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Greenness and Incident Childhood Asthma: A 10-Year Follow-up in a Population-based Birth Cohort.

Reduced access to greenspace for children has been linked to higher relative prevalence of asthma (1), and increased access to greenspace has been linked to higher physical activity (2), which is itself associated with decreased asthma development. However, in a birth cohort study conducted in New York, asthma at age 7 years and sensitization to tree pollen were significantly increased in those residing on streets with higher tree canopy coverage (3). We report on the effect of early-life exposure to surrounding residential greenness on asthma incidence, using data from a 10-year follow-up of more than 65,000 children in a population-based birth cohort study in British Columbia, Canada. Linked administrative data were used to establish the study cohort comprising all 1999–2002 single births in the metropolitan area of Vancouver, British Columbia, to mothers registered with the provincial universal health insurance program and who resided for their complete pregnancy in the study area (4). Individual covariates obtained through administrative records (5, 6) include the month/year of birth, sex, First Nations status, and maternal parity, age, smoking during pregnancy, and initiation of breastfeeding. Participants were assigned neighborhood-level socioeconomic indicators (household income and maternal education) from Census data, based on their residential census dissemination area (7), a unit representing a target population of 400–700 persons. Using linked physician billing and hospital discharge records (8), children were defined as cases after a validated case definition of asthma (9) of two physician diagnoses in a rolling 12-month period or a minimum of one hospital admission. Cases were sexand age-matched to five randomly chosen controls. Using a nested conditional logistic regression, incident asthma during preschool-age (0–5 yr old) and school-age (6–10 yr old) periods was examined in relation to surrounding greenness based on a satellite-derived normalized difference vegetation index (NDVI). As seasonal values were highly correlated (r. 0.9), average yearly greenness indices during pregnancy were calculated for 100-m areas around residential postal codes (4). This buffer size typically reflects a city block and is at a similar spatial scale as within-city air pollution variation for which exposure estimates were developed and evaluated in the study region. The mean (SD) age at time of incidence was 2.6 (1.4) and 7.0 (1.2) years in the preschool (6,948 cases) and school age (1,711 cases) groups, respectively (see Table E1 in the online supplement). Average greenness levels in metropolitan Vancouver were relatively high (median NDVI = 0.24) and inversely correlated with land use regression models estimates of air pollution (r 20.4; P = 0.07) (4). In adjusted models, we observed during preschool years a decreased risk of asthma per interquartile (0.11) NDVI increase (adjusted odds ratio [aOR] = 0.96; 95% confidence interval [CI], 0.93–0.99). Accounting for contemporaneous co-exposures to air pollutants and road proximity measures enhanced the effect of greenness (Figure 1). No such associations were observed during the school age period (see Figure E1 in the online supplement). Suggestive evidence of effect modification by birth weight was indicated, as surrounding greenness reduced the odds of new asthma onset during preschool years by 44% for children with term birth weight ,2,500 g compared with those weighing >2,500 g at birth. Children born to mothers with low educational attainment were also more likely to benefit from greenness exposure (aOR = 0.92; 95% CI, 0.86–0.97). However, no effect modification by neighborhood-level household income was found (Table 1). The lack of extensive individual level covariate data remains a limitation; however, this may be balanced by the large population-based study design. Although pediatric asthma diagnosis is challenging, especially in very young children, ascertainment bias is unlikely, as 77% of children diagnosed during preschool years remained asthmatic in later childhood. The inverse association between surrounding greenness and development of asthma in preschool-age children may support a hypothesis that biodiversity loss (10), reflected for instance by less interaction with natural spaces, leads to immune dysfunction, inflammation, and asthma. Stress reduction may be another pathway by which greenness influences asthma development. As the protective effect of greenness was enhanced in models including air pollution exposure, despite the absence of a significant interaction term between these covarying exposures, the positive confounding by air pollution may represent a case of variance suppression. Our observations of an association between greenness and incident asthma among preschool children are unlikely to be mediated by increased physical activity, as no association was found between incident asthma and distance to the nearest park (aOR = 0.98; 95% CI, 0.95–1.00). Interaction models confirmed an effect of surrounding greenness that was independent of park proximity. The possible protective effect of surrounding greenness on children weighing less than 2,500 g and for those born in areas of lower socioeconomic status raises the prospect of preventive public health interventions during pregnancy. In one of the largest birth cohort studies to examine within-community NDVI-based contrasts, we found that the The Border Air Quality Study was supported in part by Health Canada via an agreement with the British Columbia Centre for Disease Control. Additional support was provided by the Centre for Health and Environment Research at the University of British Columbia, funded by the Michael Smith Foundation for Health Research. H.S. was funded by the Canadian Institutes of Health Research Banting and Best Doctoral Award.