Impact of Green Space Exposure on Children’s and Adolescents’ Mental Health: A Systematic Review

Abstract

In recent years, the interest in the relationship between exposure to green spaces and children’s and adolescents’ mental health has risen. This systematic review aims to provide an overview of observational studies assessing the association between empirical green space exposure with standardized outcome measures of mental health problems, mental well-being and developmental problems in children, adolescents and young adults. The PRISMA statement guidelines for reporting systematic reviews were followed. A PubMed and Scopus search resulted in the inclusion of 21 studies. The evidence consistently suggests a beneficial association between green space exposure and children’s emotional and behavioral difficulties, particularly with hyperactivity and inattention problems. Limited evidence suggests a beneficial association with mental well-being in children and depressive symptoms in adolescents and young adults. These beneficial associations are resistant to adjustment for demographic and socio-economic confounders, which thus may represent independent links. Mediating factors and the variability of this association between different age groups are discussed. From a precautionary principle, evidence up to now demands the attention of policy makers, urban planners and mental healthcare workers in order to protect children’s and adolescents’ mental health in light of rapid global urbanization by providing sufficient exposure to green spaces.

Full text

International Journal of
Environmental Research
and Public Health
Review
Impact of Green Space Exposure on Children’s and
Adolescents’ Mental Health: A Systematic Review
Gert-Jan Vanaken * and Marina Danckaerts
Department of Child and Adolescent Psychiatry, University Psychiatric Centre Leuven, KU Leuven,
3000 Leuven, Belgium
*Correspondence: gertjan.vanaken@kuleuven.be; Tel.: +32-499-48-09-34
Received: 1 November 2018; Accepted: 24 November 2018; Published: 27 November 2018


Abstract:
In recent years, the interest in the relationship between exposure to green spaces and
children’s and adolescents’ mental health has risen. This systematic review aims to provide
an overview of observational studies assessing the association between empirical green space
exposure with standardized outcome measures of mental health problems, mental well-being
and developmental problems in children, adolescents and young adults. The PRISMA statement
guidelines for reporting systematic reviews were followed. A PubMed and Scopus search resulted in
the inclusion of 21 studies. The evidence consistently suggests a beneficial association between green
space exposure and children’s emotional and behavioral difficulties, particularly with hyperactivity
and inattention problems. Limited evidence suggests a beneficial association with mental well-being
in children and depressive symptoms in adolescents and young adults. These beneficial associations
are resistant to adjustment for demographic and socio-economic confounders, which thus may
represent independent links. Mediating factors and the variability of this association between
different age groups are discussed. From a precautionary principle, evidence up to now demands
the attention of policy makers, urban planners and mental healthcare workers in order to protect
children’s and adolescents’ mental health in light of rapid global urbanization by providing sufficient
exposure to green spaces.
Keywords: green space; mental health; mental well-being; children; adolescents; urban planning
1. Introduction
In recent years, academic interest in a possible relationship between exposure to green spaces
and public health has risen, as can be derived from the number of studies published. In a
systematic review of 12 reviews published between 2010 and 2016, the authors underscored the
beneficial association of exposure to green spaces with all-cause mortality, mortality by cardiovascular
disease and mental health in adults [
1
]. However only one included review, published in 2015 by
Gascon et al. assessed mental health in children, finding inadequate evidence for such an association [
2
].
This incremental interest can be understood firstly, clinically in the light of the rapid global urbanization
dynamic, triggering concerns of a decline of contact with natural green spaces [
3
–
5
], and secondly,
methodologically in the availability of new remote sensing techniques to quantify green spaces using
satellite images [
6
]. Of course urbanized regions can offer contact with green spaces as well, defined
in this context as urban vegetated spaces such as parks, grasslands, cemeteries, sports and playing
fields and near-road trees [
7
]. Following these evolutions, the importance of green spaces has also
attracted political interest over the past years. In the United Nation’s Sustainable Development Goals
for instance, member states committed themselves to the following goal, “By 2030, provide universal
access to safe, inclusive and accessible, green and public spaces, in particular for women and children,
older persons and persons with disabilities” [8].
Int. J. Environ. Res. Public Health 2018,15, 2668; doi:10.3390/ijerph15122668 www.mdpi.com/journal/ijerph

Int. J. Environ. Res. Public Health 2018,15, 2668 2 of 17
Although green spaces seem to be associated with a range of health benefits, particular interest has
arisen for the impact on children’s mental health. One of the main challenges in this field of research
is disentangling the independent role of green space exposure from other confounding variables.
Intuitively most attention was paid to the role of socio-economic status, as this factor predicts mental
health and may influence where families eventually end up living. In 2009, Maas and colleagues
published a landmark study linking data of residential land use to medical diagnoses in primary care
settings for approximately 345,000 Dutch patients from different age groups [
9
]. Patients living in
the greenest environments were significantly less frequently diagnosed with a range of physical and
mental disorders after adjustment for the most probable socio-economic and demographic confounders,
compared to their counterparts living in the least green neighborhoods. Remarkably, the strongest
associations were found for psychiatric conditions, particularly in children [9]. Since then, a growing
number of studies have assessed those associations between green space and children’s and adolescents’
mental health. As mentioned earlier, Gascon et al. conducted the first systematic review on this specific
topic, in 2015, considering the results of the four existing observational studies as inadequate evidence
at the time for a causal relationship between green spaces and children’s mental health [
2
]. More
recently, in 2017, McCormick published another systematic review in this domain [
10
]. With a broader
scope on children’s mental well-being and school functioning, including 12 articles in the review, the
author concluded that access to green space was associated with improved mental well-being and
cognitive development of children. However, it should be noted that heterogeneity in study design
was significant and sufficient replication research was not systematically available at the time of review.
As global urbanization progresses, more than ever, children, adolescents and young adults are
coming to age in neighborhoods dominated by built environments. At the same time, mental health
problems in these age groups put a significant burden on these individuals and their life course, their
families and society as a whole. Since the research line that sheds a light on the interplay of these
dynamics expanded rapidly in recent years, there is a need for a new overview of the existing evidence
to guide urban planning and public health policy. This paper aims to provide such an attempt at
answering the question if an association between green space exposure and children’s and adolescent’s
mental health and neurocognitive development exists and which domains of these outcome concepts
are possibly involved in particular.
2. Materials and Methods
2.1. Search Strategy
This systematic review was conducted following the Preferred Reporting Items for Systematic
reviews and Meta-Analyses (PRISMA) statement guidelines [
11
]. The literature search was carried
out using two frequently used search engines, PubMed and Scopus. Following an exploratory
phase to identify the essential aspects in this domain of research, the following keywords were
selected: Keywords related to green space (“green space”, greenspace, greenness) combined with
keywords related to mental health and neurocognitive development (“mental health”, “well-being”,
development *) and with keywords related to childhood and adolescence (child *, adolescen *, infan *,
“young adult *”, youth). The asterisk (*) acts as a truncation symbol to detect various results composed
of a single string of text, e.g. ‘child *’ detects terms as ‘children’ and ‘childhood’. On 2 March 2018,
the last search was conducted. The title and abstract provided the information for a first screening
of the given articles. Following this first screening, full articles were read to decide on inclusion
for this systematic review. References of the included studies were taken into account to identify
additional articles.
2.2. Selection Criteria
The selection criteria for inclusion that all studies needed to fulfill were: (a) The article described
an original research study; (b) the article was written in English; (c) the article was published in a

