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A study of more than 2.5 million pregnant people in California found that those exposed to wildfire smoke for at least one day faced a higher risk of giving birth prematurely.

The findings were presented Saturday at the Society for Maternal-Fetal Medicine’s annual meeting and are currently undergoing peer review. They’re set to be published in the American Journal of Obstetrics and Gynecology.

The researchers collected data from hospital records of pregnant people from 2007 to 2012, then analyzed daily estimates of wildfire smoke in the participants’ ZIP codes during their pregnancies, based on satellite images.

The results suggested that just one day of smoke exposure slightly raised the risk of spontaneous preterm birth — defined as before the 37th week of pregnancy. But the odds of preterm birth increased by 0.3% with each additional day of smoke exposure.

“Most pregnant persons are having well over one day of exposure, and the chronicity of this exposure, which continues to increase, is really the worrisome relationship with wildfire smoke,” said Dr. Anne Waldrop, the study’s lead author and a maternal-fetal medicine fellow at Stanford University.

Waldrop’s team estimated that 86% of the pregnant people studied in California had been exposed to fine particulate matter from wildfire smoke during their first or second trimesters, or during the four weeks leading up to conception. On average, the study participants were exposed to more than seven days of smoke during that time.

Exposure was only associated with preterm birth within the first 20 weeks of pregnancy, however, not before conception.

The term fine particulate matter refers to tiny particles in the air that are less than 4% of the diameter of a human hair. Once inhaled, the particles can penetrate into the lungs and enter the bloodstream. They can increase a person’s risk of asthma, lung cancer and other chronic lung diseases, particularly among vulnerable groups like older people, infants and children, and those who are pregnant.

“Not all smoke exposures are these dramatic smog- and smoke-filled days,” Waldrop said. “Most likely a lot of these [people] didn’t recognize the days that they were being exposed.”

Since smoke from fires in the West can travel thousands of miles, she added, “it’s not an isolated issue with the Western United States anymore.”

Dr. Leonardo Trasande, a pediatrics professor at New York University who researches children’s environmental health, said the risk of preterm birth could worsen over time as wildfires become more frequent and intense.

“Insofar as wildfires are a byproduct of climate change, it’s common sense to put two and two together and ultimately relate preterm births to climate change,” he said.

Trasande’s own research has focused on the link between air pollution and preterm births. He previously estimated that nearly 16,000 preterm births in the U.S. in 2010 alone were attributable to exposure to air pollution.

On a global scale, another analysis found that air pollution likely contributed to nearly 6 million premature births in 2019. Rakesh Ghosh, the author of that analysis and an epidemiologist at the University of California, San Francisco, said that exposure to wildfire smoke during the first trimester can cause the amniotic sac to rupture early due to inflammation.

“Any abnormality at that stage, in the early stage of the development of the placenta, can cause a very long-term condition,” he said.

Premature birth is a risk factor for death in the first 28 days to one year of an infant’s life. Research also suggests that preterm birth can be associated with developmental delays in childhood.

Trasande said it’s difficult to compare the risk of wildfire smoke to other forms of air pollution, say from cigarettes or vehicles.

“What distinguishes certain types of smoke is what’s being burned,” he said. “We know, for example, that, increasingly, plastic is a component of fires, period. And wildfires lead to damaged homes, which lead to emissions of what’s burned in those homes.”

Waldrop said emerging evidence suggests wildfire smoke might be particularly harmful because it contains many different pollutants, including carbon monoxide.

“There’s actually now data that suggests that wildfire smoke pollutants can be more biologically active and worse for human health than other forms of pollutants,” she said.

Aside from leaving an area where there’s wildfire smoke or staying indoors with an air filter running, it’s difficult to avoid exposure, health experts said. N95 masks may help prevent high levels of exposure, Ghosh said, though experts disagree as to how effective masks can be in smoky air.

Waldrop said it’s also hard to determine the precise line beyond which smoke becomes a serious health threat to an individual.

“What are the thresholds for smoke exposure that are safe? That’s really not clearly defined,” she said.

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