Findings from published research suggest that exposure to and interactions with green spaces are associated with improved psychological wellbeing and have cognitive, physiological and social benefits, but few studies have examined their potential effect on the risk of mortality.

Publishing their findings in the October issue of The Lancet Planetary Health, Dr Dan Crouse, an epidemiologist and sociology research associate at the University of New Brunswick, and a team of North American researchers set out to see how everyday exposures to greenness affected mortality patterns.

“We found shockingly protective effects associated with increased exposure,” said Dr Crouse. “The size of effects we found were much stronger than what we expected.”

The researchers found that living in greener areas in Canadian cities was associated with an 8–12 per cent reduced risk of dying from several common causes of death, including cardiovascular and respiratory diseases.

The study, ‘Urban greenness and mortality in Canada’s largest cities: a national cohort study’, used a national cohort of 1.3 million Canadians living in 30 cities across the country over an 11-year period. It measured greenness with images from NASA’s Aqua satellite, and derived estimates of exposure to greenness from the remotely sensed Normalized Difference Vegetation Index (NDVI) – said to be the most widely used satellite-derived indicator of the quantity of green vegetation on the ground.

Taking into account the amount of trees, plants, shrubs, and various other vegetation within 250 metres of each individual’s home, the researchers created estimates of daily “greenness” exposure. Personal characteristics of the subjects were taken into account, as well as data related to the communities where they lived, such as socioeconomic makeup and environmental characteristics – for example, air pollution and population density.

Income and education factors
Interestingly, researchers found that the protective effects of green space exposure weren’t the same for everyone. People on higher incomes and with more education tended to live in the greenest urban areas, and the researchers also found notably more protective effects among these groups. This suggests that more affluent and better educated people tended to experience not only greater exposures to greenness, because they could afford it, but also greater benefits to their health.

For these high-income individuals, more green means cleaner air, less noise from traffic, and cooler temperatures, which all leads to reduced stress, better sleep and higher levels of mental functioning.

In contrast, findings showed that low-income individuals with the same amounts of exposure to greenness would not necessarily receive the same benefits. Among the poorest people, greenness seemed to offer no benefit and no reduction in risk of dying; however, further study would be necessary to ascertain the reasons why this is the case.

“The idea that urban green space may not benefit everyone equally could have some interesting implications for public health policy,” said Dr Crouse.

Implications
Such implications may relate to both the quantity and quality of green spaces in different parts of cities, or to facilitate more equal access and potential exposures. The findings may also be of interest to urban and city planners who are responsible for building and managing cities’ green spaces; for example, planting new trees throughout a city, on medians and along sidewalks, could produce benefits to health different from those associated with expanding parks or other localised green spaces.

The study didn’t look at the effects of green space on people living in rural areas, as urbanites tend to be healthier than rural dwellers because of higher levels of education, lower smoking rates and body mass indexes, and more access to jobs and services.

“What we were trying to get at with this study was how greenness might act as a buffer against the stresses of urban living, like air pollution, traffic, noise and heat,” explained Dr Crouse. “The study suggests that there’s a real health benefit to general community greening. That investment certainly seems worth it if it can be made wisely.”