Live long in nature and long live nature!

The Lancet Planetary Health Volume 1, Issue 7, October 2017, Pages e265-e266 open access The Lancet Planetary Health Comment Author links open overlay panelMatildavan den Boscha https://doi.org/10.1016/S2542-5196(17)30122-5 Get rights and content Under a Creative Commons license Refers to Dan L Crouse, Lauren Pinault, Adele Balram, Perry Hystad, Paul A Peters, Hong Chen, Aaron van Donkelaar, Randall V Martin, Richard Ménard, Alain Robichaud, Paul J Villeneuve Urban greenness and mortality in Canada's largest cities: a national cohort study The Lancet Planetary Health, Volume 1, Issue 7, October 2017, Pages e289-e297 Download PDF Referred to by Dan L Crouse, Lauren Pinault, Adele Balram, Perry Hystad, Paul A Peters, Hong Chen, Aaron van Donkelaar, Randall V Martin, Richard Ménard, Alain Robichaud, Paul J Villeneuve Urban greenness and mortality in Canada's largest cities: a national cohort study The Lancet Planetary Health, Volume 1, Issue 7, October 2017, Pages e289-e297 Human beings are an integral part of ecological systems and human health is dependent on the health of the planet. These interdependencies have been recognised for centuries within the discipline of traditional ecological knowledge of indigenous populations.1 In occidental science, these ideas were conceptualised in the 1950s by environmental psychologists. The theory of biophilia—an inherent love of nature—suggests that human beings feel safe and recover from stress in natural settings because of our evolutionary origins from the African savannah.2 Other theories propose that our capacity for directed attention is depleted unless we can achieve effortless attention in nature (ie, a fascination) as a way of avoiding mental fatigue.3 Although a few early studies provided some empirical support for these theories, it was not until the 21st century that the field gained interest among environmental health scientists, and findings from epidemiological studies4 have now indicated an association between green spaces and various health outcomes or health-promoting behaviours such as physical activity. However, the physiological effects of nature exposure are still not fully known. In the Lancet Planetary Health, Dan Crouse and colleagues5 showed a relation between increased urban greenness and decreased cause-specific mortality among urban Canadians in a national cohort of approximately 1·3 million adults. This sample size made it one of the larger studies in the field and it improved on existing evidence by including a follow-up of 10 years during which patterns of residential mobility were tracked. The researchers assigned estimates of greenness exposure to participants derived from a remotely sensed Normalized Difference Vegetation Index (NDVI), then estimated hazard ratios (HR) and corresponding 95% CIs per IQR (0·15) increase in NDVI, showing a significantly decreased risk for all causes of mortality (non-accidental HR 0·915, 95% CI 0·905–0·924; cardiovascular plus diabetes 0·911, 0·895–0·928; cardiovascular 0·911, 0·894–0·928; ischaemic heart disease 0·904, 0·882–0·927; cerebrovascular 0·942, 0·902–0·983; and respiratory 0·899, 0·869–0·930), even with adjustment for covariates that might influence longevity, such as age, sex, and marital status. Can this reduced mortality be attributed to recovery from stress or mental fatigue, as postulated in earlier theories? Chronic stress is indeed a significant risk factor for several non-communicable diseases and related mortality.6 A prolonged stress reaction results in a release of pro-inflammatory cytokines and interleukins resulting in chronic inflammation.7 This eventually causes oxidative stress, damaging the DNA of mitochondria, and increases the risk for premature death.8 If green space exposure does prevent stress, it might explain the reduced mortality from several of the causes that the investigators describe. Most studies have shown that exposure to nature might contribute to acute stress relief,9 but the effect on chronic stress remains to be explored. Urban green spaces also provide ecosystem services10 from biodiverse ecosystems that are the ultimate premises for human health and survival, possibly explaining the association between green spaces and health. For example, exposure to microbial biodiversity is necessary for the development of many organ systems and for regulation of the immune system. In modern urban environments, with scarce access and exposure to biodiverse green spaces, there is a risk of depletion of this immunoregulatory service.11 The