Live long in nature and long live nature!

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.1Finn S Herne M Castille D The value of traditional ecological knowledge for the environmental health sciences and biomedical research.Environ Health Perspect. 2017; 125: 085006Crossref Scopus (26) Google Scholar 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.2Wilson EO Biophilia: the human bond with other species. Harvard University Press, Cambridge1984Google Scholar 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.3Kaplan R Kaplan S The experience of nature: a psychological perspective. Cambridge University Press, Cambridge1989Google Scholar 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 studies4Mitchell R Popham F Effect of exposure to natural environment on health inequalities: an observational population study.Lancet. 2008; 372: 1655-1660Summary Full Text Full Text PDF PubMed Scopus (1027) Google Scholar 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 colleagues5Dan L Crouse Lauren Pinault Adele Balram et al.Urban greenness and mortality in Canada's largest cities: a national cohort study.Lancet Planet Health. 2017; 1: e289-e297Summary Full Text Full Text PDF Scopus (92) Google Scholar 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.6McEwen BS Stress, adaptation, and disease: allostasis and allostatic load.Ann N Y Acad Sci. 1998; 840: 33-44Crossref PubMed Scopus (2588) Google Scholar A prolonged stress reaction results in a release of pro-inflammatory cytokines and interleukins resulting in chronic inflammation.7Cohen S Janicki-Deverts D Doyle WJ et al.Chronic stress, glucocorticoid receptor resistance, inflammation, and disease risk.Proc Natl Acad Sci USA. 2012; 109: 5995-5999Crossref PubMed Scopus (615) Google Scholar This eventually causes oxidative stress, damaging the DNA of mitochondria, and increases the risk for premature death.8Epel ES Psychological and metabolic stress: a recipe for accelerated cellular aging.Hormones. 2009; 8: 7-22Crossref PubMed Scopus (294) Google Scholar 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,9Annerstedt M Jönsson P Wallergård M et al.Inducing physiological stress recovery with sounds of nature in a virtual reality forest—results from a pilot study.Physiol Behav. 2013; 118: 240-250Crossref PubMed Scopus (233) Google Scholar but the effect on chronic stress remains to be explored. Urban green spaces also provide ecosystem services10Millennium ecosystem assessment programEcosystems and human well-being. Island Press, Washington DC2005Google Scholar 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.11Rook GA Regulation of the immune system by biodiversity from the natural environment: an ecosystem service essential to health.Proc Natl Acad Sci USA. 2013; 110: 18360-18367Crossref PubMed Scopus (309) Google Scholar 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.12Dunder T Tapiainen T Pokka T Uhari M 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. 2007; 161: 972-977Crossref PubMed Scopus (32) Google Scholar 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 colleagues5Dan L Crouse Lauren Pinault Adele Balram et al.Urban greenness and mortality in Canada's largest cities: a national cohort study.Lancet Planet Health. 2017; 1: e289-e297Summary Full Text Full Text PDF Scopus (92) Google Scholar 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 colleagues5Dan L Crouse Lauren Pinault Adele Balram et al.Urban greenness and mortality in Canada's largest cities: a national cohort study.Lancet Planet Health. 2017; 1: e289-e297Summary Full Text Full Text PDF Scopus (92) Google Scholar 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,4Mitchell R Popham F Effect of exposure to natural environment on health inequalities: an observational population study.Lancet. 2008; 372: 1655-1660Summary Full Text Full Text PDF PubMed Scopus (1027) Google Scholar Crouse and colleagues5Dan L Crouse Lauren Pinault Adele Balram et al.Urban greenness and mortality in Canada's largest cities: a national cohort study.Lancet Planet Health. 2017; 1: e289-e297Summary Full Text Full Text PDF Scopus (92) Google Scholar 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 colleagues5Dan L Crouse Lauren Pinault Adele Balram et al.Urban greenness and mortality in Canada's largest cities: a national cohort study.Lancet Planet Health. 2017; 1: e289-e297Summary Full Text Full Text PDF Scopus (92) Google Scholar 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 Urban greenness and mortality in Canada's largest cities: a national cohort studyIncreased amounts of residential greenness were associated with reduced risks of dying from several common causes of death among urban Canadians. We identified evidence of inequalities, both in terms of exposures to greenness and mortality risks, by personal socioeconomic status among individuals living in generally similar environments, and with reasonably similar access to health care and other social services. The findings support the development of policies related to creating greener and healthier cities. Full-Text PDF Open Access

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