Int. J. Environ. Res. Public Health 2018,15, 2668 3 of 17
peer-reviewed scientific journal; (d) the study had an observational design, interventional designs were
excluded; (e) green space exposure was analyzed empirically either by objective methods based upon
land cover maps or remote sensing data, or by subjective methods using standardized questionnaires;
(f) green space exposure was assessed in residential or school surroundings; (g) mental health and
neurocognitive development were assessed using standardized instruments or through existing
medical records or databases; (h) the study focused primarily on children, adolescents or young adults
with an age limit of 25 years, articles primarily focusing on adults over 25 years of age and were
excluded even when the study population existed in part of participants under the age of 25 and (i) no
restriction on publication date was put in place.
2.3. Data Collection and Evaluation of Evidence
Following the methodology used in a previous systematic review on green space and mental
health by Gascon et al. [
2
], we extracted the ensuing basic characteristics, methodological features and
outcome results of the studies included in our systematic review: Author, year of publication, country,
study design, study population, sample size, exposure assessment, outcome assessment, confounding
factors and main results. These extraction processes were conducted manually.
As children’s mental health and development are broad concepts, with a multitude of existing
methods to capture these concepts, the available evidence for any impact of exposure to green
spaces was evaluated based on the studied outcome parameter. These outcome parameters were
divided into three categories, i.e., (1) emotional and behavioral difficulties; (2) mental well-being and
(3) neurocognitive development. The evaluation of evidence for an association between green space
exposure and any studied outcome parameter was based on the number of studies finding a beneficial
association in comparison to the total number of studies assessing this parameter.
3. Results
3.1. Literature Search Results
The literature search using the described keywords yielded 89 results in PubMed and 197 results
in Scopus, as presented in Figure 1. A first screening based on title and abstract resulted in 48 articles
after the elimination of duplicates. After full-text read-through, 21 studies were selected for inclusion.
No additional articles fitting the predefined criteria were detected via the references.
3.2. Study Characteristics and Methods
As presented in Table 1, more than half of the included studies were published in 2016 or later
(n= 12), while the earliest publication matching the inclusion criteria only dates back to 2013. Eleven
of the selected studies were conducted in Europe, others were conducted in North America (n= 6)
and Oceania (n= 4). Twelve studies had a cross-sectional design, seven studies were longitudinal
and two studies were ecologically designed, i.e., relying on exposure and outcome data at the district
instead of the individual level [
12
,
13
]. The sample size of the examined populations ranged between
72 [
14
] and approximately 3,000,000 participants [
12
]. Exposure to green space was measured in the
majority of studies with at least one objective parameter. Land cover maps providing data on land
use (n= 10) or remote sensing data based on satellite images providing the Normalized Difference
Vegetation Index (NDVI) (n= 9) and derivations, were the most commonly used objective green space
parameters. The NDVI is a parameter based on the difference in reflected visible and infrared light
between green spaces and the built environment. One study used geolocation data obtained by study
participants wearing digital watches with GPS-functionality [
14
]. Two studies assessed green space
exposure solely with a subjective parameter, i.e., the score on a questionnaire gauging time spent in
green spaces or the distance to the nearest green space [
15
,
16
]. Seven studies combined objective and
subjective parameters in the exposure assessment.

