Exposure to green spaces and all-cause mortality: limitations in measurement and definitions of exposure
Authors: Samuel P Trethewey, Ella K M Reynolds
Journal: The Lancet Planetary Health (2021)

In The Lancet Planetary Health, David Rojas-Rueda and colleagues' systematic review and meta-analysis investigated the relationship between exposure to green spaces and all-cause mortality.1Rojas-Rueda D Nieuwenhuijsen MJ Gascon M Perez-Leon D Mudu P Green spaces and mortality: a systematic review and meta-analysis of cohort studies.Lancet Planet Health. 2019; 3: e469-e477Summary Full Text Full Text PDF PubMed Scopus (108) Google Scholar We applaud the authors for their meticulous investigation; however, we have several comments regarding limitations in measurement and definitions of exposure. Studies were included if they measured green space exposure using the normalised difference vegetation index (NDVI). The NDVI is calculated using satellite image data and provides a gross, indirect estimate of the density of green vegetation in a defined area.2Rhew IC Stoep AV Kearney A et al.Validation of the Normalized Difference Vegetation Index as a measure of neighborhood greenness.Ann Epidemiol. 2011; 21: 946-952Crossref PubMed Scopus (142) Google Scholar The authors recognise two key limitations of this measure: namely, that NDVI does not quantify the quality of the green spaces nor does it provide an estimate of accessibility. In addition, NDVI does not differentiate between types of green space, such as woodland, parks, sports fields, gardens, or allotments, making it difficult to interpret usability. The relationship between exposure to different types of green space and mortality, considering the quality, accessibility, and usability of green spaces, warrants further research. Complicating the matter further is the fact that green vegetation is inherently dynamic, in that it can change over time, modifying the exposure itself; considering the date of satellite image acquisition is therefore crucial to interpretation of findings. Using a random-effects model, the authors calculated a pooled hazard ratio (HR) for all-cause mortality per increment of 0·1 NDVI within a buffer zone of 500 m or less from a participant's residence. The pooled HR was 0·96 (95% CI 0·94–0·97; I2=95%). Studies showed a very high level of statistical heterogeneity, suggesting that variation in study results is due to underlying heterogeneity rather than chance, affecting the validity of the conclusions and raising the question of whether it was appropriate to do a meta-analysis.3Imrey PB Limitations of meta-analyses of studies with high heterogeneity.JAMA Netw Open. 2020; 3e1919325Crossref PubMed Scopus (22) Google Scholar Evidence suggests that the way green space is described in the literature is complex and nuanced, lacking a single unifying definition.4Taylor L Hochuli DF Defining greenspace: multiple uses across multiple disciplines.Landsc Urban Plan. 2017; 158: 25-38Crossref Scopus (206) Google Scholar This can make it hard for results from different studies to be compared and thus might preclude meta-analysis in studies with contrasting definitions. Crucially, the definition of green spaces used can substantially alter observed relationships between exposure and outcome—eg, the relationship between green spaces and being overweight.5Klompmaker JO Hoek G Bloemsma LD et al.Green space definition affects associations of green space with overweight and physical activity.Environ Res. 2018; 160: 531-540Crossref Scopus (85) Google Scholar The effect of using different definitions of green space when investigating the relationship between exposure to green spaces and mortality thus warrants further evaluation. We declare no competing interests. Exposure to green spaces and all-cause mortality: limitations in measurement and definitions of exposure – Authors' replyWe thank Samuel Trethewey and Ella Reynolds for their Correspondence about our Article.1 We appreciate their comments because they offer the opportunity to clarify some issues on the exposure definition of green spaces and the complex relationship between green spaces and health. Full-Text PDF Open AccessGreen spaces and mortality: a systematic review and meta-analysis of cohort studiesWe found evidence of an inverse association between surrounding greenness and all-cause mortality. Interventions to increase and manage green spaces should therefore be considered as a strategic public health intervention. Full-Text PDF Open Access