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Wildfire smoke may be contributing to premature births.

A new study highlights the dangers of wildfire smoke. Follow here for the latest on extreme weather, including floods, wildfires and more.

Wildfire smoke obscured the view of San Francisco last September.Credit...Jeff Chiu/Associated Press

As wildfires ravage the West, burning through millions of acres, they are producing blankets of smoke that are spreading far beyond the boundaries of the fires themselves. Now, new research indicates that the air pollution is endangering some of the most vulnerable: the unborn.

The findings, published this month in Environmental Research, suggested that from 2007 to 2012 in California, about 7,000 preterm births, or nearly 4 percent of all such births during those years, were associated with exposure to wildfire smoke.

It is the latest sign of the potential health risks of smoke from wildfires, which can include not only the soot and ash from burning trees and undergrowth but also the chemicals that are released when homes, cars and countless other things go up in flames when wildfires race through towns and neighborhoods.

Wildfire smoke can blunt the body’s immune response, causing anything from mild but annoying sore throats or coughing to serious cardiovascular and respiratory problems. Research published this month found that exposure to wildfire smoke last summer could be associated with thousands of additional Covid-19 infections and hundreds of deaths in the pandemic.

And by nearly every metric, wildfires in the United States are worsening. They are growing larger, spreading faster and reaching higher elevations. Their plumes are also reaching farther. Last month wildfire smoke from Canada and the West stretched across the United States, prompting health alerts in cities as far east as Toronto and Philadelphia.

The new research into premature infants, which focused only on California, found that a week of exposure was associated with a 3 percent increase in the risk of a preterm birth. In 2008 — the worst smoke year in the study period — the researchers found that wildfire smoke exposure was associated with more than 6 percent of all preterm births in California.

“We knew air pollution increased the risk of preterm birth, but this new work highlights the importance of pollutants associated with wildfire smoke, which might be different from other sources of air pollution, and are becoming more of an issue with climate change,” said Lara Cushing, an environmental health scientist at the U.C.L.A. Fielding School of Public Health who was not involved with the research.

Wildfire smoke contains high levels of the smallest, most dangerous type of soot. Exposure to these particles, known as PM 2.5, is believed to cause inflammation within the body, putting strain on the immune system and decreasing blood flow to organs, including the placenta, which can trigger contractions and delivery.

Preterm births, or births that occur between 20 and 36 weeks of pregnancy, are associated with a range of developmental delays and respiratory, vision and hearing problems and can contribute to chronic diseases into adulthood. They account for 10 percent of all births in the United States and are one of the leading causes of infant mortality.

Wildfires have intensified in the years since the study period. “2020 was about two and a half times as bad as 2008, and four of the last five years have been worse smoke than 2008,” said Sam Heft-Neal, a research scholar at the Stanford University Center on Food Security and the Environment and the lead author of the study.

The findings build on a well-established link between air pollution and fetal health problems.

To arrive at their conclusion, the researchers used satellite data from smoke plumes to identify the locations and days affected by wildfires. They paired those readings with ground-level PM 2.5 data and California birth records.

Wildfire smoke may contribute up to half of the PM 2.5 in some parts of the western United States. It is unclear whether wildfire smoke is more or less toxic than particulate matter from diesel combustion or power plants.

Rupa Basu, the chief of air and climate epidemiology at the California Environmental Protection Agency’s Office of Environmental Health Hazard Assessment, said that in addition to thinking about the effects of wildfire exposure on infant health, the effects on mothers also had to be considered.

“There’s mental health issues that go along with the stress of having a premature infant,” said Dr. Basu, who has studied the effects of climate change and environment on pregnant women and noted that preterm births could also happen more rapidly and more spontaneously than expected, adding to the potential for trauma.

Winston Choi-Schagrin is a reporter covering climate and the environment. More about Winston Choi-Schagrin

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