be asking, are there things we can do to reduce the risk of climate change in general?”