Int. J. Environ. Res. Public Health 2018,15, 2668 4 of 17
Int. J. Environ. Res. Public Health 2018, 15, x 4 of 17
Figure 1. Search results. Figure based on Preferred Reporting Items for Systematic reviews and Meta-
Analyses (PRISMA) guidelines by Moher et al. [11].
The majority of studies (n = 16) assessed green space exposure only in the residential
neighborhood of children or adolescents, two studies measured exposure exclusively in the school
environment while three studies combined residential and school exposure to green spaces.
Mental health outcome was assessed by a range of different instruments. Eight studies used the
parent-based version of the Strengths and Difficulties Questionnaire (SDQ), while one study relied
on the SDQ filled out by multiple informants [17]. Eight studies relied on other standardized
questionnaires. Computerized neuropsychological tests were used to assess aspects of
neurocognitive development in three studies. Medical records and derived prevalence data provided
an outcome measure in two studies. Outcome results were described heterogeneously as the
percentile change in average score on a given test, the odds ratio, the rate ratio or the beta coefficient
of correlation. Associations that are presented in Table 1 or described below always consist of
significant and confounder-adjusted associations, unless stated otherwise. All studies assessed at
least one measure of the socio-economic status (SES), i.e., family income, parental education and/or
parental employment, except for two studies [14,15]. Adjustments for ethnicity, parental mental
health and neighborhood socio-economic status (nSES), i.e., safety, walkability, house value, average
household income of the neighborhood, were conducted less systematically, as presented in Table 1.
Records
identified
through
PubMed search
(n = 89)
Screening
Inclusion
Eligibility
Identification
Additional records
identified through
references
(n = 0)
Records identified
(n = 286)
Full-text articles
assessed for eligibility
(n = 48)
Full-text articles
excluded
(n = 27)
Studies included in
systematic review
(n = 21)
Records
identified
through
Scopus search
(n = 197)
Records screened after
removal of duplicates
(n = 208)
Excluded records based
on title/abstract
(n = 160)
Removed
duplicates
(n = 78)
Figure 1.
Searchresults. Figure based on Preferred Reporting Items for Systematic reviews and Meta-Analyses
(PRISMA) guidelines by Moher et al. [11].
The majority of studies (n= 16) assessed green space exposure only in the residential neighborhood
of children or adolescents, two studies measured exposure exclusively in the school environment while
three studies combined residential and school exposure to green spaces.
Mental health outcome was assessed by a range of different instruments. Eight studies used
the parent-based version of the Strengths and Difficulties Questionnaire (SDQ), while one study
relied on the SDQ filled out by multiple informants [
17
]. Eight studies relied on other standardized
questionnaires. Computerized neuropsychological tests were used to assess aspects of neurocognitive
development in three studies. Medical records and derived prevalence data provided an outcome
measure in two studies. Outcome results were described heterogeneously as the percentile change in
average score on a given test, the odds ratio, the rate ratio or the beta coefficient of correlation.
Associations that are presented in Table 1or described below always consist of significant and
confounder-adjusted associations, unless stated otherwise. All studies assessed at least one measure of
the socio-economic status (SES), i.e., family income, parental education and/or parental employment,
except for two studies [
14
,
15
]. Adjustments for ethnicity, parental mental health and neighborhood
socio-economic status (nSES), i.e., safety, walkability, house value, average household income of the
neighborhood, were conducted less systematically, as presented in Table 1.

Int. J. Environ. Res. Public Health 2018,15, 2668 5 of 17
Table 1. Study characteristics, methodology and results.
Author, Year,
Country
Study
Design N, Age Exposure Data
Source
Exposure
Area
Exposure Concept
(Scale)
Outcome
Instrument
Confounders Adjusted for
in Model Significant Results in Adjusted Model
Aggio et al., 2015,
Scotland [15]
cross-sectional
3586
5–6 years questionnaire residence walking time to
nearest GS SDQ no adjusted model in publication (Not adjusted) more than 20 min walking
time to GS associated with higher scores
on TDS.
Amoly et al., 2014,
Spain [18]
cross-sectional
2111
7–10 years NDVI,
questionnaire residence,
school
average greenness
(100 m, 250 m, 500 m);
GS playing time;
proximity to major GS
SDQ, DSM
IV/ADHD
gender, school level, ethnicity, SES,
parental marital status, nSES
preterm birth, breastfeeding,
environmental tobacco smoke,
gestational maternal smoking,
responding person
Average greenness inversely associated
with TDS, H/I & DSM IV-ADHD score;
green space playing time inversely
associated with TDS, emotional and peer
problems; proximity to major GS not
associated with outcome parameters.
Balseviciene, 2014,
Lithuania [19]
cross-sectional
1468
4–6 years NDVI, land
cover map residence average greenness
(300 m); distance to
city park SDQ age, gender, parenting stress, SES
Distance to city park positively associated
with TDS, H/I, peer and conduct problems
in low SES subgroup; no associations with
average greenness.
Bezold et al., 2017,
USA [20]
cross-sectional
9385
12–18 years NDVI residence average greenness
(250 m, 1250 m) McKnight Risk
Factor Survey
age, gender, ethnicity, grade level,
SES, maternal history of
depression, nSES, PM2.5
IQR increase in average greenness
associated with 11% lower odds of high
depressive symptoms.
Bezold et al., 2018,
USA [21]
longitudinal
11,346
9–25 years NDVI residence cumulative average
greenness (1000 m)
Mc Knight Risk
Factor Survey;
CES-D
age, gender, ethnicity, SES,
maternal history of depression,
population density, nSES, PM2.5
IQR increase in cumulative residential
greenness associated with 6% lower
incidence of high depressive symptoms;
stronger associations for under-18 year olds
and in more densely populated areas.
Dadvand et al.,
2015, Spain [22]
longitudinal
2593
7–10 years NDVI residence,
school,
commuting
average greenness
(250 m residence, 50 m
commuting route, 50 m
school)
Compu-terized
n-back, ANT age, gender, SES, nSES, air
pollution
Average greenness positively associated
with 12 m development of two-back,
three-back and ANT results.
Dadvand et al.,
2017, Spain [23]
longitudinal
1875
4–7 years NDVI, VCF residence
cumulative average
greenness (100 m,
300 m, 500 m), tree
canopy cover
computerized
K-CPT, ANT
age, gender, preterm birth,
maternal cognitive performance,
gestational smoking,
environmental tobacco exposure,
SES, nSES
Cumulative average greenness inversely
associated with K-CPT omission errors and
HRT-SE at 4–5 years, and with ANT HRT-SE
at 7 years.
Dzhambov et al.,
2018, Bulgaria [24]
cross-sectional
399
15–25 years
NDVI, SAVI,
TCI, GIS,
questionnaire residence
average greenness
(500 m), tree canopy
cover,
GS access/quality/usage
GHQ
age, gender, ethnicity, SES
orientation of rooms duration of
residence, time spent at home, air
pollution, noise, population
density
No direct associations; positive association
via serial mediation (restoration, physical
activity, social cohesion).
Feng and
Astell-Burt, 2017,
Australia [25]
longitudinal
4968
4–13 years land cover map,
question-naire residence
amount of GS (SA2), GS
quality SDQ age, gender, ethnicity, SES, nSES,
urbanicity
Amount and quality of GS inversely
associated with TDS, IS and ES, for all age
groups; for older children GS quality more
strongly inversely associatied with IS.
Feng, 2017,
Australia [17]
cross-sectional
3083
12–13 years land cover map,
question-naire residence amount of GS (SA2),
GS quality SDQ
age, gender, SES, nSES, geographic
remoteness
Amount of GS inversely associated with the
parent-reported TDS and IS; GS quality
inversely associated with both parent- and
child-reported TDS, IS and ES; stronger
associations found for
parent-reported scores.

