Fine particle air pollution, which can be inhaled deeply, is linked to the greatest health risks, researchers cautioned. These particles can be found in smoke and haze.
"We know that mothers' exposure to air pollution during pregnancy can affect lung development of their babies and lead to subsequent respiratory disorders, including asthma, although little is known about whether timing of the exposure is important to consider," said the study's lead author, Yueh-Hsiu Mathilda Chiu, from the department of pediatrics at Icahn School of Medicine at Mount Sinai in New York City.
"In our study, we assessed whether higher exposure to particulate air pollution at more specific time windows in pregnancy were particularly linked to higher asthma risk in urban children," Chiu said in an American Thoracic Society news release.
The researchers followed over 400 children to age 7 years and their mothers. Daily exposure to air pollution from traffic, power plants and other sources in the prenatal period was estimated based on where the mothers lived.
The study, scheduled for presentation Monday at the annual meeting of the American Thoracic Society in San Diego, revealed exposure to higher levels of fine particles in the second trimester of pregnancy had the strongest association with greater prevalence of asthma. This was particularly true for the children born to mothers who were not obese, the researchers found.
"It is possible that the effect of maternal obesity, another known risk factor of childhood asthma onset, may be so strong that it was difficult to determine additional effects of air pollution among children born to obese mothers in this setting," the researchers wrote.
Dr. Rosalind Wright, the study's senior investigator, said it's important to "continue to improve air quality and minimize exposure to pregnant women throughout the entire pregnancy for a host of health reasons. [But] pinpointing the gestational period during which air pollution has the greatest effects on the developing lung may add to our understanding of the mechanisms underlying this relationship," she said in the news release.
Data and conclusions presented at meetings are typically considered preliminary until published in a peer-reviewed medical journal.
The U.S. Environmental Protection Agency has more about particle pollution.