Exposure to green spaces as a modifiable risk factor in attention-deficit hyperactivity disorder
Authors: Julia J Rucklidge
Journal: The Lancet Planetary Health (2019)

Geoffrey Donovan and colleagues1Donovan GH Michael YL Gatziolis D 't Mannetje A Douwes J 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-e234Summary Full Text Full Text PDF PubMed Scopus (32) Google Scholar should be commended for their creative exploration of the role that the natural environment plays in the development of attention-deficit hyperactivity disorder (ADHD). Exposure to green spaces has been associated with numerous health benefits for children's and adolescents' mental health,2Vanaken GJ Danckaerts M Impact of green space exposure on children's and adolescents' mental health: a systematic review.Int J Environ Res Public Health. 2018; 15: E2668Crossref PubMed Scopus (134) Google Scholar including positive stress reduction.3Hunter MR Gillespie BW Chen SY-P Urban nature experiences reduce stress in the context of daily life based on salivary biomarkers.Front Psychol. 2019; 10: 722Crossref PubMed Scopus (129) Google Scholar However, Donovan and colleagues' study is the first to extensively explore the role of the environment in the development of a chronic mental health condition that poses a substantial financial burden in terms of individual and societal costs. Donovan and colleagues1Donovan GH Michael YL Gatziolis D 't Mannetje A Douwes J 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-e234Summary Full Text Full Text PDF PubMed Scopus (32) Google Scholar identified all children (n=57 450) born in New Zealand in 1998, of which 49 923 were included in the study sample (after exclusion of those with no address or those who had left the country). The researchers assessed those children's exposures to green spaces yearly from gestation to 18 years of age on the basis of the rurality and greenness (normalised difference vegetation index [NDVI], calculated from satellite imagery) of their residential location. ADHD diagnoses were defined as hospital diagnosis or at least two recorded prescriptions for ADHD drugs. The results showed that living in a rural area between 2 years and 18 years of age had a protective effect against the development of symptoms associated with ADHD (odds ratio [OR] 0·670 [95% CI 0·461–0·974]), as did increased minimum yearly greenness exposure for the same period (standardised OR 0·841 [0·707–0·999] for second quartile vs first quartile of minimum NDVI exposure). Rurality or greenness exposure before 2 years of age did not affect the development of ADHD symptoms. The observed protective effect was evident even after controlling for known risk factors such as sex, ethnicity, maternal education, and maternal smoking. The authors speculated that green spaces probably exert their effect by moderating the expression of the symptoms of ADHD, rather than by preventing their initial development. This study is the most comprehensive cohort study to date to investigate this relationship. In addition to the large and mostly intact sample, the use of technology to assess greenness (ie, calculating NDVI from satellite imagery) and to link those data with a child's residential location throughout life, and thus with ADHD diagnoses, is a strength of the paper. Mental disorders constitute a considerable burden for modern communities, and evidence indicates that, despite access to evidence-based treatments, this burden is likely to escalate further.4Mulder R Rucklidge J Wilkinson S Why has increased provision of psychiatric treatment not reduced the prevalence of mental disorder?.Aust N Z J Psychiatry. 2017; 51: 1176-1177Crossref PubMed Scopus (35) Google Scholar, 5Jorm AF Patten SB Brugha TS Mojtabai R Has increased provision of treatment reduced the prevalence of common mental disorders? Review of the evidence from four countries.World Psychiatry. 2017; 16: 90-99Crossref PubMed Scopus (261) Google Scholar For children with ADHD, even when they have received the best standard of care, long-term financial, legal, psychological, and educational outcomes remain suboptimal.6Swanson JM Arnold LE Molina BSG et al.Young adult outcomes in the follow-up of the multimodal treatment study of attention-deficit/hyperactivity disorder: symptom persistence, source discrepancy, and height suppression.J Child Psychol Psychiatry. 2017; 58: 663-678Crossref PubMed Scopus (168) Google Scholar Focusing on variables that can reduce this burden is essential to prevent the development of such neurodevelopmental disorders. Donovan and colleagues' study1Donovan GH Michael YL Gatziolis D 't Mannetje A Douwes J 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-e234Summary Full Text Full Text PDF PubMed Scopus (32) Google Scholar highlights one controllable environmental variable that could easily be modified in modern home designs and through providing education on the important health benefits for children of spending time in natural environments. Their findings also fit comfortably within the context of a growing body of research showing the detrimental effects of other modifiable environmental factors on the expression of ADHD, such as poor nutrition7Taylor MR Chuang C Carrasco KD Nagatomo S Rucklidge JJ Dietary and micronutrient treatments for children with neurodevelopment disorders.Curr Dev Disord Rep. 2018; 5: 243-252Crossref Scopus (3) Google Scholar and early-life exposure to persistent organic pollutants.8Lenters V Iszatt N Forns J et al.Early-life exposure to persistent organic pollutants (OCPs, PBDEs, PCBs, PFASs) and attention-deficit/hyperactivity disorder: a multi-pollutant analysis of a Norwegian birth cohort.Environ Int. 2019; 125: 33-42Crossref PubMed Scopus (95) Google Scholar Although other risk factors have been identified for the development of ADHD that are difficult to change, such as genetics,9Faraone SV Larsson H Genetics of attention deficit hyperactivity disorder.Mol Psychiatry. 2019; 24: 562-575Crossref PubMed Scopus (375) Google Scholar Donovan and colleagues' study underscores that a focus on modification of the environment is essential to reduce the burden of mental illness. The link between the rural environment and microbial exposure, as postulated in the study, is intriguing. Some preliminary evidence suggests that microbial diversity might affect the expression of ADHD symptoms.10Prehn-Kristensen A Zimmermann A Tittmann L et al.Reduced microbiome alpha diversity in young patients with ADHD.PLoS One. 2018; 13: e0200728Crossref PubMed Scopus (120) Google Scholar We do not yet know whether manipulation of this diversity can improve the symptoms of ADHD; however, Donovan and colleagues' study suggests that greater exposure to green spaces might promote such changes. The opportunities for both prevention and intervention are obvious, but might require government policy to be adequately targeted. Some assumptions made in conducting the study are limitations to the findings. Children living in rural areas might be less likely to seek an ADHD diagnosis or to be able to access specialised professionals. Furthermore, a child's home environment does not constitute the entire living environment, and we cannot assume that reduced access to green space is associated with less time spent outside. Donovan and colleagues' study shows that linking satellite data with a mental health diagnosis is achievable, and the authors discovery of this novel way of exposing a modifiable risk factor for ADHD is commendable. As the technology becomes more sophisticated, so will the certainty in the findings. I declare no competing interests. Association between exposure to the natural environment, rurality, and attention-deficit hyperactivity disorder in children in New Zealand: a linkage studyRurality and increased minimum greenness were strongly and independently associated with a reduced risk of ADHD. Increasing a child's minimum lifetime greenness exposure, as opposed to maximum or mean exposure, might provide the greatest increment of protection against the disorder. Full-Text PDF Open Access