Int. J. Environ. Res. Public Health 2018,15, 2668 6 of 17
Table 1. Cont.
Author, Year,
Country
Study
Design N, Age Exposure Data
Source
Exposure
Area
Exposure Concept
(Scale)
Outcome
Instrument
Confounders Adjusted for
in Model Significant Results in Adjusted Model
Flouri et al., 2014,
UK [26]
longitudinal
6383
3–7 years land cover map,
question-naire residence
amount GS (LSOA), use
of GS SDQ
age, gender, ethnicity, SES, family
structure, use of GS, access to
private garden, life adversity, nSES,
maternal (mental) health, physical
activity
Amount of GS inversely associated with
emotional problems score for age 3–5 years.
Huynh et al., 2013,
Canada [27]
cross-sectional
17,249
11–16 years land cover map school amount GS (5000 m) Cantrill ladder age, gender, ethnicity, SES, nSES Amount of GS not associated
with well-being.
Kabisch et al., 2016,
Germany [13]ecological 30,427
5–6 years land cover map residence amount GS (LEA)
(/capita) health visit
sub-district level: SES, ethnicity,
measles immunization,
participation in check-up,
kindergarten attendance, tobacco
exposure
Amount of GS inversely associated with
deficits in visuo-motoric development.
Kim et al., 2016,
USA [28]
cross-sectional
92
9–11 years
remote sensing
data
(NDVI-like) residence
amount, number, size,
distance to,
cohesiveness of GS (400
m, 800 m)
PedsQL age, gender, SES, nSES, BMI,
physical activity
Larger and more tree areas positively
associated with children’s health related
quality of life.
Markevych et al.,
2014, Germany [29]
cross-sectional
1932
10 years land cover map,
NDVI residence distance to nearest GS SDQ age, gender, SES, maternal age at
birth, parental marital status,
screen/outdoors time
Residence > 500 m away from nearest GS
positively associated with TDS, peer
relationship and H/I problems (after
stratification; only association with H/I for
boys), no associations for residential
average greenness.
Richardson et al.,
2017, Scotland [30]
longitudinal
2909
4–6 years land cover map,
question-naire residence amount GS and public
parks (500 m), access to
private garden SDQ age, gender, SES, parental mental
health, nSES, hours of screen time
Private garden access strongly associated
with TDS and H/I and to lesser extent with
peer and conduct problems, neighborhood
amount of GS associated with prosocial
behavior scores, little evidence of influence
on developmental trajectory.
Saw et al., 2015,
Singapore [31]
cross-sectional
426
18–25 years land cover map,
question-naire residence distance from nearest
GS, number of GS’s
(1200 m), use of GS
LSS, Pos.and
Neg. Affect
Scale, PSS
age, gender, SES, physical activity,
serious health problems,
personality traits
Neither access to or use of GS associated
with well-being.
Ward et al., 2016,
New Zealand [14]
cross-sectional
72
11–14 years geolocation and
timing all locations time spent in GS
LSS; TDIW; HS;
comp.
CNS-Vital Signs
test
age, gender, school, physical
activity
Time spent in GS and physical activity
positively associated with greater emotional
wellbeing, no associations with
neurocognitive development measures.
Wu et al., 2017,
USA [12]ecological ~3 ×106
5–12 years land cover map school
district
amout forest and
grassland (school
district), amount
(near-road) tree canopy.
prevalence
autism district level: ethnicity, gender,
SES, road density
Amount of GS and tree cover metrics
inversely associated with autism prevalence
in high road density districts.

Int. J. Environ. Res. Public Health 2018,15, 2668 7 of 17
Table 1. Cont.
Author, Year,
Country
Study
Design N, Age Exposure Data
Source
Exposure
Area
Exposure Concept
(Scale)
Outcome
Instrument
Confounders Adjusted for
in Model Significant Results in Adjusted Model
Younan et al., 2016,
USA [32]
longitudinal
1287
9–18 years NDVI residence average greenness
(250 m, 350 m, 500 m,
1000 m)
CBCL-Agression
age, gender, ethnicity, SES, nSES,
ambient temperature, traffic
density and proximity to freeways
and roads, maternal depression,
gestational smoking
IQR increase in average greenness inversely
associated with aggresive behaviour.
Zach et al., 2016,
Germany [16]
cross-sectional
6206
6–12 years questionnaire residence availablity of GS SDQ gender, country of birth, SES,
single parenthood, crowding,
traffic load
Non-accessibility of green space associated
with TDS and hyperactivity/
inattention problems.
Note: ADHD, Attention Deficit and Hyperactivity Disorder; ANT, Attention Network Task; CBCL, Child Behavior Checklist; CES-D, Center for Epidemiologic Studies—Depression Scale;
comp. CNS-Vital Signs test, computerized Central Nervous System-Vital Signs test; DSM IV, Diagnostic and Statistical Manual Fourth Edition; ES, Externalizing Scale; GHQ, General
Health Questionnaire; GIS, Geographical Information System; GS, Green Space; H/I, hyperactivity/inattention; HRT-SE, Hit Reaction Time Standard Error; HS, Happiness with life
as whole Scale; IQR, Interquartile Range; IS, Internalizing Scale; LEA, Living Environment Area; LSOA, Lower layer Super Output Area; LSS, Life Satisfaction Scale; K-CPT, Kiddies
Continuous Performance Task; NDVI, Normalized Difference Vegetation Index; nSES, neigborhood Socio-Economic Status; PedsQL, Pediatric Quality of Life scale; PM
2.5
, Particulate
Matter > 2.5
µ
m; PRS, Perceived Restorativeness Scale; SA2, Statistical Area 2; SAVI, Soil-Adjusted Vegetation Index; SDQ, Strengths and Difficulties Questionnaire; SES, Socio-Economic
Status (i.e., family income, parental education and/or parental employment); TCI, Tree Cover Index; TDIW, Ten Domain Index of Happiness; TDS, Total Difficulties Scale; VCF, Vegetation
Continuous Fields.

