Prior findings of neighborhood greenness's protective effect against asthma may not have considered differing asthma phenotypes

Childhood asthma is a complex disease exacerbated by such environmental exposures as air pollution. Another type of exposure – residential greenness -- may have a protective effect against asthma in the first 5 years of life. The purpose of this investigation was to identify trajectories of childhood asthma and to further understand the potential impact of residential greenness and air pollution on asthma trajectory subgroups.

Data for this study included information about the annual occurrence and re-occurrence of asthma symptoms over a ten-year period of more than 65,000 children born between 1999 to 2002 to mothers living in the metropolitan area of Vancouver, British Columbia. Group-based trajectory modeling – a form of statistical analysis – identified four childhood trajectories: one with no asthma (representing 88.8% of the cohort), one with transient (temporary) asthma (5.6%), and two trajectories with chronic asthma during early childhood. The chronic asthma trajectories consisted of early (after birth) onset (1.5%) and late (after 3) onset (4.1%). Additional analysis of the data assessed the relationship between asthma trajectories and known risk factors for asthma, including air pollution and residential greenness.

Children in the non-asthma trajectory were in families with significantly higher socioeconomic status compared with the three other asthma trajectories. Children in both chronic asthma trajectories were more likely to be born to a mother who had not given birth to previous children. Exposure to nitrogen dioxide (present in car exhaust) increased the risk of having chronic asthma. Exposure to residential greenness showed a modest effect on children with transient asthma and no associations with those with chronic asthma. This finding doesn't necessarily contradict earlier research, in that previous studies suggesting a protective effect against asthma in the first five years of life did not distinguish between asthma trajectory subgroups.

Since traffic-related air pollution is a predictor of persistent asthma in children, public health interventions would do well to include initiatives that reduce traffic in residential areas.