Environmental Research 215 (2022) 114387 Contents lists available at ScienceDirect Environmental Research journal homepage: www.elsevier.com/locate/envres The evaluation of the 3-30-300 green space rule and mental health Mark J. Nieuwenhuijsen a, b, c, *, Payam Dadvand a, b, c, Sandra Márquez a, b, c, Xavier Bartoll d, e, Evelise Pereira Barboza a, b, c, Marta Cirach a, b, c, Carme Borrell b, c, d, e, Wilma L. Zijlema a, b, c a Barcelona Institute for Global Health (ISGlobal), Doctor Aiguader 88, 08003, Barcelona, Spain b Universitat Pompeu Fabra (UPF), Doctor Aiguader 88, 08003, Barcelona, Spain c CIBER Epidemioloǵıa y Salud Pública (CIBERESP), Melchor Ferńandez Almagro, 3-5, 28029, Madrid, Spain d Agència de Salut Pública de Barcelona, Avinguda Pŕıncep D’Astúries, 63 (1-1), 08012, Barcelona, Spain e Institut de Recerca Biomèdica Sant Pau (IIB-Sant Pau), 08041, Barcelona, Spain A R T I C L E I N F O A B S T R A C T Keywords: Background and aims: Urban green space has many health benefits, but it is still unclear how much actually is Greenspace needed for better health. Recently a new 3-30-300 rule of thumb for urban forestry and urban greening has been Greenness proposed, but this rule has not been evaluated for benefits on health. The rule requires that every citizen should Tree cover be able to see at least three trees from their home, have 30 percent tree canopy cover in their neighbourhood and 3-30-300 Anti-depressants not live more than 300 m away from the nearest park or green space. The aim of this study was to evaluate the Sedatives relationship between the 3-30-300 green space rule and its components in relation to mental health. Tranquilizer Methods: We conducted a cross-sectional study based on a population-based sample of 3145 individuals aged Medication use 15–97 years from in Barcelona, Spain who participated in the Barcelona Health Survey (2016–2017). We created Psychologist visits 3-30-300 green space indicators using questionnaire data, GIS, remote sensing and land cover maps. Mental Psychiatrist visits health status was assessed with the 12-item General Health Questionnaire (GHQ-12) and also the use of tran­ Mental health quilizer/sedatives or antidepressants and psychiatrist or psychologist visits. Analyses were conducted using mixed effects logistic regression models with districts as the random effect, adjusted for relevant covariates. Results: We found that people in Barcelona had relatively little exposure to green space, whether through window view, living in an area with sufficient greenness, or access to a major green space, and only 4.7% met a surrogate 3-30-300 green space rule. Residential surrounding greenness, but not tree window view or access to major green space, was significantly associated with better mental health, less medication use, and fewer psychologist or psychiatrist visits. Meeting the full surrogate 3-30-300 green space rule was associated with better mental health, less medication use, and fewer psychologist or psychiatrist visits, but only for the latter combined the association was statistically significant (Odds ratio = 0.31, 95% CI: 0.11, 0.91). Conclusion: Few people achieved the 3-30-300 green space in Barcelona and we used a surrogate measure. We observed health benefits when the full surrogate rule was met. 1. Introduction increase biodiversity in cities, particularly through well-designed green infrastructure and networks throughout the city (Coutts and Hahn, Urban green space has many health benefits including longer life 2015; Nieuwenhuijsen, 2021). expectancy, fewer mental health problems, improved cognitive func­ The availability of green space varies quite considerably between tioning, better mood and healthier babies (Gascon et al., 2015; Nieu­ cities and is also not equally distributed within cities, with some people wenhuijsen et al., 2017a; Markevych et al., 2017; Rojas-Rueda et al., having easy access to green space, while many others have no or little 2019). It also mitigates air pollution, heat and noise levels, is associated access (Schüle et al., 2017; Pereira Barboza et al., 2021; Mushangwe with lower air pollution emissions and contributes to CO2 sequestration et al., 2021). Green space does not only include parks, but also green and therefore helps with mitigation and adaptation to the climate crisis such as trees in streets or gardens (Salmond et al., 2016; Wolf et al., (Nieuwenhuijsen, 2020). Green space can improve ecosystems and 2020) and green