Int. J. Environ. Res. Public Health 2018,15, 2668 8 of 17
3.3. Evaluation of Evidence
3.3.1. Emotional and Behavioral Difficulties
As mentioned earlier, nine studies evaluated emotional and behavioral difficulties in children using
the internationally validated Strengths and Difficulties Questionnaire (SDQ), giving opportunities for
the comparison of respective results. All but one study relied on the parent-rated version, although
a teacher-rated version and a self-rated variant suitable from age eleven, were equally available.
The SDQ consisted of five domains, i.e., emotional symptoms, peer problems, hyperactivity and
inattention symptoms, conduct problems and prosocial behavior. The first four domains could
be summed up resulting in the Total Difficulties Score (TDS), a measure of general mental health.
Additionally, an internalizing (emotional symptoms and peer problems) and externalizing subscale
(hyperactivity and inattention symptoms and conduct problems) could be derived. However, not all
studies reported separately on these different domains or subscales. Studies differed substantially
in design, study population and assessed exposure parameters as presented in Table 1. This group
of studies described pre-school and school-aged children between the ages of 3 and 13. Seven out
of those nine studies reported on the Total Difficulties Score after adjustment for demographic and
socio-economic confounders. All of those seven studies documented a significant confounder-adjusted
association between a measure of green space exposure and the Total Difficulties Score [
16
–
19
,
25
,
29
,
30
].
Adjusting for the assessed confounders did not systematically shift the results in one direction, i.e.,
it depended on the specific city or country context whether taking into account demographic and
socio-economic confounders would strengthen or weaken the associations. Quantitative comparison of
the size of association was limited because of different exposure mapping and variation in measures of
presentation of the results, as mentioned earlier. However, where sufficient data were available,
intra-study comparison highlighted that confounder-adjusted associations between green space
exposure and the Total Difficulties Score were comparable in effect size to the associations of the
family socio-economic status (SES) with this same outcome measure [16,17,25,30].
Regarding the different domains of the SDQ, the strongest results were found for the hyperactivity
and inattention domain. Five out of six studies documented significant associations with hyperactivity
and inattention problems [
16
,
18
,
19
,
29
,
30
]. Smaller associations were found in four out of five studies
for peer problems [
18
,
19
,
29
,
30
] and less consistently in two out of five studies for conduct [
19
,
30
]
and emotional problems [
18
,
26
]. Richardson and colleagues [
30
] found a modest correlation between
prosocial behavior with the amount of residential public green space as compared to private garden
access, being the only study assessing this domain in the light of a different exposure between public
and private green spaces. Two other studies did not find associations between prosocial behavior and
measures of residential green space [18,19].
Next to the Total Difficulties Score, Feng et al. [
17
] and Feng and Astell-Burt [
25
] presented scores,
of 4 to 13 year old children, on the internalizing and externalizing subscale of the SDQ, being the only
two studies to do so. Green space quality and quantity were associated with both of the parent-rated
subscales scores for all age groups. For the oldest group in this population, i.e., age 12 to 13 years,
the associations with the internalizing subscale were tested and confirmed on the self-rated version of
the SDQ. Longitudinal results showed that associations with green space quality grew notably stronger
compared to green space quantity, in this older age group of the study population. Green space quality
was defined subjectively in this study as the parents’ answer on the question to what extent good
parks, playgrounds and spaces to play were available in their neighborhood.
Studies that analyzed different types of green space exposure showed that a shorter distance to
the nearest green space and access to a private garden were stronger associated with the SDQ than the
average amount of green space in the neighborhood [
16
,
19
,
29
,
30
]. Although beneficial correlations to
the SDQ scores were described for all studied age groups, longitudinal studies are not equivocally
presenting an impact of green space on the trajectory of these scores [25,26,30].

