Researcher found a link between the spatial distribution of urban greenspace and mortality risk

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Some community parks are square, a reflection of the city block where they’re located – but irregularly shaped parks reduce the mortality risk of residents who live near them, concluded a study by Huaquing Wang, a Ph.D. Urban and Regional Sciences student and Lou Tassinary, professor of visualization.

“Nearly all studies investigating the effects of natural environments on human health are focused on the amount of a community’s green space,” said the scholars in a paper describing their project.

“We found that the shape or form of green space has an important role in this association.”

Their paper was published in the Nov. 2019 issue of The Lancet Planetary Health.

In the study, Wang and Tassinary performed statistical analyses of Philadelphia land cover data to assess links between landscape spatial metrics and health outcomes.

This is a drawing of different shaped parks

They found that residents in census tracts with more connected, aggregated, and complex-shaped greenspaces had a lower mortality risk. The image is credited to Texas A&M.

They found that residents in census tracts with more connected, aggregated, and complex-shaped greenspaces had a lower mortality risk.

“Our results suggest that linking existing parks with greenways or adding new, connected parks might be fiscally accessible strategies for promoting health,” said Wang and Tassinary.

“We showed that the complexity of the park shape was positively associated with a lower risk of mortality,” they said in the paper.

“This association might be attributable to the increased number of access points provided by complex-shaped green spaces.”

Irregularly shaped parks are either designed that way or shaped by the parcel they’re established in, said Wang.

Lower mortality risk wasn’t associated with any particular form, but the data supports the idea that the more complex the park shape, the better, she said.

The relationship between park shape and mortality is important to city designers and planners who seek to create healthier living environments, they said in the paper.

“Our findings bring us closer to understanding the mechanisms underlying the protective effects of green space on mortality,” they said.


Discussion

Our findings show that residents who live in urban areas characterised by a larger total percentage of greenspace and larger mean area of greenspace seem to have a reduced risk of all-cause mortality, and lower fragmentation of greenspace, better connectivity and aggregation of greenspace distribution, and increased complexity of greenspace shape also seem to decrease the risk of deaths from heart disease, chronic lower respiratory disease, and neoplasms.

These significant relations were identified by analysis of high-resolution landcover data, established (yet generally unfamiliar) landscape metrics, and valid health outcomes from reliable data sources.The negative association we recorded between the total percentage of greenspace area (PLAND) and all-cause mortality was consistent with previous studies. We did not note any relation between PLAND and deaths from heart disease, lower respiratory disease, or neoplasms.

This finding was expected because studies have long been reporting conflicting results when using PLAND in capturing greenness to predict health. Some studies have shown significant inverse associations whereas others have reported null findings.5222324 

Our study showed that measures of greenspace morphology predict mortality independent of greenness, because all statistical models in our analysis simultaneously controlled the total area of greenspace and total land area of a census tract.

We believe these underlying greenspace morphology characteristics, which were not captured in previous studies, might help to reconcile those conflicting findings previously reported.We also noted that the mean area of greenspaces (AREA_MN) had an inverse association with all-cause and cardiac mortality. We believe the mean size of greenspaces is an important indicator because it reflects the type of greenspace to which residents could be exposed.

High values indicate residents might have an increased likelihood of exposure to large parks; low values suggest that residents could encounter small green land parcels mainly near their residence, which might not afford diverse types of health-related activities. When taking all greenspaces into consideration (including smaller areas), an increase in AREA_MN led to a decrease in neoplasm-based mortality risk.

The underlying mechanism for this reduction could entail an increase in human NK cell activity, intracellular anticancer proteins, an optimised microbial input to our immune systems gained by contact with nature,25 or even a preferred microclimate.26 

Residents have to be present inside greenspaces to gain beneficial phytoncides and microbial inputs and the size of greenspace must be large enough to provide necessary facilities and to afford desired activities, because residents are less likely to walk into and stay in small green land parcels compared with larger parks.272829 

Moreover, AREA_MN indexes a greenspace’s ecosystem service (ie, biodiversity) and, therefore, might also affect the presence of beneficial phytoncides and microbes.

