Exposure to wildfire smoke could be contributing to preterm births, according to a new study that evaluated wildfire smoke patterns and records of more than 3 million births in California.

The study, which was published earlier this month in the peer-reviewed journal Environmental Research, evaluated every birth in California from 2007-2012 and estimated that about 3.7% of all preterm births — nearly 7,000 — could be attributed to wildfire smoke exposure.

The finding adds to a growing volume of research that suggests the long-term effects of wildfire smoke pollution are pronounced and costly to people’s health. Smoke represents a threat to many communities in the West, including Seattle, because wildfires have become more widespread, more frequent and more intense than in the past due to unhealthy forests and higher temperatures from climate change.

“It really emphasizes to us the societal cost of unmitigated wildfire exposure,” said Marshall Burke, associate professor of Earth system science at Stanford University, adding that pregnant women should be among those taking precautions on smoky days. “Our research shows clearly: Pregnant moms are a key vulnerable group.”

Scientists have known for years that air pollution can affect pregnancy outcomes. 

A birth is considered preterm when a baby is born before 37 weeks of pregnancy (a full-term birth is about 40 weeks). Babies born too early can have respiratory problems, feeding difficulties and developmental delays, among other issues, according to the Centers for Disease Control and Prevention. Babies born before 32 weeks are at increased risk of death and disability.


A study published in Environmental Health Perspectives found that more than 3.3% of all preterm births in the U.S. could be attributed to air pollution in 2010 at a cost of more than $5 billion.

But scientists hadn’t produced much research on the role wildfire smoke plays or how its impacts compare to pollution from car tailpipes or coal power plants. 

Wildfires spew tiny particles that can enter the lungs and even the bloodstream. Researchers pay particularly close attention to particles less than 2.5 micrometers, which is small enough that about 20 could fit into the width of a human hair.

Wildfires have become increasingly important to those studying air pollution in Washington and other western states.

“Wildfire smoke has become — if not the most important — one of the most important air pollution sources in the region, which is a change from 25 years ago,” said Dr. Joel Kaufman, a University of Washington professor of medicine, epidemiology and environmental health sciences who was not involved in the study. Kaufman said there was no reason to think the study’s implications wouldn’t apply in Washington state.

To determine how smoke was affecting births, the California scientists used satellite data and air quality readings to model which ZIP codes were affected by smoke and at what intensity.


Then, they compared birth outcomes in identical areas when they were affected by smoke and in other years when smoke wasn’t a problem. The researchers controlled for other factors that could affect the likelihood of preterm births, such as a mother’s age and race. 

The approach is not perfect. A mother who is exposed to smoke pollution at home could work elsewhere and not experience the same exposure. Modeling smoke concentrations is complex and the data sources are imperfect.  

Kaufman said he was impressed with the study. He said authors had been careful in analyzing the data and built a “compelling” case with statistical strength because of the large number of births included in the research.

“Assuming that’s right, that’s an enormous amount and every preterm birth is a large amount of direct expenses and a huge emotional toll,” Kaufman said of the researchers’ estimate that nearly 7,000 preterm births could be attributed to wildfire smoke.  

It remains something of a mystery exactly how this pollution affects a pregnancy. 

“We can’t say precisely why smoke in the body is causing that outcome,” Burke said. “We’re still convinced we’re identifying the effect of smoke.” 


Wildfire smoke is known to trigger inflammatory responses throughout the body. The researchers suspect such responses could somehow trigger a preterm birth, though the biology of that relationship is uncertain. 

The researchers saw several interesting patterns in their data.

Exposure to less frequent, but higher intensity smoke was correlated with increased risk of preterm birth. Impacts appear to be more significant in a mother’s second trimester. White mothers in poorer ZIP codes made up the group most often exposed to smoke. 

Other forms of pollution disproportionately affect certain groups, such as people of color or those living in low-income areas.

People with high incomes have more housing choices and can avoid living near sources of pollution like highways and power plants, said Sam Heft-Neal, a research scholar at Stanford’s Center on Food Security and the Environment.

“It’s not possible for wealthy households to avoid wildfire smoke,” he said. “With wildfire smoke, we see high-income households or white households are exposed just as much as other households.” 

Because the researchers analyzed data from 2007-2012, when wildfire smoke exposure levels were lower, they suspect smoke exposure’s recent impacts on preterm births could be more pronounced. 


“Our study was conducted in a relatively low smoke period,” Heft-Neal said, adding that 2020 smoke exposure levels exceeded the worst year in the study by about 2.5 times.

Scientists have been ramping up their study of wildfire smoke after several years in which population centers in western states have stewed in hazy, polluted air.

Wildfires are projected to intensify and become more frequent as the climate warms in the western U.S., according to a comprehensive climate evaluation produced by the U.S. Global Change Research Program.

As exposure increases, Kaufman, the UW professor, said researchers are beginning to better understand that wildfire smoke’s effects must be studied not only in terms of its acute effects, but also its contribution to chronic conditions. 

A study published earlier this month linked smoke to spikes in COVID-19 cases in West Coast communities. Recent analysis of California’s 2018 Camp Fire, which killed at least 85 people and burned many homes and other structures, showed wildfire smoke contained high concentrations of toxic heavy metals like lead. Another recent study suggests wildfire particles could be more toxic than equal amounts of particle pollution from other sources like vehicle emissions. 

“The more people looked, the more people have found these health effects occur just like they do with other sources of air pollution,” Kaufman said.   

Kaufman said this research illustrates the urgency of reducing worldwide emissions and how important it is for people to stay out of unhealthy, smoky air when wildfire smoke arrives in the Pacific Northwest.