Int. J. Environ. Res. Public Health 2018,15, 2668 9 of 17
In a large, prospective twins birth cohort study, Younan et al. [
32
], found short—(one to three
months) and long-term (one to three years) exposure to neighborhood green space to be associated
with reduced aggressive behavior in Californian children and adolescents, as reported by parents on
the aggression subscale of the Child Behavior Checklist (CBCL). The authors described a decrease in
the CBCL-Aggression score of 0.4 points for an average of 4.86 (SD 5.03) when increasing surrounding
greenness over an interquartile range, i.e., the difference in exposure between the lowest versus highest
quartile of the population. When compared to the decline in aggressive behavior over time seen during
normal development, the decrease in aggression scores associated with higher levels of exposure to
green space was equivalent to approximately two years of behavioral maturation.
In an American cohort of adolescents, Bezold and colleagues [
20
] documented an 11% decrease in
depressive symptoms on the self-rated McKnight Risk Factor Survey in relationship to an interquartile
range increase in residential neighborhood greenness. In a following study, partially based on the
same data, the authors found the cumulative greenness exposure during childhood and adolescence to
be associated with a 6% decrease in the incidence of high depressive symptoms in adolescents and
young adults, with even stronger associations in highly urbanized neighborhoods [21].
No direct association between indicators of green space exposure and mental health as measured
on the self-reported 12-item General Health Questionnaire was found by Dzhambov and colleagues [
24
]
in their study based on a sample of 399 15- to 25-year olds. However, the authors detected indirect
associations of green space exposure with mental health via serial mediation of physical health,
neighborhood social cohesion and perceived restorative quality, a “measure of psychologically
restorative qualities of environmental experience grounded in attention restoration theory” [24].
3.3.2. Mental Well-Being
In the city of Houston, USA, Kim et al. [
28
] found that for 9- to 11-year old children, more and
larger urban green spaces in the neighborhood were positively associated with their health-related
quality of life, as reported by themselves and their mothers, on the Pediatric Quality of Life Inventory
(PedsQL). Ward et al. [
14
] collected objective data on 72 adolescents on time spent in green spaces
and physical activity relying on data from GPS-facilitated sports watches. Both exposure measures
showed robust associations with adolescents’ self-rated life satisfaction, well-being and happiness,
but not with their results on a computerized battery of neurocognitive tests. Time spent in green
spaces and physical activity proved to be partially interdependent but were also both independently
related to increased scores on the emotional well-being scales, with green space exposure having the
strongest relationships. It should be noted however, that this study was the only one that did not
include a measure of SES as a possible confounder in the statistical adjustment model. Huynh and
colleagues [
27
] did not find any significant associations for the average greenness in a 5000 m buffer
surrounding Canadian adolescents’ schools with self-reported emotional well-being on the Cantrill
ladder. Neither were Saw and colleagues [
31
] able to show that access or use of green spaces affected
emotional well-being of older adolescents and young adults in the tropical city of Singapore.
3.3.3. Neurocognitive Development
In the city of Berlin, Germany, Kabisch and colleagues [
13
] analyzed the results of community-organized
medical check-ups of pre-school in children in relation to green space exposure on a subdistrict level.
The authors found an inverse correlation between the percentage of naturally covered land and
deficits in visuo-motor and, to a lesser extent, language development. Dadvand and colleagues [
22
]
documented an enhanced twelve-month developmental progress in working memory and superior
working memory, and a reduction in inattentiveness in primary school children that were exposed
to higher levels of surrounding greenness. The authors relied on repeated computerized n-back
tests (respectively two- and three-back) and the Attention Network Task (variability of reaction time).
Results on these tests differed by +5%, +6% and
−
1%, respectively per interquartile range change
in total, i.e., school-based and residential, greenness exposure. Concentrations of elemental carbon,

Int. J. Environ. Res. Public Health 2018,15, 2668 10 of 17
a traffic-related air pollutant were assessed and explained 20–65% of the variances in neurocognitive
outcome. No association was found when only residential greenness was considered in contrast to the
school-based or the combined exposure. In a following study, comprising a younger population of age
four to seven, Dadvand et al. [
23
] found an association between specifically greater lifelong residential
green space exposure and enhanced development of sustained attention, derived from the results of
the computerized Attention Network Task and Continuous Performance Task.
Within the pool of identified articles, only one study looked at a possible relationship betwen
green space exposure and the prevalence of autism. Wu et al. [
12
] conducted an ecological study in
the state of California, United States, finding negative correlations between different types of green
spaces on elementary school district-level, i.e., forests, grasslands and (near road) tree canopy, and
autism prevalence, derived from an existing prevalence database. In stratified analyses based on the
road density of the school districts, these correlations remained intact in the quartile of districts with
the highest road density, but they did not in other quartiles. In these districts with the highest road
density, the authors described for every 10% increase in forest, grassland, average tree canopy and
near-road tree canopy, a decrease in autism prevalence of respectively 10%, 10%, 11% and 19%.
4. Discussion
4.1. Evidence for an Association between Green Space Exposure and Mental Health
The aim of this review was to provide an overview of observational studies assessing the
association between green space exposure and a variety of mental health outcomes in children and
adolescents. The seven studies reviewed regarding emotional and behavioral difficulties, present
credible evidence for an association between green space exposure and the Total Difficulties Score
of the Strengths and Difficulties Questionnaire, a measure of general mental health in children.
Regarding the subdomains that compose this general score, there is considerable evidence for a
consistent and significant correlation between green space exposure and hyperactivity and inattention
problems, suggested by four out of five studies assessing this subdomain. This finding is supported
by two longitudinal studies demonstrating the association between green space exposure and the
neuropsychological development of attention and working memory, relying on computer-based
tests. Furthermore, both studies that investigated the parent- and self-rated emotional well-being
and quality of life of children in the light of green space exposure did find a beneficial correlation.
Less research is available for emotional and behavioral difficulties in adolescents. However, two
studies that assessed self-rated depressive symptoms in this age group, did find an inverse correlation
with green space exposure. One study that investigated aggressive behavior in adolescents in a
longitudinal design, also found a significant inverse relationship. Emotional well-being in adolescents
was assessed in one study without finding a beneficial association. The authors of this early study,
published in 2013, assessed the effects of green spaces in a rather large exposure, i.e., 5000 m around
the adolescent’s schools [
27
]. This may possibly explain the absence of finding a clear association.
Later studies systematically assessed smaller exposure ranges. Two studies in this review assessed
mental well-being in young adults, however, both did not find a beneficial association with green
space exposure. A small sample size [
24
] and a tropical climatological setting [
31
] could explain these
findings. It was noted that young adults were hardly defined as a separate population, explaining
why a small number of studies focusing on 18- to 25-year olds, were identified using the predefined
inclusion criteria.
4.2. Independency of the Association
These described relationships with mental health outcomes are all associations that were adjusted
for demographic (age, gender, ethnicity) and socio-economic confounders (parental education, income,
employment), as those are suggested predictors of mental health. Results on the SDQ and other
outcome parameters after adjustment for these confounders were not systematically shifted towards

