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10 May 2019

Donovan GH et al. Lancet Planet Health 2019; 3: e226-e234

Exposure to the natural environment may be protective against ADHD or moderate ADHD symptoms in children. Most cross-sectional studies have focused on the short-term effects of the natural environment on children already diagnosed with ADHD, and have small sample sizes. The aim of this longitudinal study was to determine the relationship between exposure to the natural environment and the risk of ADHD in a large linkage study of children in New Zealand.

Data from all children born in New Zealand in 1998 were used in this study.* Statistics New Zealand’s Integrated Data Infrastructure (IDI) was used to identify children with ADHD and to define covariates. The IDI also provided residential history for individuals based on residential meshblocks (the smallest geographical unit for which the New Zealand Consensus reports data), and maternal data were used to supplement a child’s residential history if their residential status suggested cohabitation. Exposure to green space for each year of a child’s life was estimated at the meshblock level using normalised difference vegetation index (NDVI), and land-use data from Landcare Research New Zealand. Controlling for covariates, logit models were used to assess associations between ADHD prevalence, rural living, and minimum, maximum and mean lifetime NDVI.

There were 57,450 children born in New Zealand in 1998 and 49,956 (87.0%) were eligible for study inclusion. Many children were not of European descent (n = 22,079 [44.2%]), and 14,149 (28.3%) were Māori. Only a small number of children had always lived in a rural area (n = 3419 [6.8%]); 6061 (12.1%) had a low birthweight, and 8770/39,157 (22.4%) were born to mothers who had not completed high school or equivalent.

The study results were as follows:

  • The final analytical sample consisted of 49,923 children, as 33 children had missing data on covariates that were not imputed.
  • Children (aged >2–18 years) who had always lived in a rural meshblock were protected against ADHD (odds ratio [OR] 0.670, 95% confidence interval [CI] 0.461–0.974).
  • Increasing minimum NDVI was also protective against ADHD (OR 0.834, 95% CI 0.773–0.901).
  • When NDVI was split into quartiles, a possible dose-response relationship between greenness and ADHD was identified (quartile 2: OR 0.841, 95% CI 0.707–0.999, p = 0.049; quartile 3: OR 0.809, 95% CI 0.680–0.963, p = 0.017; quartile 4: OR 0.664, 95% CI 0.548–0.805, p < 0.001 [vs quartile 1 for each]).
  • In early life (prenatal to age 2 years), rural living was protective against ADHD (results not shown).
  • Maximum NDVI exposure in children (aged >2–18 years) was a risk factor for ADHD but only for quartile 3 (OR 1.224, 95% CI 1.016–1.475, p = 0.034 vs quartile 1).
  • Land cover within a child’s residential meshblock, neighbourhood deprivation, NO2 concentration and proximity to roads were not significantly associated with ADHD.
  • For the associations between mean NDVI covariates and ADHD, female sex (OR 0.314, 95% CI 0.270–0.364, p < 0.001), non-European ethnicity (Asian: OR 0.143, 95% CI 0.074–0.278, p < 0.001; Māori: OR 0.451, 95% CI 0.383–0.532, p < 0.001; Pacific: OR 0.098, 95% CI 0.058–0.167, p < 0.001), and no maternal smoking (OR 0.686, 95% CI 0.592–0.795, p < 0.001) were linked to a reduced risk of ADHD.
  • Low birthweight (OR 1.454, 95% CI 1.229–1.720, p < 0.001) and increasing birth order (OR 1.156, 95% CI 1.082–1.235, p < 0.001) were associated with the increased risk of ADHD and mean NDVI.

This study had several limitations. Firstly, this study was a single, observational study; therefore, a causal link between greenness and ADHD could not be established. Secondly, as coarse overall greenness metrics were used, elements of the natural environment that are most protective against ADHD could not be identified. Thirdly, ADHD diagnoses were not obtained from primary care doctors, and the results were not sensitive to different ADHD definitions. Finally, exposure metrics in this study were based on residential meshblocks and not residential addresses, which could introduce measurement errors, especially in rural areas sparsely populated.

The authors concluded that this longitudinal study identified a relationship between exposure to the natural environment and ADHD, with rurality and increased minimum greenness strongly and independently associated with a reduced risk of ADHD. The authors suggested that exposing children to the natural environment or increasing their minimum greenness exposure may moderate ADHD symptoms and could protect against ADHD.

Read more about exposure to the natural environment, rurality and children with ADHD here

*Children without an address history, those who were not singleton births, and those who died or emigrated before 18 years of age were excluded
A child was defined as having ADHD if one of two conditions occurred at <18 years of age: (1) a child was diagnosed with ADHD in hospital (code F90.9 in the International Classification of Diseases 10th Revision [ICD-10]); or (2) a child received ≥2 prescriptions for methylphenidate hydrochloride, atomoxetine or dexamfetamine sulfate
Covariates included: sex; ethnicity; underweight or premature birth; birth order; parental smoking; parental education and occupation; infections and antibiotic use; and parental age at childbirth

Donovan GH, Michael YL, Gatziolis D, et al. Association between exposure to the natural environment, rurality, and attention-deficit hyperactivity disorder in children in New Zealand: a linkage study. Lancet Planet Health 2019; 3: e226-e234.

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