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When Rosalind Wright started analyzing data on prenatal exposure to air pollution in mothers and children in Boston, she had a notion that ultra-fine particles could be even more harmful than the slightly larger particles she’d studied before.

“If the effect is strong enough, we’ll find it,” Wright, a pulmonary physician and professor of pediatrics at the Icahn School of Medicine at Mount Sinai, remembered thinking.


They did: In recently published results in the American Journal of Respiratory and Critical Care Medicine, she and her colleagues reported that children whose mothers had prenatal exposure to ultra-fine particle pollution, which is not subject to federal regulation, were at an increased risk of being diagnosed with asthma.

Factors from air pollution to smoking to drug use can shape fetal and childhood development, but the new study is one of the first to specifically link ultra-fine particle exposure in utero to a higher chance of kids developing asthma.

Ultra-fine particles are less than 0.1 microns in diameter; a typical human hair is about 50 to 80 microns. They can come from a number of sources, including car exhaust, industrial processes, burning wood or coal, and airplanes taking off and landing. Unlike other air pollutants and slightly larger particles, they aren’t regulated by the Environmental Protection Agency — and their minuscule size amplifies their danger, Wright said. 


Ultra-fine particles “can get deeper down into the lungs. And it also then can more readily pass through the lungs into the bloodstream,” said Wright, the study’s lead author. The particles not only cause inflammation in mothers’ lungs, she said, but are able to get through the placenta to the developing fetus.

Wright and her colleagues examined data from 376 pregnant women living in Boston that they had already been following for years as part of other studies, as well as the health records of their children, with follow-up every four months between birth and 30 months, and every year after. Most of the asthma diagnoses were made just after age 3. Wright worked with Tufts University researchers, who used special monitors to detect the particles around Boston, including a mobile laboratory that they could drive to different communities. 

They found that every time the number of ultra-fine particles in the air doubled, the odds were about four times greater that a mother who was exposed to the pollution during pregnancy would give birth to a child who would be diagnosed with asthma. Overall, 18% of the children in the study developed asthma, more than twice the incidence of childhood asthma in the U.S., which is 7%. Of the women in the study, many of whom lived near roadways with heavy traffic, 38% were Black, 44% were Hispanic, and 53% had lower than a high school education.

Wright said further research is needed to see how ultra-fine particles might affect mothers and children in different places, for instance in parts of the western U.S. that are affected by wildfires. In addition, she said there should be more study of how the pollution could disproportionately impact people of color.

“We want to understand, why are they at higher risk, what are the environmental factors that might be driving that so that we can hopefully minimize and reduce that going forward?” said Wright.

Wright and her team’s research builds upon a 2019 study in Toronto that looked at data on more than 160,000 mothers and children and used a similar model to measure ultra-fine particle pollution. The Canadian scientists reported theirs was the first study to evaluate the association between prenatal exposure to ultra-fine particles and childhood asthma.

“Every time the amount of exposure to ultra-fine particles was increasing by 10,000 [particles], there was an additional 5% risk of asthma,” said Éric Lavigne, a senior epidemiologist with Health Canada, an adjunct professor in the school of epidemiology and public health at the University of Ottawa, and the lead author of the study. 

Lavigne and his colleagues adjusted their model to control for the effects of other air pollutants, like slightly larger particles called PM2.5s and nitrogen dioxide, as did Wright and her team. Wright’s study also adjusted for temperature. Both studies found that prenatal exposure to ultra-fine particles was separately associated with childhood asthma. Lavigne’s study found a particular risk from exposure during the second trimester of pregnancy; Wright’s results may indicate a slightly heightened risk during the third trimester. 

Other research has found an association between prenatal exposure to PM2.5s and harmful gases like sulfur dioxide and increased incidence of asthma in children. One study published last month examined children who were born in and spent their first year of life in Sarnia, a city in Ontario that’s home to Canada’s “Chemical Valley,” an area in which 40% of Canada’s chemical processing takes place. The researchers compared the rates of childhood asthma there to two other areas in Ontario with less air pollution, and found that children born in Sarnia were more likely to have asthma.

“It was the sulfur dioxide, and to a lesser extent, the PM2.5, that seems to be correlated with differences in rates of asthma between the cities,” said Dhenuka Radhakrishnan, a pediatric respirologist at Children’s Hospital of Eastern Ontario, an assistant professor of pediatrics at the University of Ottawa, and the lead author of the study.

Gretchen Goldman, research director for the Center for Science and Democracy at the Union of Concerned Scientists, led a 2017 study on air pollution exposure in seven communities in an industrial corridor around Wilmington, Del., which had larger Black and Hispanic populations and/or higher poverty levels compared to the state as a whole. She and her colleagues reported that these communities were exposed to significantly more air pollution, which included particulate matter from diesel, than a wealthy, predominantly white community near Wilmington called Greenville and than state residents overall. Although the study did not look at asthma specifically, it found that residents of these communities were at a higher risk of cancer and respiratory health hazards. 

“The environmental justice impacts of air pollution are well-documented,” Goldman said.

Goldman said some of this disparity can be linked directly to lower income, since neighborhoods farther away from sources of pollution tend to cost more to live in, but it also isn’t that simple.

“This is because of a variety of issues,” she said, “some of them being systemic racism, and the siting of pollution sources in communities of color, or lower rates of enforcement in these places.” In particular, Goldman said, redlining of neighborhoods has left many communities of color located closer to sources of pollution, whether that is highways, landfills, or chemical plants.

Dora Williams, a community activist with the Delaware Concerned Citizens for Environmental Justice, said she isn’t at all surprised to see an association between exposure to air pollution during pregnancy and childhood asthma. Williams, who is Black, lives in New Castle, Del., off a major highway and near multiple sources of air and water pollution. Her community was one of the ones examined in the study Goldman and others conducted. Williams said her granddaughter has a dust allergy that requires her to use an inhaler, and a friend’s child spent much of her early life in the hospital because of severe asthma, although no one else in the girl’s family had the condition.

“These kinds of things kept happening with people that we knew and lived in certain areas,” said Williams.

Wright emphasized that before any sort of regulation of ultra-fine particles can happen, there needs to be more research like her team’s that shows the potential harms. 

“The hope is really to build the evidence base that says, hey, we have to routinely monitor this [pollution], invest in the technology to do so, and to start looking at this,” she said. 

Asthma is “the most common chronic childhood condition in the world,” said Radhakrishnan. “Kids can have very mild and minimal symptoms, but some children really suffer with significant symptoms and recurrent visits to hospital to treat exacerbations.” 

Wright agreed. “Childhood asthma is one of the most costly diseases of pediatrics,” she said. 

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