Int. J. Environ. Res. Public Health 2018,15, 2668 11 of 17
stronger, weaker or similar associations. This can be understood by the fact that in some cities or
countries green space is spread out equally between the strata of socio-economic status or ethnical
background, such as in the city of Barcelona, Spain [
18
]. In other settings, inequalities do exist,
e.g., in Scotland the socio-economically most deprived children had lower odds of having private
garden access or a public park in their neighborhood [
30
]. On the other hand, in the city of Kaunas,
Lithuania, families of higher socio-economic status lived further away from green spaces than their
counterparts of lower socio-economic status [
19
]. This suggests that depending on the city or country
of interest, these confounding variables work out differently. As mentioned earlier, confounding
by ethnicity, neighborhood socio-economic status and parental mental health were assessed less
systematically. However, it could be noted that associations in studies that adjusted for these concepts
did still find significant associations in line with studies with a more modest adjustment methodology.
Therefore, based on these results, it can be presumed that there is strong evidence for an association
between green space exposure and mental health in children and adolescents, independent of the
aforementioned confounders.
4.3. Direction of the Association
Although our main hypothesis was that green space exposure causes better mental health, it could
be hypothesized that the described association between green spaces and mental health works out in
the opposite direction of what is generally presumed. This alternative, inverse hypothesis would imply
a mechanism that makes parents of a child with more mental health problems, more likely to reside in a
less green neighborhood. Longitudinal studies such as those by Dadvand et al. showed that from birth
to age 7, about 25 % of the children changed addresses, a significant percentage [
23
]. A clear rationale
to explain a theoretical mechanism is lacking, but since none of the included longitudinal studies
presented data on a possible association between children’s mental health problems and their odds of
moving towards less green neighborhoods, this possibility cannot yet be dismissed. Another theoretical
explanation for this alternative hypothesis could imply a gene-environment interaction making parents
with hyperactivity and attention deficits more likely to choose more stimulating neighborhoods and
thus possibly less green environments to live in. If true, this would mean that finding a greater share
of children with hyperactivity and inattention problems in less green neighborhoods is explained by a
genetic rather than an environmental risk factor. However, based on the presented evidence, parental
mental health does not seem to attenuate the documented associations fully, making this hypothesis
less plausible. Other contra-arguments regarding this alternative hypothesis can be found, as we
discuss below, in the evidence for mediating mechanisms that explain an association in the direction
from more green space exposure towards better mental health. Nonetheless, it would be valuable for
future research to assess the alternative hypothesis in depth by adjusting more profoundly for parental
mental health and specifically for relevant domains such as hyperactivity and inattention problems.
Additionally, in longitudinal research, data could be collected on children’s mental health problems
and their odds of moving towards less green environments.
4.4. Mediation of the Association
In previous work, a multitude of mediators of the association between green space and mental
health were suggested [
2
]. The included studies in this review, provide some evidence for the roles
of air pollution, physical activity and social interaction, adding plausibility to the causal role of
green space exposure. Firstly, the protective role of green spaces against air pollution was especially
examined for neurocognitive development outcome parameters such as (superior) working memory,
inattentiveness and autism prevalence [
12
,
22
]. Although this evidence is limited, it adds consistency
to the growing evidence for the concerning, detrimental role of air pollutants on children’s brain
development, increasing the odds of having an impaired attentional function and autism spectrum
disorder [
33
–
35
]. Research in adult populations also suspects that exposure to poor air quality increases
the odds of depression [
36
]. Bezold et al. found no such role for fine particulate matter as a mediator

Int. J. Environ. Res. Public Health 2018,15, 2668 12 of 17
in the association between green space exposure and depressive symptoms in adolescents [
21
]. In this
study, however, average levels of this subgroup of air pollutants hardly differed between the strata of
green space exposure, possibly explaining the negative findings. Buffering of noise annoyance, which
often correlates with traffic load and indirectly with air pollution, was not assessed separately as a
mediating mechanism in the included studies. However, this factor is known to be associated with
behavioral problems in children [
37
] and thus could be another plausible mediator that future research
could investigate.
Secondly, Ward et al. provided objective data on time spent in green spaces and physical activity
relying on data by GPS-facilitated sports watches. Spending time in green spaces was related to higher
levels of physical activity and to better mental health. Associations of green space exposure with
children’s well-being were in part attenuated by separately adjusting for their physical activity in
those green spaces, suggesting a partial mediating role for physical activity [
14
]. Lastly, some evidence
is available for the role of social interaction as another plausible mediator of the association. Green
space playing time, in contrast to the mere availability of green spaces [
18
], was strongly associated to
less peer problems, and access to public instead of private green space was modestly associated with
increased prosocial behavior [30].
Since evidence is available for the beneficial effects of green spaces on adults’ mental health [
1
],
parental mental health is a tentative mediating factor as well. Some studies adjusted for possible
confounding effects of parental mental health, however in the included studies, mediation analysis of
this factor was lacking.
4.5. Variability by Age Group
Green space exposure seems to be correlated with children’s and adolescents’ mental health in
different ways depending on their developmental level. In children, especially, the availability of
a private garden and a shorter distance from their residential address to a green space seems to be
beneficial [
15
,
19
,
30
]. This observation may reflect the smaller environmental circle that is accessible
at this age. For older children and adolescents, associations with mental health are found more
consistently with average greenness in the surrounding neighborhood and with the quality of green
spaces, suggesting that this age group benefits not merely from the closest green space, but rather from
green spaces of their choice, reflecting their growing autonomy [25].
4.6. Relevance of the Association
In general, concluding a causal pathway between green space exposure and children’s and
adolescents’ mental health might be preliminary. However, the presented associations are resistant
to adjustment for the most important demographic and socio-economic confounders and there is
considerable evidence for plausible mediators between exposure and outcome. Therefore, these results
warrant, from a precautionary principle, attention from policy makers, urban planners and health care
providers. The importance of green space exposure for public mental health is indicated by Younan
et al. and Dadvand et al. Based on their data, they suggest that if surrounding greenness would be
increased over an interquartile range, then respectively 12% and almost 9% percent of children in a
clinical range of aggressive behavior and superior working memory impairment, would move out of
this category [
22
,
32
]. This represents the fact that although individual effects can be rather small and
do not explain the majority of variance in mental health problems, the effects are apparent across the
full dimension of urban-dwelling youth and not only in the clinical range. Therefore, the effects of
green spaces are of societal importance.
4.7. Strengths and Limitations
The strengths of this systematic review lie in the number of identified articles providing
opportunities to compare results of similarly designed studies. Additionally, this review could
include some longitudinal studies, studies on adolescent populations and studies relying on objective

