“Nature is, after all, the only book that offers important content on every page.” Johann Wolfgang von Goethe
Yang et al1 report a rigorously conducted cross-sectional epidemiological study of 59 754 children and adolescents residing near 94 schools and kindergartens in Liaoning province, in northeastern China. They evaluated the association between greenness, as identified from satellite imaging, and increased prevalence of symptoms of attention-deficit/hyperactivity disorder (ADHD), using 2 indices of the proportion of land and 2 parent reports of ADHD symptoms.1 Greenness indices varied widely across sites. For example, values for the normalized difference vegetation index within 500 m of schools or kindergartens, which has a range of −1 (representing barren areas) to 1 (representing dense vegetation),1,2 ranged from −0.09 to 0.77. In terms of both greenness indices and parent measures, Yang et al1 found significant associations between greenness near schools and increased ADHD symptom prevalence. In fully adjusted models, each 0.1-unit increase in normalized difference vegetation index within 500 m of a school and soil-adjusted vegetation index within 500 m of a school was associated with reduced odds of being reported to have at least 6 symptoms of ADHD (odds ratios, 0.87 [95% CI, 0.83-0.91] and 0.80 [95% CI, 0.74-0.86], respectively; P < .001). Thus, by our calculations, a 0.1-unit increase in greenness was associated with 13% to 20% decreased odds of having ADHD symptoms. Although ADHD symptoms were only ascertained from parents, the pattern of phenotypic findings is consistent with other epidemiological results in ADHD, supporting the overall validity of their phenotypic strategy.1
As Yang et al1 relate in their study, their effort builds on prior clues. The widespread availability of increasingly detailed satellite data is converging with the global acceleration of urbanization and questions about its effects on health. A 2014 study3 of 2111 children from the Barcelona BREATHE project found that the normalized difference vegetation index for greenness was inversely associated with ADHD symptoms, particularly when home and school greenness were combined. A longitudinal epidemiological study4 using normalized difference vegetation index and clinical data examined the risk of ADHD among 49 923 children born in 1998 in New Zealand. Donovan and colleagues4 found that rurality and increased minimum greenness across development from ages 2 to 18 years were both strongly and independently associated with reduced risk of ADHD. Interestingly, mean or maximum normalized difference vegetation index levels were largely not associated with ADHD risk in the New Zealand data, suggesting that targeting deprivation of access to green spaces may be the most effective ameliorating strategy.
Questions regarding exposure to nature and health are becoming more pressing as increased urbanization leads to decreased opportunities to experience nature. As population distribution shifts progressively toward less-natural environments, each generation has a decrease of nature experience, especially during childhood. Such “environmental generational amnesia”5 can shift the baseline for what is “acceptable quality, richness and variation of nature experiences.”5 The study by Yang et al1 breaks important new ground in extending the association between greenness and ADHD symptoms to non-Western populations. Beyond confirming and extending these findings in longitudinal approaches, future studies will need to characterize the features of nature (eg, size, composition, and structure of vegetation and other aspects of nature experience) to determine which features exert which effects at what periods of the life span.5 Individuals’ experience of nature should also be considered, including the types of interaction (eg, swimming in the ocean vs walking in a city park), dose-response of nature exposure, and how personal characteristics affect response.
Delineation of the mechanisms involved in mental health remains the most ambitious task, given that multiple physiological pathways are invariably involved, each of which may interact in transducing the effects of nature exposure. Although air pollution and level of physical activity likely contribute, experimental studies suggest that simply looking at city-based natural landscapes might be beneficial. For example, in young adults, pictures of green urban spaces appear to produce greater recovery after stressful tasks than pictures of urban built spaces, as measured by subjective rating scales and physiologically by variation in respiratory sinus arrhythmia.6 This phenomenon is likely mediated by increased vagal activity in response to simply viewing green urban spaces.
Finally, the data emerging on the impact of nature on mental health have the potential to inform psychiatric treatment and social policies.5 Documenting the beneficial effect of green spaces should lead to increased inclusion of green spaces in and near schools. The work reported by Yang et al1 is important in highlighting the effect of green spaces near schools as places for nature access, especially for economically disadvantaged children, who are among those most likely to have nature amnesia or deprivation. Fortunately, scientific interest in the interface between nature exposure and mental health is growing quickly5,7 even as the quest to understand this elusive association remains daunting and wondrous.
Published: December 18, 2019. doi:10.1001/jamanetworkopen.2019.17880
Open Access: This is an open access article distributed under the terms of the CC-BY License. © 2019 Baroni A et al. JAMA Network Open.
Corresponding Author: Francisco Xavier Castellanos, MD, Department of Child and Adolescent Psychiatry, Hassenfeld Children’s Hospital at NYU Langone, 1 Park Ave, Seventh Floor, New York, NY, 10016 (firstname.lastname@example.org).
Conflict of Interest Disclosures: Dr Castellanos reported receiving nonfinancial support from Greenwich Biosciences and personal fees from BOL Pharma outside the submitted work. No other disclosures were reported.
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Baroni A, Castellanos FX. Emerging Insights Into the Association Between Nature Exposure and Healthy Neuronal Development. JAMA Netw Open. 2019;2(12):e1917880. doi:10.1001/jamanetworkopen.2019.17880
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