Our results showed, however, that the association between AREA_MN and neoplasm-related mortality disappeared when greenspaces of 83·6 m2 or smaller were removed from the analysis.

This finding suggests that the current standard minimum size for a pocket park could be sufficient to support the health benefits mediated by biodiversity and microclimate.Our findings showed a modest but positive association of greenspace fragmentation (PD) with mortality. When controlling for the total area of greenspace, increasing PD captures the growing number and decreasing size of greenspaces.

This change will result in a more fragmented distribution, which was associated with higher mortality risk. When considered in conjunction with the effect of mean size, this finding supports the idea that a few large parks are better at reducing mortality risk than are many small green parcels in living environments.

The potential mechanisms could at least partly entail positive psychophysiological affordances of larger size greenspaces and the improved ecosystem afforded by less fragmented greenspace.

The relation we noted between greenspace fragmentation, regardless of the size of each green region, and deaths from chronic lower respiratory diseases accords with previous findings from Taiwan, where fragmentation of green structures increased primary and secondary air pollutants and led to a higher mortality risk.30 

This association in our study disappeared after removing smaller greenspaces, again, suggesting that the greenspace’s ecosystem function (eg, reducing air pollution) might need to be of a specific size to be effective.

Future studies will be needed to unravel such relations.It is important to note that our study counted small green parcels down to 1 m2 as greenspace and reported a positive relation between greenspace density and all-cause and cause-specific mortality. Previous studies have shown negative associations between the number of greenspaces and health.31

 It is necessary to differentiate the idea of density used in this study and the number of greenspaces frequently used in previous studies. PD is a fragmentation spatial distribution measure. The metric itself has no relation with the total area of greenspace or percentage of greenspace.

In our study, when holding the total green area constant, the mean size of greenspace decreases with increases in density.

Although the number of greenspaces is a discrete continuous measure, it is highly correlated with measures of total greenspace area, particularly in studies based on land use data, which considered a much larger size of green area as greenspace.More connected and aggregated distributed greenspace was inversely associated with all-cause and cause-specific mortality. Such distributions might increase the likelihood of residents’ exposure to or use of natural elements in cities. A more continuous greenspace might afford residents the opportunity for physical activity (eg, walking, biking, or jogging).

Our results suggest that linking existed parks through greenways or adding new connected parks might be fiscally accessible strategies for promoting health. Finally, with respect to urban design, our findings showed that the complexity of the park shape was positively associated with a lower risk of mortality.

This association might be attributable to the increased number of access points provided by complex shape greenspace. Further study is necessary to identify the mechanism in operation.

Most experts who have investigated relations between greenspace and health have concluded that the associations are complex. Our evidence suggests there are at least two potential mechanisms that deserve further scrutiny. First, the morphology of greenspace might affect the likelihood of residents encountering natural elements in their daily life.

Greenspace exposures have been previously linked to improved cognition, reduced stress, shorter hospital stays, and increased physical activity, less obesity, higher quality sleep, and improved cardiovascular health.3233343536 Affecting the likelihood of routine encounters could play a part in these associations.

For instance, unevenly distributed greenspaces might aggravate the health inequity in cities, via unequal accessibility to greenspace resources,5 and could be linked to racial discrimination and policies that privilege economic growth over equity.373839 

Alternatively, linear-shaped parks could increase accessibility compared with parks with compact shapes.40 And large parks might provide additional benefits when compared with small parks, by providing alternative recreational options.4142 

The second potential pathway is via the effect of landscape morphology on the ecological function of greenspace.43 

Several studies have found that, for a fixed amount of greenspace, the size, fragmentation, and aggregation affect the urban microclimate, including land surface temperature and air pollution,2644454647 variables that are associated with respiratory disease48 and mortality risk.495051 

Future studies should examine whether these mediating effects exist, so we can better understand interactions between humans and nature in cities.The association of greenspace morphology with all-cause mortality was moderated by the percentage, in census tracts, of older adults and residents with a bachelor’s degree or higher.

