Elsevier

Environmental Research

Volume 115, May 2012, Pages 51-58
Environmental Research

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

https://doi.org/10.1016/j.envres.2012.03.003Get rights and content

Abstract

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.

Highlights

► Few studies, primarily ecological, have evaluated the relationship between green space and mortality. ► We studied associations between green space and death in a 22 year follow-up of 575,000 adults. ► Green space was assigned using the remote-sensing derived Normalized Difference Vegetation Index. ► Green space was inversely associated with mortality, particularly, for non-malignant respiratory disease. ► Additional research is needed to confirm these findings, and evaluate the potential role of self selection bias.

Introduction

Urban parks and green space areas are important as they improve the quality of life for urbanites, (Maas et al., 2006, Mitchell and Popham, 2007) and can produce tangible health benefits (Handy et al., 2002, Hansmann et al., 2007, Hartig et al., 2003, Pretty et al., 2005, Takano et al., 2002, Ulrich, 1984, van den berg et al., 2007). Of its many advantages, green space has been shown to reduce stress and headaches, (Hansmann et al., 2007, Hartig et al., 2003) positively influence recovery from surgery, (Ulrich, 1984) moderate temperatures by producing shade and cooling, (Cummins and Jackson, 2001) absorb ambient air pollution, (Nowak et al., 2006) and provide opportunities for individuals to participate in physical activity (Handy et al., 2002, Pretty et al., 2005, van den berg et al., 2007). Physical activity in green spaces appears to be more beneficial to people on emotional, cognitive and physiological levels when compared to activity done in non-green urban environments (Hansmann et al., 2007, Hartig et al., 2003, Pretty et al., 2005). Increased access to green space and parks may play a role in developed countries to fight the epidemics of obesity and lack of exercise (Carlson et al., 2008, Finucane et al., 2011).

A series of cross-sectional European studies found that a higher percentage of green space was associated with improvements in self-perceived health, (de vries et al., 2003, Maas et al., 2006) and with lower mortality, particularly for circulatory disease (Mitchell and Popham, 2008). However, these studies did not evaluate directly the long-term benefits of green space. As far as we know, the only longitudinal study that has investigated associations between green space and mortality found improved five-year survival rates among elderly residents who lived near parks and tree-lined spaces (Takano et al., 2002).

Herein, we investigated the association between green space and mortality in an urban cohort of approximately 575,000 adults who were followed prospectively for up to 22 years. We investigated associations between green space and mortality from all non-accidental causes, cardiovascular and respiratory diseases for which there are plausible biological mechanisms whereby green space may play a role in conferring a health benefit, such as improving air quality (Nowak et al., 2006), as well as reducing stress, and providing greater opportunities for participation in leisure-time physical activities (Lee and Maheswaran, 2011), all of which are accepted risk factors for the health outcomes investigated in this paper.

Section snippets

Study population

The cohort was assembled by randomly selecting Canadians who resided in one of 10 urban areas in the province of Ontario (Hamilton, Kingston, London, Ottawa, Sarnia, St. Catharines, Sudbury, Thunder Bay, Toronto, Windsor). The primary objective of the cohort study was to evaluate associations between long term exposure to ambient air pollution and mortality, and these ten urban areas were selected because they differed with respect to ambient concentrations of fine particulate matter. The

Results

Values of NDVI were assigned to a total of 574,840 subjects in the cohort. A total of 186,990 deaths were identified over the 22-year follow-up (Table 1). Within our study population, the value of the interquartile range for the NDVI for the 30 m buffer was 0.38, and was 0.24 for the 500 m buffer. The Pearson correlation coefficient between the two different spatial resolutions of green space was 0.68. Those who were married and those with higher household incomes were more likely to reside in

Discussion

Our findings reveal that individuals who lived in areas with more green space had lower mortality rates over a follow-up interval that spanned nearly two decades. Detailed analysis on a subset of the cohort suggests that this finding is not confounded by ambient air pollution concentrations of NO2 and PM2.5 at intra-urban and inter-urban levels, respectively. The inverse associations between green space and mortality persisted after adjusting for a variety of sociodemographic and neighborhood

Acknowledgments

We are grateful to Lana Marjama of Statistics Canada for overseeing the record linkage of the study, and to Sonia Demers and Hung Pham for their assistance with the management of data from the T1 Family File database. The authors have no conflicts of interest to declare.

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    Funding: Funding for the research was provided by the Canadian Institutes for Health Research and Health Canada.

    Ethics approval: This study was approved by the Research Ethics Boards at the University of Toronto, and Health Canada.

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