A cohort study relating urban green space with mortality in Ontario, Canada

Parks and green space areas are important to human health for psychological and physiological reasons. There have been few evaluations of access to green space on mortality. This paper describes a cohort study of approximately 575,000 adults, 35 years of age and older, who resided in 10 urban areas in Ontario, Canada, between 1982 and 1986. Individuals were identified from income tax filings, and vital status was determined up to December 31, 2004 through record linkage to the Canadian Mortality Data Base. Place of residence was defined by postal code data that were extracted from income tax filings. Urban green space was defined by Landsat satellite retrievals with the Normalized Difference Vegetation Index and this was assigned to individuals' place of residence at inception into the cohort using both a 30 m grid cell and a 500 m buffer. The proportional hazards model was used to estimate rate ratios (RRs) and their corresponding 95% confidence intervals (CI) for selected underlying causes of death. The rate ratios were adjusted for income, marital status, ambient air pollution, and contextual neighborhood characteristics. About 187,000 subjects died during follow-up. An increase in the interquartile range of green space, using a 500 m buffer, was associated with reduced non-accidental mortality (RR=0.95, 95% CI=0.94–0.96). Reductions in mortality with increased residential green space were observed for each underlying cause of death; the strongest association was found for respiratory disease mortality (RR=0.91, 95% CI=0.89–0.93). Risk estimates were essentially unchanged after adjusting for ambient air pollution. Our study suggests that green space in urban environments was associated with long-term reduction in mortality although this finding should be interpreted cautiously as this association may be influenced by residual confounding of sociodemographic and lifestyle factors. Further research is needed to: confirm these findings, better understand the relationships between access to green space and behavioral risk factors for mortality, and identify what green space characteristics may confer the greatest health benefit.