Slightly more than 18 percent of the children born to these mothers developed asthma in their preschool years, compared to 7 percent of children overall in the United States identified as having asthma by the Centers for Disease Control and Prevention.

Other types of pollutants are routinely monitored and regulated to reduce potential health effects, such as larger-size particulate pollution and gaseous pollutants such as nitrogen dioxide. These have been associated with asthma risk in children in prior research. This study controlled for exposure to these other types of pollution and exposure to pollutants following birth, and it still found an elevated risk of asthma in children born to mothers with heightened exposure to ultra-fine particles in pregnancy.

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