Children’s exposure to natural environments is associated with positive mental health outcomes but the overall evidence is limited by methodological differencesAlthough many studies have linked exposure to natural environments (NEs) with children’s healthy development, challenges in measuring nature exposure remain a limitation. Research has relied on various NE metrics, including “remotely sensed indices, land use/land cover (LULC) datasets, on-site evaluations, and self-reports” to assess children’s exposure to green and blue spaces. This review aims to evaluate different NE assessment methods and determine their associations with childhood mental health and developmental outcomes.
A systematic search of the literature was conducted to identify peer-reviewed, quantitative studies of children (up to 12 years old) which investigated the relationship between NE exposure and mental health and development. Studies with children over the age of 12 were included only if 80% of the participants were under the age of 15 years. The final review included 45 studies which were conducted in ten different countries, the majority in the U.S. (42%). Most studies were published from 2015–2020. Researchers assessed and numerically scored each study based on the quality of NE and health outcome measurements. Studies were scored as poor quality (28%), moderate quality (60%), and good quality (11%). Researchers also assessed and classified the overall level of evidence of an association between the health outcomes and NE exposure. The classifications were: 1) Sufficient: significant relationships were found by studies of moderate to good quality; 2) Limited: significant relationships were found by studies of varying quality, indicating a need for further research; 3) Insufficient: inconclusive evidence due to a lack of good quality studies; and 4) Lack of evidence: associations between outcomes and NE exposure were not found by good quality studies.
Across studies, an array of methods were utilized to assess exposure to NEs, which often varied by outcome assessed. The most commonly used metrics were LULC (35%) and normalized difference vegetation index (NDVI) (31%) datasets. To a lesser extent studies used survey reports of exposure to NEs or reports of participants’ use of NEs. Studies in the review commonly assessed multiple outcomes, which included: academic achievement (n = 9), prevalence of doctor diagnosed disorders (n = 3), emotional and behavioral functioning (n = 22), well-being (n = 9), social functioning (n = 11), and cognitive skills (n = 5). Overall, increased exposure to NEs was associated with improved childhood mental health and development in 82% of reviewed studies’ findings.
Studies on academic achievement assessed school NEs and the majority detected positive associations between NE and measures of academic achievement (standardized test scores or attendance). Researchers classified the evidence of association between NEs, as measured by LULC datasets, and academic achievement as limited due to the limitations of research designs and some instances of mixed findings. Evidence from studies using NDVI to assess NEs was determined to be insufficient, as was the evidence regarding school attendance. NE exposures were associated with reduced doctor-diagnosed mental disorders (ADHD and autism) consistently across the three studies investigating this outcome; however, due to the small number of studies, the evidence was classified as insufficient. Increased exposure to NEs was linked with reduced emotional and behavioral problems in all studies that utilized NDVI, which was determined to provide a sufficient level of evidence for this association. When other measures of NEs were used in relation to emotional and behavioral functioning, the evidence ranged from limited to insufficient due to differences in study design or study quality. Well-being was also found to increase as NE exposure increased in the far majority of studies investigating this outcome. However, evidence of association was rated as insufficient due to differences in NE measurements and variation in study quality. Overall, improved social functioning was associated with more NE exposure. This evidence was classified as limited when measurements were based on actual use of NE and insufficient in studies using NDVI, LULC, and other measures, due to the small number of studies. Finally, studies found cognitive function (working memory and cognitive performance) was better with more NE exposure at home or school. Again, due to variations in NE metrics and a small number of studies, the evidence for cognitive function was classified as insufficient.
The review suggests that previous findings on the association between NEs and childhood health are somewhat dependent on the metrics employed to assess nature exposure. Importantly, researchers determined the level of evidence to be sufficient or limited for several mental health outcomes. In particular, research that utilized NDVI as a measure of NE was found to provide sufficient evidence of an association with children’s emotional and behavioral outcomes, due in part to the number of high-quality studies. Clearly, further research is needed to more fully understand the relationship between NE and health outcomes where limited and insufficient evidence was found. Future research should employ high-quality designs and consistent measurements of NE and health outcomes across studies.
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