Wildfire smoke increases risk of preterm birth: study

Exposure to wildfire smoke during pregnancy increases the risk of preterm birth — a risk that is only getting worse, a new study from Stanford University has found.

The study, published in Environmental Research, found as many as 7,000 additional preterm births in California could be attributed to wildfire smoke exposure between 2007 and 2012. Preterm births, according to the authors, are those that occur before 37 weeks of pregnancy, when incomplete fetal development can increase the risk of neurodevelopmental, gastrointestinal and respiratory complications, as well as death.

During the smokiest season covered in the study, 2008, the authors found that wildfires may have contributed to more than 6 percent of preterm births. That year, a severe lightning storm, coupled with powerful winds and high temperatures, set the stage for particularly intense conditions, a news release that accompanied the study said.

But the devastation that struck in 2008 has already been surpassed by that of 2020, a season in which smoke exposure was 2.5 times worse than in 2008, according to study co-author Marshall Burke. And the authors fear that this phenomenon will repeat itself in 2021.

“2020 does not look so unusual from what we might expect in the future,” Burke, a Stanford environmental economist, told The Hill. “Unfortunately, we should think of it as a new normal.”  

California is grappling with one of its worst wildfire seasons in history. The Dixie fire, northeast of San Francisco, was still expanding after five weeks and only 37 percent contained as of Sunday, according to The Associated Press. The Caldor fire, which broke out on August 14, scorched the Sierra Nevada and had zero containment by Sunday, while the French fire in Southern California was only 10 percent contained, the AP reported.

These fires are ravaging a state still reeling from what Cal Fire described as the “2020 Fire Siege,” which burned some 4 million acres and generated some of the worst daily air pollution ever documented in California.

Wildfire smoke is particularly dangerous due to its high levels of PM 2.5 — particulate matter with a diameter of 2.5 micrometers, or 30 times smaller than the diameter of a single human hair, according to the Environmental Protection Agency. These particles can get deep into the lungs and sometimes even into the bloodstream, the EPA said.

The Stanford study builds upon an established link between particle pollution and adverse birth outcomes, but it is the first to pinpoint the effects of wildfire smoke on early births, according to the news release.

To establish this correlation, the authors analyzed smoke plume satellite data from the National Oceanic and Atmospheric Administration in 2,610 ZIP codes for the 2007-2012 window. They then paired that data with ground-level PM 2.5 pollution estimates, developed using a machine learning algorithm that melded data from air quality sensors, satellite observations and computer models that track how pollutants move through the atmosphere.

Preterm birth data came from California birth records, and excluded twins and higher multiples, which commonly arrive early due to other factors, the news release said. After accounting for other factors linked to preterm birth risk, the authors investigated how patterns of such birth in each ZIP code varied alongside surges in the number and intensity of smoke days.

“We see a remarkable consistency across households,” Burke told The Hill.

Ultimately, the authors found that each day of smoke exposure during pregnancy heightened the risk of preterm birth, regardless of race, ethnicity or income, the news release said. A full week of exposure meant a 3.4 percent greater risk in comparison to a mother who experienced no wildfire smoke exposure, while the greatest impact occurred during the second trimester.

Scientists have yet to determine the precise health impacts of wildfire smoke, which has a different chemical makeup from other PM 2.5 sources, such as agriculture, tailpipe emissions and industry, according to the news release.

But one potential explanation for the surge in early births, according to the authors, is that smoke may trigger an inflammatory response that sets the delivery process in motion. They did acknowledge that this risk is small in comparison to other factors that contribute to a healthy, full-term birth.

“However, against a backdrop where we know so little about why some women deliver too soon, prematurely, and why others do not, finding clues like the one here helps us start piecing the bigger puzzle together,” co-author Gary Shaw, a professor of pediatrics and co-primary investigator of Stanford’s March of Dimes Prematurity Research Center, said in the news release.

Sam Heft-Neal, lead author and a research scholar at Stanford’s Center on Food Security and the Environment, warned that in the future, intense exposure to such smoke will continue to rise, “due to a confluence of factors” like climate change and fire suppression.

“As a result, the health burden from smoke exposure – including preterm births – is likely to increase,” Heft-Neal said in the news release.

A short-term solution for pregnant women is to stay indoors or wear an appropriate mask while outdoors, according to Shaw. Long-term, the authors said that their research demonstrates the value in wildfire mitigation investments, such as prescribed burns and forest thinning.

“There’s a lot of evidence that doing prescribed burns is a cheap way in which we reduce the likelihood of these really bad fires,” Burke told The Hill.

Since premature births cost the national health care system about $25 billion each year, Burke contended that even a small reduction in preterm birth risk could garner “enormous societal benefits.”

The average societal cost of each preterm birth is about $65,000 — meaning that in 2008, the societal cost linked to smoke exposure in California was about $130 million, Burke estimated. He calculated that in 2020, with about 2.5 times the exposure, associated costs would be closer to $325 million.

Assessing costs for prescribed burning is a bit more “all over the place,” Burke explained, noting that $1,000 per acre is one metric often used. If California needs about 1 million acres to be treated, then this would require an approximately $1 billion investment, he said.

With these figures in mind, Burke explained that preterm births alone could justify about one-third of what California would need to spend to mitigate wildfire risks.

“But needed expenditures cannot be justified on just preterm birth impacts alone,” he added.