This effect is consistent with findings of previous studies showing that, in cities, older people and those who are less educated seem to benefit more from contact with nature.525354 

These individuals are also the population who tend to have limited access to greenspace.

As the morphology of greenspace has a role in these associations, the optimal spatial arrangement of greenspace might be used as a strategy to promote health equity.

The satellite images in figure 4 show urban areas and greenspace in Philadelphia, PA, USA, and how greenspace area, connectedness, fragmentation, and shape are important. 

Figure 4A represents an area with small areas of greenspace that are not connected, have a uniform shape, and are fragmented. Figures 4B and 4C represent what our findings suggest are more health beneficial greenspaces, with large areas of greenspace that are connected and have complex shapes. The greenspace in figure 4B most likely affords residents a wide range of accessible activities, albeit at the expense of larger front yards. 

Figure 4C shows a single-family home area with large connected green cover, which is thought to support more sustainable and healthy ecosystems. 

Figure 4A shows a single-family home area with an equivalent total amount of greenspace to that shown in figure 4B, yet the area seems barren, there are no greenspace areas large enough to support diverse common activities, and ecosystems are uncoupled because of their small size and fragmented distribution.

Overall, these findings support public health and urban planning practice by showing the health effects of failing to consider the shape and connectedness of urban greenspace.

Figure thumbnail gr4
Figure 4Satellite images of urban areas and greenspace in Philadelphia, PA, USAShow full captionView Large ImageFigure ViewerDownload Hi-res imageDownload (PPT)

Our study had several limitations.

First, although an ecological approach done at the population level provides data at precisely the right resolution for urban planners making decisions at the neighbourhood level, the level of analysis is still too coarse-grained to discern the multiple mechanisms entailed.

We do not have a direct measure of greenspace exposure and, therefore, do not know if residents visited or indeed spent time in any of these greenspaces.

To ascertain this information would require additional studies done at the individual level. Second, the study site was restricted to Philadelphia, which is a large city located in the northeast of the USA in the State of Pennsylvania. We included all land with vegetation cover in the city without controlling for type, quality, or features.

Finally, our study is cross-sectional, which restricted our analysis to mortality data in 2006. We had no means of knowing the extent to which residents were interacting with the various distribution of green environments throughout their lifetime. Migration before death might also place residents into distinctly different environments.

Further studies are needed to more fully understand the complex relations between greenspace morphology and human health. Mortality risk has been predicted by greenspace morphology in our study.

If the underlying mechanisms that are mediating the likelihood of exposure to greenspace and ecological services are operative, additional health outcomes should vary in predictable ways as a function of greenspace morphology.

Effects of the spatial distribution of varying types, qualities, and features of greenspace can be investigated to further clarify what type of green area is the most effective operational surface for practice.

For example, future research should investigate the effect of park and non-park greenspace morphology on health outcomes, with the intention to test recreational opportunity as one of the pathways, because parks are one of the important accessible venues for public recreation.

The analytical unit in our study was the census tract, which is on a small geographical scale. This choice of unit could help in accounting for why previous studies that used greenness alone in predicting mortality did not find a significant result at a larger city scale.55 

Studies done at a range of scales, therefore, will be necessary to fully explicate the role of greenspace morphology.In conclusion, nearly all extant studies at the community level that have investigated the effects of natural environments on human health have focused primarily on the amount of greenspace.

We show that the shape or form of such greenspace has an important role in this association. The effect of greenspace morphology on mortality is significant, modest, independent of greenness level, and varies by age and education.

Our study hints at what greenspace spatial layout is most salubrious and provides insights into the reason for conflicting results in previous work.

We believe that particular spatial morphologies increase the likelihood of routine exposure to greenspace and, thereby, positively affect health outcomes.

If these findings are replicated, such relations will be of importance to city designers and planners as they seek to create healthier living environments through the intentional layout of the cityscape.

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Source:
Texas A&M
Media Contacts:
Richard Nira – Texas A&M
Image Source:
The image is credited to Texas A&M.

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