Shutting down power plants that burn fossil fuels can almost immediately reduce the risk of premature birth in pregnant women living nearby, according to research published Tuesday.

Researchers scrutinized records of more than 57,000 births by mothers who lived close to eight coal- and oil-fired plants across California in the year before the facilities were shut down, and in the year after, when the air was cleaner.

The study, published in the American Journal of Epidemiology, found that the rate of premature births dropped from 7 to 5.1 percent after the plants were shuttered, between 2001 and 2011. The most significant declines came among African American and Asian women. Preterm birth can be associated with lifelong health complications.

The results add fresh evidence to a robust body of research on the harmful effects of exposure to air pollution, especially in young children—even before they're born.

"The ah-ha moment was probably just seeing what a large, estimated effect size we got," said lead author Joan Casey, who is a post-doctoral fellow at UC Berkeley. "We were pretty shocked by it—to the point that we did many, many additional analyses to try to make it go away, and didn't succeed."

Coal- and oil-fired power plants emit a bevy of air pollutants that have known negative impacts on public health—including fine particulate matter (or PM 2.5), nitrogen oxides, sulfur dioxides, benzene, lead and mercury.

Using birth records from the California Department of Public Health, the researchers found mothers who lived within 5 kilometers, 5-10 kilometers and 10-20 kilometers of the eight power plants. The women living farthest away provided a control group, since the authors assumed their exposure would be minimal.

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The authors controlled for many socioeconomic, behavioral, health, race and ethnicity factors affecting preterm birth. "That could account for things like Obamacare or the Great Recession or the housing crisis," Casey said.

The study found that the women living within 5 kilometers of the plants, those most exposed to the air pollution, saw a significant drop in preterm births.

Greater Impact on African American Women

In an accompanying commentary in the journal, Pauline Mendola, a senior investigator with the Eunice Kennedy Shriver National Institute of Child Health and Human Development, wrote that the methods and creative design of the study add to its importance.

"The authors do an excellent job of testing alternative explanations for the observed associations and examining social factors that might increase vulnerability," she wrote.

Noel Mueller, an epidemiologist at Johns Hopkins University who also studies health impacts of air pollution, said one particularly notable and complicated finding was the greater impact on non-Hispanic African American and Asian women. African American women, in particular, are known to have higher rates of preterm childbirth.

"Studies like this highlight a potential role that environmental exposure might have in driving that disparity," he said. "I think that's really important."

What Happens When Air Pollution Continues

In a separate article published last week in the American Heart Association's journal Hypertension, Mueller examined what can happen when the pollution source is not eliminated.

In a study that looked at 1,293 mothers and their children in the Boston area, Mueller and his coauthors found that babies who were exposed to higher levels of particulate matter during the third trimester were significantly more likely to have high blood pressure in childhood.

Particulate matter can come from cars and the burning of coal, oil and biomass.

Casey, the author of the California study, said the findings from the two studies are related. "We know that preterm birth isn't the end of the outcomes for a child that is born early," she said.

Mueller said the same factors that can cause preterm labor, such as higher intrauterine inflammation, also could be causing higher blood pressure in children who have been exposed.

"It raises serious questions about whether we want to roll back any environmental regulations," Mueller said.

In her commentary on the California study, Mendola made a similar observation.

"We all breathe. Even small increases in mortality due to ambient air pollution have a large population health impact," she wrote. "Of course, we need electricity and there are costs and benefits to all energy decisions, but at some point we should recognize that our failure to lower air pollution results in the death and disability of American infants and children."