Long-term exposure to traffic-related pollution may significantly increase the risk of asthma in early childhood, a study warns.
The study, published in the Journal of Allergy and Clinical Immunology, analysed data from 1,522 Boston-area children born between 1999 and 2002.
"Our previous research demonstrated that living close to a major roadway and lifetime exposure to air pollutants were associated with lower lung function in seven- to ten- year-old children," said Mary B Rice from Beth Israel Deaconess Medical Center (BIDMC) in the US.
"We suspected that these exposures would also be associated with pediatric asthma," said Rice.
Researchers used mapping technologies to determine the distance between each child's home address and the nearest major roadway.
They also linked home addresses to census data and satellite-derived atmospheric data to calculate each participant's daily exposure to fine particulate matter (PM) - tiny particles suspended in the air that when inhaled deposit in the terminal sacs of the lung.
Fine PM originates from fuel combustion, including traffic, power plants, and other pollution sources.
The study found that living close to a major road was linked to childhood asthma at all ages examined.
"Children living less than 100 meters from a major road had nearly three times the odds of current asthma - children who either experience asthma symptoms or use asthma medications daily - by ages seven to 10, compared with children living more than 400 meters away from a major road," said Rice.
"Even in the Boston area, where pollution levels are relatively low and within Environmental Protection Agency standards, traffic-related pollutants appear to increase the risk of asthma in childhood," said Rice, who is also an assistant professor at Harvard Medical School.
Researchers said that lifetime exposure to black carbon and fine PM were also linked to the asthma in early childhood (ages three to five years).
However, in mid-childhood (ages seven to 10 years), these pollutants were associated with asthma only among girls.
"Younger children spend a larger proportion of their time at home than school-aged children, and their airways are smaller and may be more likely to wheeze in response to pollution," said Rice.
"This may explain why pollution exposure was most consistently linked to asthma in young children," he said.
The stronger link between lifetime pollution and asthma among school-age girls was somewhat surprising, researchers said.
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