This review examined scientific studies published from January 22, 2015 through the end of 2017 investigating associations between greenness and human health. Findings from a previous review conducted by the same authors found strong evidence of associations for higher greenness with healthier birth weights, higher levels of physical activity, and lower mortality rates. There was also some evidence of an association between greenness exposure and lower levels of obesity/overweight. While only a few studies in the previous review addressed cardiovascular disease, those that did showed that higher greenness was related to lower levels of this disease Most of the studies in the previous review were based on cross-sectional methodologies. While the current review also included cross-sectional studies, it included a number of longitudinal and prospective studies, as well.
Most of the studies in this current review used the normalized difference vegetation index (NDVI) as the metric to quantify greenness. NDVI uses remote sensing measurements to obtain readings of live green vegetation in a targeted area. A limited number of studies quantified exposure to greenspace by using land cover datasets and/or the distance between a residence and the nearest green space environment. At times, greenness measures were based on subjective perceptions of greenness exposure. The authors of this review recognize that the field of exposure assessment in greenness and health is still evolving and offer some suggestions on how future research might address some of the limitations associated with these metrics.
Findings of this current review provide “mounting evidence demonstrating associations between greenness and health.” Areas where the evidence is consistent include physical activity and mental health. Higher levels of greenness were associated with higher levels of physical activity and lower levels of depression and depressive symptoms. The review also found higher levels of greenness associated with healthier birth weights, particularly for mothers of lower socioeconomic status. This association, however, varied somewhat by geographic region. Findings related to other birth outcomes, including pre-term birth, were less clear. Other areas where the association was less clear include cardiovascular disease and asthma and allergies (the latter can be positively affected perhaps as a result of less air pollution or negatively affected, likely as a result of allergens possibly specific to vegetation type). Recent investigations have considered possible associations between greenness and less-studied outcomes, such as spectacle use in children, autism, prostate cancer, malaria, and gun assaults.
Overall, this review adds support to the idea that nature exposure positively affects human health. More research is needed, however, to identify the mechanisms through which this occurs.