The Association between Residential Green Space in Childhood and Development of Attention Deficit Hyperactivity Disorder: A Population-Based Cohort Study

Background: Access to green space has been hypothesized to have a beneficial impact on children’s mental well-being and cognitive development. The underlying mechanisms of the mental health benefits of green space are not fully understood, but different pathways have been suggested, such as the psychologically restoring capacities of green space, the ability to facilitate physical activity and social cohesion, and the mitigation of exposure to air pollution. Objectives: In this nationwide cohort study, we investigated associations between residential green space in early childhood and a clinical diagnosis of attention deficit hyperactivity disorder (ADHD). Methods: The cohort included individuals, who were born in Denmark between 1992 and 2007 (n=814,689) and followed for a diagnosis of ADHD from age 5, during the period 1997–2016. We used the normalized difference vegetation index (NDVI) as a measure of vegetation greenness surrounding each residential address in a quadratic area of 210m×210m in which the residence was located in the center of the quadrate. Individual exposure to green space was calculated as the average of NDVI surrounding each individual’s residential address (or addresses if more than one) between birth and the fifth birthday. Multilevel modeling was used to estimate the incidence rate ratios (IRRs) with 95% confidence intervals (CI) for ADHD, according to exposure level and adjusted for calendar time, age, sex, parental socioeconomic status, neighborhood level socioeconomic status, and urbanicity. Results: Individuals living in areas defined by sparse green vegetation (lowest decile of NDVI) had an increased risk of developing ADHD, compared with individuals living in areas within the highest decile of NDVI (IRR=1.55; 95% CI: 1.46, 1.65). Adjusting for the known confounders attenuated the result, but the association remained (IRR=1.20; 95% CI: 1.13, 1.28). Conclusion: Our findings suggest that lower levels of green space in residential surroundings, during early childhood, may be associated with a higher risk of developing ADHD. https://doi.org/10.1289/EHP6729


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Figure S1. Definition of study population. Table S1. IRRs for ADHD by NDVI within the first five years of life and stratified by year of birth, sex, region, urbanicity, parental SES and neighborhood level SES (with interaction term). Table S2. IRRs for ADHD by NDVI within different proximities around residential address. Table S3. Incidence rate ratios (IRRs) for ADHD by NDVI at different ages within an exposure zone of 210 × 210 m.  Mother was resident in Denmark 9 months before birth

N=824,778
The cohort member was alive and resident in Denmark on the 5 th birthday

N=816,098
Mother was not resident in Denmark 9 months before birth

N=3,223
The child had died or was not resident in Denmark on the 5 th birthday

N=814,689
The cohort member did not have an ADHD diagnosis before the 5 th birthday

N=814,834
Missing information about NDVI  Abbreviations: NDVI; Normalized Difference Vegetation Index. IRR; Incidence rate ratio. ₸ Multilevel modeling was used to estimate the association between NDVI in numeric deciles measured at 210 × 210 m around an individual's residential address between age 0 to 5 years and the outcome of ADHD in a cohort of 814 689 individuals born in Denmark 1992 to 2007 and who were followed from 1997 until 2017 and adjusted for age, calendar year, sex β Multilevel modeling was used to estimate the association between NDVI in numeric deciles measured at 210 × 210 m around an individual's residential address between age 0 to 5 years and the outcome of ADHD in a cohort of 814 689 individuals born in Denmark 1992 to 2007 and who were followed from 1997 until 2017 and adjusted for age, calendar year, sex, mother's and father's level of education and income, urbanicity and proportion of low income, low education and unemployment at municipal level Quadrat size is the different exposure zones of green space around the residential address. Multilevel modeling was used to estimate the association between NDVI in numeric deciles and ADHD All estimates were adjusted for age, calendar year, sex, mother's and father's level of education and income, urbanicity and proportion of low income, low education and unemployment at municipal level Multilevel modeling was used to estimate the association between NDVI in deciles measured at 210 × 210 m around an individual's residential address between age 0 to 5 years and the outcome of ADHD in a cohort of 814 689 individuals born in Denmark 1992 to 2007 and who were followed from 1997 until 2017. All models were adjusted for age, calendar year, sex and mother's and father's level of education and income, urbanicity and proportion of low income, low education and unemployment at municipal level Multilevel modeling was used to estimate the association between NDVI in deciles measured at 210 × 210 m around an individual's residential address between age 0 to 5 years and the outcome of ADHD in a cohort of 814 689 individuals born in Denmark 1992 to 2007 and who were followed from 1997 until 2017.