Convenient access to urban green space could improve resistance to diabetes, heart or respiratory diseases. Canadian researchers recently completed what’s believed to be the first large-scale study of links between residential proximity to greenery and mortality rates from those chronic illnesses, and concluded there’s evidence that municipal policy makers and land-use planners may want to consider.
“Increased amounts of residential greenness were associated with reduced risks of dying from several common causes of death among urban Canadians,” they state in their report, published in the international medical journal, Lancet Planetary Health, earlier this month. “The findings support the development of policies related to creating greener and healthier cities.”
Peer reviewers commend the work, led by Dr. Dan Crouse, an epidemiologist with the New Brunswick Institute for Research, Data and Training, for its expansive sample size of more than 1.26 million respondents to the 2001 Canadian census and follow up of 11 subsequent years of data. This measured exposure to greenery — derived from the study subjects’ postal codes and satellite images supplying what’s known as the normalized difference vegetation index (NDVI) — then linked it to Statistics Canada’s mortality database and factored in several socio-economic and ambient air quality variables.
“By linking the NDVI, which can be standardized across the world, to mortality data in a well-designed statistical study, Crouse and colleagues have contributed to further advancing the field, as it will now be possible to replicate their study in various geographical regions with different socioeconomic and cultural conditions,” Dr. Matilda van den Bosch of University of British Columbia’s School of Population and Public Health, observed in an accompanying Lancet commentary.
The Canadian research team derived average 10-year values for the impact of green space within 500 metres and 250 metres of each study subject’s residence. Risks of mortality were found to be lower in all cases for residents living within 500 metres of green space, with best result exhibited in fewer deaths from non-malignant respiratory disease and the most minimal benefit seen in deaths from cerebrovascular disease.
Various cohorts within the overall sample, which was drawn from 30 Canadian cities, also appear to enjoy greater protective benefits. Notably, men aged 35 to 74 with higher incomes who were married or in common law relationships had the most markedly better health outcomes compared to similar subjects who lived at a greater distance from green space.
“Middle age might be an etiologically important window during which living close to greenness might yield the largest benefits to health,” the researchers hypothesize.
Income-related results also proved intriguing since other studies of the links between access to green space and emotional and physiological well-being have found lower-income groups generally record the most distinctive health gains. In those cases, restorative natural environments are deemed to have incrementally less influence on stress and other physical conditions for research subjects who typically begin with a better base level of health.
In this case, Canadian researchers postulate subjects with higher incomes may have more leisure time to enjoy the outdoors and/or superior choices. “The quality of the green space around the homes of more affluent populations might be different from those around people in more deprived populations, which is a factor that we were unable to assess or describe with our existing datasets,” they acknowledge.
Subjects who lived within 500 or 250 metres of green space were benchmarked only within their own urban communities to ensure that other potential variables — climate, access to health care, etc. — matched closely. van den Bosch calls the research findings particularly telling when accounting for air pollutants.
“Crouse and colleagues were able to adjust for several environmental covariates, including PM2.5 (fine particulate matter), ozone and nitrogen dioxide, but noted these only had a small confounding effect on the green space hazard risks. Thus, in this case, it seems as if greenness has a protective effect that is independent of air quality,” she notes.
However, researchers do underline a few intangibles. The NDVI describes the presence and amount of vegetation at a given location, but not whether it is publicly accessible and/or easily navigable. Nor does the research focus, which is solely on place of residence, contemplate subjects’ possible exposure to greenery in their daily travels and workplace or school settings.
Self-selection among the research subjects could also colour results if, for example, it can be shown that people who have healthy lifestyles are more predisposed to choose homes with convenient access to green space. Both the research team and peer reviewers concur that further exploration is warranted.
“Future studies should build on the approaches of Crouse and colleagues to assess the potential of urban green spaces as inherently health-promoting assets in cities,” van den Bosch maintains.