Greenness measured at larger distances (up to 2000 meters) was a better predictor of physical health than greenness within smaller buffersThis systematic review of the literature was designed to identify at what distance greenness best predicts physical health. Studies included in the analyses (1) used Geographic information system (GIS) buffers to estimate residential greenness; (2) used statistical analyses that calculated the significance of the greenness-physical health relationship; and (3) were peer-reviewed articles published in English between 2007 and May 2017. GIS software packages, when used as research tools, can provide empirical evidence on how different distances in which greenness is measured impact physical health. Remote sensing and GIS datasets (including the Normalized Difference Vegetation Index) can be uploaded into these packages to provide objective measures of green cover in countries around the world. Buffer tools can then calculate the percentage of greenspace (or relative “greenness”) within a specified geographic area, such as the area surrounding a person’s residence.
The Web of Science database was used to identify relevant articles for this review. Search terms included “greenness” measures, GIS datasets, and physical health outcomes. While the initial search produced 311 records, only 68 were considered relevant to this study. Of these, 47 met the criteria for inclusion. Most of the included articles were published in the last four years. To capture multiple findings from a single article, the unit of inquiry – not the article -- was selected for analysis. The final sample included 260 analyses.
The analyses used a wide range of sample sizes and ages. The smallest sample was 61; the largest 345,143. While all ages -- infants to elders – were included in the analyses, adults were the most commonly studied age range. Only 35% of analyses demonstrated statistically significant relationships between greenness and improved physical health; 62% demonstrated no significant relationship; and 3% found a significant negative relationship, linking more green to worse physical health. These findings were similar across the lifespan. Of the 75 analyses that studied only children, youth, and/or infants, 29% reported significant positive effects; 7% reported negative effects.
Three outcomes accounted for over one-half of the health-related variables studied in the analyses: physical activity, birth and developmental outcomes, and cardiovascular outcomes. Obesity and atopy (asthma, allergies, and eczema) were also commonly studied. The most commonly used buffer sizes were between 1000–1999 meters. The relative number of analyses that tied greenness to physical health grew as the buffer size increased, but only up to 1999 meters.
Findings from this review indicate that the likelihood of greenness predicting physical health increased as the size of the buffer increased. However, this trend plateaued at buffers between 1000–1999 meters in size. These findings do not indicate that nearby greenspace – such as a park located within a 250-meter buffer from a house -- is less predictive of physical health than distant greenspace, as the 1000-meter buffer includes smaller, nested buffers around that house. The researchers recommend that future analyses use nested rather than overlapping buffers to evaluate to what extent greenness, not immediately around a person’s home, predicts physical health, as well as network buffers (which estimate greenness that is physically accessible to people along walking routes) rather than radial buffers.
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