Int. J. Environ. Res. Public Health 2018,15, 2668 13 of 17
neuropsychological tests, which is a fairly recent evolution in this field. In contrast to the recent
systematic review of McCormick [
10
], this review differed in search terms and inclusion criteria,
focusing on observational studies and more prominently on reliable mental health and developmental
outcome parameters. In comparison to the earlier systematic review on the topic by Gascon et al. [
2
],
published in 2015, this review relied on a largely similar methodology, but could include 17 new
studies examining the impact of green spaces on children’s and adolescents’ mental health, proving
the emerging interest in this field of research. In addition to previous work, this review looked
at green space exposure beyond a residential environment, including the school environment and
focused in particular on the available evidence for different outcome domains. It can be noted as
a strength that beneficial associations of green space exposure with mental health are presented,
independent of the measurement instrument, i.e., multiple behavioral and quality of life questionnaires
and neuropsychological tests, and independent of the information source, i.e., parents and children.
Limitations can be identified at the study-level, noting that the assessment of green space
exposure differed largely between studies, although this difference also provided opportunities
to assess which green space exposure concepts might be beneficial in particular. Additionally, the
comparison of outcome results for high and low exposure groups were generally conducted within a
given geographical region, making it difficult to define an optimal, absolute amount of green space
exposure. Furthermore, it can be noted that mental health problems in adolescents such as addiction,
that are not easily detected by general questionnaires as the SDQ or CBCL, fell out of the scope of
this review.
At review-level, despite the thorough exploration of relevant keywords prior to the final search, an
incomplete retrieval of results relying on the selected search terms and databases cannot be excluded.
However, no additional studies were detected via the references of the search results and all studies
matching the inclusion criteria mentioned in earlier systematic reviews were identified, suggesting
that the conducted search was fairly adequate. Grey literature, i.e., publications by organizations or
governments published outside of the traditional academic journals, was not systematically searched.
Although urbanization occurs most quickly in terms of geography—in Africa and Asia—, and —in
terms of economics—, in low- and middle-income countries [
38
], none but one study collected data in
these settings.
4.8. Future Research Suggestions
Based on our discussion and the aforementioned limitations, it would be valuable if future
research would assess differential effects of varying green space exposure concepts (i.e., time spent
in, proximity and quality of green spaces etc.) and different exposure ranges simultaneously,
taking into account non-residential exposure such as exposure in school and leisure environments.
This future research could be particularly valuable in guiding policy-making and urban planning
more concretely. Analogously, future reviewers of literature might benefit from using a broader
set of keywords concerning non-residential green spaces. Keywords such as ‘natural environment’,
‘natural playground’, ‘green schoolyard’ etc., were up to now used to a rather limited extent within
the field. However, these keywords might gain traction in the coming years as can be suspected
from publications that appeared after the final search for this review had been conducted [
39
,
40
].
For reasons of comparability, use of the same outcome measures as in earlier research, such as the
SDQ, is recommended. This instrument can be used for adolescent populations as well, since the
questionnaire is validated for individuals up to 17 years old. The parent-based CBCL or the Youth
Self Report are other valuable alternatives, since they provide broader and more clinical information.
To overcome biases introduced by single informant questionnaires, the use of multi-informant variants
are recommended. Research that addresses mental health problems outside the scope of the frequently
used questionnaires would be welcomed. Gaming and internet addiction could be such a topic
of interest since it can be hypothesized that screen time and green space exposure are inversely
correlated [
15
] and since horticultural therapy has been mentioned as a relevant intervention in the

Int. J. Environ. Res. Public Health 2018,15, 2668 14 of 17
context of addiction [
41
]. Studying the role of green spaces seems particularly valuable in rapidly
urbanizing and highly urbanized countries in Northeast Asia where internet addiction is reported to
be highly prevalent [42].
Additionally, it could be valuable for this line of research to add biological correlates of mental
health to the outcome parameters for comparison with questionnaires and neurocognitive tests.
Recently, hair cortisol analysis in children [
43
] and neuro-immunological measures [
44
] and mobile
neuro-imaging [45] in adults were introduced in this field.
Furthermore, to shed light on other confounding and/or mediating variables of the established
associations, future studies could try to build models assessing the separate roles of air and noise
pollution, physical activity, social interaction and parental mental health. Longitudinal studies with
broad screening tools adapted to different age groups are welcomed to investigate the impact on the
developmental process and to raise the probability of causality. Attention should be paid to change of
address during follow-up as mentioned earlier.
Finally, studies in rapidly urbanizing African and Asian countries seem to be very relevant for
policy guidance.
5. Conclusions
In children and adolescents, there is significant evidence for an inverse relationship between green
space exposure and emotional and behavioral problems. These beneficial associations are resistant to
the adjustment for demographic and socio-economic factors and thus may represent an independent
link. The presented evidence suggests potential partial mediation via physical activity, buffering of
air pollution and social interaction. Future research is needed to shed more light on confounders,
mediators, varying impacts during development and causality. The presented findings warrant the
attention of policy makers, urban planners and health care workers in order to protect the mental health
of children and adolescents in an urbanizing world, by providing sufficient exposure to green spaces.
Author Contributions:
Conceptualization, G.-J.V.; methodology, G.-J.V.; analysis, G.-J.V.; Writing—original draft
preparation, G.-J.V.; Writing—review and editing, M.D.; supervision, M.D.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflicts of interest
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