• Hind Sbihi, Mieke Koehoorn, Lillian Tamburic, and Michael Brauer.
• School of Population and Public Health, University of British Columbia, Vancouver, British Columbia, Canada.
• Am. J. Respir. Crit. Care Med. 2017 Mar 1; 195 (5): 607-613.

RationaleThe heterogeneity of asthma phenotypes may explain inconsistencies in observed associations with environmental exposures.ObjectivesTo identify trajectories of childhood asthma and to characterize the potential impact of residential greenness and air pollution on asthma trajectory subgroups.MethodsLinked administrative databases of medical visits were used to define the occurrence and recurrence of asthma over a 10-year follow-up period within a population-based birth cohort of more than 65,000 children. Group-based trajectory modeling was used to identify unique asthma trajectories. Weighted multinomial regression was used to assess the relationship between asthma trajectories and risk factors, including environmental exposures.Measurements And Main ResultsGroup-based trajectory modeling distinguished four trajectories: one with no asthma representing 88.8% of the cohort, one with transient asthma (5.6% of the cohort), and two trajectories with chronic asthma with early (<1 yr; 1.5%) and late (<3 yr; 4.1%) onset during early childhood. These trajectories differed with respect to socioeconomic markers and modifiable risk factors, including maternal smoking and breastfeeding initiation. After accounting for sex, parity, breastfeeding, term birth weight, household income, maternal education, delivery mode, and smoking, an interquartile increase in nitrogen dioxide exposure increased the risk of membership in the early and late-onset chronic asthma trajectories, relative to subjects without asthma, by 50% and 20%, respectively (weighted risk ratio, 1.5 and 1.2; 95% confidence interval, 1.2-1.9 and 1.0-1.4). Greenness was not associated with any of the asthma trajectories.ConclusionsTraffic-related air pollution increased the probability of a chronic asthma trajectory.

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