Babies born to mothers exposed to air pollution from traffic sources had an increased risk for developing asthma during their first 5 years of life, according to a study of Canadian children followed from birth.

In one of the largest population-based, birth cohort studies ever to examine within-community pollution differences, researchers found a 25% increased odds of developing asthma in children of mothers living near highways during pregnancy.

Asthma risk increased with greater exposure to the vehicle emission pollutants nitrogen dioxide and carbon monoxide, and children with low birth weights had a greater asthma risk than those whose weight was normal at birth, Hind Sbihi, a PhD candidate at the University of British Columbia in Vancouver, and colleagues. Children diagnosed with asthma between the ages of 6 and 10 did not show consistently elevated asthma risk associated with in utero vehicle emissions exposure, they wrote in the European Respiratory Journal.

"Air pollution from traffic sources increased the risk of developing asthma during early years before children reached school age, even in an urban area with relatively low levels of air pollution," Sbihi noted in a written press statement.

The study included more than 65,000 children born in Vancouver from 1999 to 2002, who were followed from birth until age 10 through linked administrative health databases.



Children with asthma were sex- and age-matched to five randomly chosen controls and associations between in utero exposure to air pollutants and incident asthma before age 5 and in the early school years were estimated using conditional logistic regression.

Assessment of maternal exposure to air pollution during pregnancy was determined using land use regression models, which linked traffic-related air pollution monitoring with the mother's home address. Measurements of air pollutants from monitoring stations near the mother's residence were also included in the analysis, as were assessments of residential proximity to a major road or highway.

The average age at first asthma diagnosis among offspring was 2.6 years (1.4 years) for children who met the case definition of asthma between birth and age 5 and 7 years (1.2 years) for those diagnosed during their early school years.

Compared to children in the cohort who did not develop asthma, those who did were more likely to be born to younger mothers, have shorter gestation periods, and smaller birth weights. They were also more likely to live in poverty and less likely to have been breastfed.

Following adjustment for birth weight, gestational period, household income parity, breastfeeding at discharge, and maternal age and education, asthma risk during the preschool years was found to be increased by exposure to nitric oxide (NO, adjusted OR using interpolation per interquartile increase=1.06, 95% CI 1.01-1.11), nitrogen dioxide (NO2, 1.09, 95% CI 1.04-1.13), and carbon monoxide (CO, 1.05, 95% CI 1.01-1.10).

Positive associations were also seen with exposure to sulfur dioxide (So2) and particulate matter 10 micrometers or less in diameter (PM10). The researchers noted that findings from several other recent birth-cohort studies examining the role of air pollution on early asthma incidence have been mixed. A study from Britain found no association between NO2 and PM10 exposure and asthma in children followed until the age of 11, while a Swedish birth-cohort study found a positive association between air pollution exposure and asthma, but only in older children.
However, a Dutch study published in 2010 showed a positive association between NO2 and asthma at the ages of 2 and 4.

A study limitation was the lack of data on exposures in micro-environments other than the home during pregnancy, which could have lead to potential exposure misclassification. The authors also noted that "in the absence of linked residential histories throughout the follow-up period, no formal comparison of pregnancy and postnatal exposures was conducted."

The researchers found that preschool-age children who weighed less than 2,500 g (about 5.5 lbs) at birth had a consistently higher risk for asthma associated with air pollution exposure, as did children born to nulliparous mothers.

"This observation may reflect the importance of the in utero environment, given evidence of reduced proliferative responses in cord blood mononuclear cells with increasing parity," they wrote.