failure of immune system regulation has consequences in the form of a wide range of inflammatory diseases, which are currently increasing in urbanised high-income countries.12 Regulatory and cultural ecosystem services in cities contribute to reduction of the urban heat island, better air quality, and opportunities for social and physical activities. These services can act as mediators between green spaces and health and might contribute to reduced mortality. Crouse and colleagues5 were able to adjust for several environmental covariates, including PM2·5, ozone, and nitrogen dioxide, but noted these only had a small confounding effect on the green space HRs. Thus, in this case, it seems as if greenness has a protective effect that is independent of air quality. The evidence around green spaces and health is still fragmented for most specific outcomes and is often difficult to interpret because of the different metrics used. By linking the NDVI, which can be standardised across the world, to mortality data in a well designed statistical study, Crouse and colleagues5 have contributed to further advancing the field, as it will now be possible to replicate their study in various geographical regions with different socioeconomic and cultural conditions. The sociocultural, and possibly genetic, mosaic in multiethnic Canada provides an interesting ground for forthcoming studies in specific populations. By contrast with previous research,4 Crouse and colleagues5 showed the strongest protective effects were in people who could already be assumed to be in a healthier population segment—ie, people in relationships with higher incomes and education. The investigators hypothesise that this result could be from a higher exposure to greenness in these groups, due to easier access to green spaces in wealthier areas and self-selection. Future studies should build on the approaches of Crouse and colleagues5 to assess the potential of urban green spaces as inherently health-promoting assets in cities. We also need to invest in controlled, experimental research, to investigate pathways and biological mechanisms behind green space exposure and human health outcomes. By combining these kinds of studies, a better understanding can be reached of how and why nature is important for health and longevity. I declare no competing interests References 1 S Finn, M Herne, D Castille The value of traditional ecological knowledge for the environmental health sciences and biomedical research Environ Health Perspect, 125 (2017), p. 085006 View Record in Scopus 2 EO Wilson Biophilia: the human bond with other species, Harvard University Press, Cambridge (1984) 3 R Kaplan, S Kaplan The experience of nature: a psychological perspective, Cambridge University Press, Cambridge (1989) 4 R Mitchell, F Popham Effect of exposure to natural environment on health inequalities: an observational population study Lancet, 372 (2008), pp. 1655-1660 ArticleDownload PDFView Record in Scopus 5 L Crouse Dan, Pinault Lauren, Balram Adele, et al. Urban greenness and mortality in Canada's largest cities: a national cohort study Lancet Planet Health, 1 (2017), pp. e289-e297 6 BS McEwen Stress, adaptation, and disease: allostasis and allostatic load Ann N Y Acad Sci, 840 (1998), pp. 33-44 CrossRefView Record in Scopus 7 S Cohen, D Janicki-Deverts, WJ Doyle, et al. Chronic stress, glucocorticoid receptor resistance, inflammation, and disease risk Proc Natl Acad Sci USA, 109 (2012), pp. 5995-5999 CrossRefView Record in Scopus 8 ES Epel Psychological and metabolic stress: a recipe for accelerated cellular aging Hormones, 8 (2009), pp. 7-22 CrossRefView Record in Scopus 9 M Annerstedt, P Jönsson, M Wallergård, et al. Inducing physiological stress recovery with sounds of nature in a virtual reality forest—results from a pilot study Physiol Behav, 118 (2013), pp. 240-250 ArticleDownload PDFView Record in Scopus 10 Millennium ecosystem assessment program Ecosystems and human well-being, Island Press, Washington DC (2005) 11 GA Rook Regulation of the immune system by biodiversity from the natural environment: an ecosystem service essential to health Proc Natl Acad Sci USA, 110 (2013), pp. 18360-18367 CrossRefView Record in Scopus 12 T Dunder, T Tapiainen, T Pokka, M Uhari Infections in child day care centers and later development of asthma, allergic rhinitis, and atopic dermatitis: prospective follow-up survey 12 years after controlled randomized hygiene intervention Arch Pediatr Adolesc Med, 161 (2007), pp. 972-977 CrossRefView Record in Scopus © 2017 The Author(s). Published by Elsevier Ltd.