infrastructure like green roofs and walls * Corresponding author. ISGlobal, Doctor Aiguader 88, 08003 Barcelona, Catalonia, Spain. E-mail address: mark.nieuwenhuijsen@isglobal.org (M.J. Nieuwenhuijsen). https://doi.org/10.1016/j.envres.2022.114387 Received 6 June 2022; Received in revised form 15 September 2022; Accepted 17 September 2022 Available online 23 September 2022 0013-9351/© 2022 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). M.J. Nieuwenhuijsen et al. Environmental Research 215 (2022) 114387 (Nieuwenhuijsen, 2021). Agency (2018), close visual inspection showed that the map missed out a We know that green space is good for health, but it is still not clear lot of tree canopy cover in Barcelona, and using the map only three how much exactly and in which form, but urban planners need some subjects had more than 30% tree canopy cover. Instead, we used resi­ guidance. Recently Cecil Konijnendijk, an urban forester, proposed the dential surrounding greenness, as a surrogate for tree canopy cover, new 3-30-300 rule of thumb for urban forestry and urban greening, or in which was calculated as the average Normalized Difference Vegetation short 3-30-300 green space rule, where he suggested that every citizen Index (NDVI) in 500 m buffers. NDVI imagery (30 m × 30 m resolution) should be able to see at least three trees (of a decent size) from their was obtained from Landsat 8 image taken on August 14, 2016 and home, have 30 percent tree canopy cover in their neighbourhood and processed to remove the influence of negative pixels from water bodies not live further than 300 m away from the nearest park or green space and atmospheric attenuations. The NDVI can be calculated since healthy (IUCN, 2021; Konijnendijk et al., 2022). vegetation absorbs most visible light and reflects large parts of There is some good evidence that 30 percent tree canopy cover in the near-infrared light, and sparse vegetation reflects more visible light and neighbourhood is beneficial for health, but for access to a major green less near-infrared light. Based on this distinction, a value between 0 and space and visibility of trees from the window, the evidence is more 1 indicates photosynthetically active vegetation with higher values mixed or has been less reported (Gascon et al., 2015; Triguero-Mas et al., indicating higher density of green vegetation (Weier and Herring, 2000). 2015; Labib et al., 2020; Astell-Burt et al., 2021; Astell-Burt and Feng, The correlation between the tree canopy cover map and NDVI was 2019; Astell-Burt and Feng, 2020a; Astell-Burt et al. 2020b; Callaghan strong (r = 0.88) but it was driven by some extreme values and the tree et al., 2021). A number of recent studies showed that green views from canopy cover map did not perform well at the lower end. We dichoto­ windows showed stronger associations with lower depressive symptoms mized the NDVI into <0.3 and =>0.3 assuming the latter coincides with and better mental health, particularly during the COVID19 pandemic a significant amount of surrounding greenness similar to a 30% tree (Dzhambov et al., 2021; Ribeiro et al., 2021; Soga et al., 2021; Spano canopy cover although in reality the canopy cover may have been lower. et al., 2021). No study has evaluated all the three components combined. - 300 m from the nearest park or green space: The linear distance to The 3-30-300 green space rule is appealing and widely promoted by the nearest major green space was calculated with data inputs from urban foresters and some urban planners, but there is an urgent need to Catalan and European land use maps (Land Cover Map of Catalonia, test whether the 3-30-300 rule is actually associated with better health. MCSC version 4 for amount of green space; and Urban Atlas, 2012 for The aim of this study was to evaluate the relationship between the 3-30- distance to green space) (CREAF, 2009; European Environment Agency, 300 green space rule and its components in relation to mental health. 2014) and dichotomized into distance of≤300 m and >300 m. We developed a 3-30-300 green space score (range 0–3) by adding up 2. Methods the number of times that they met one of the components of the 3-30- 300 green space rule, with a score of 3 meaning that the full 3-30-300 2.1. Study sample rule was met. This cross-sectional study was based on a population-based sample of 2.3. Mental health status 3145 individuals aged 15–97 years in Barcelona, Spain who participated in the Barcelona Health Survey (2016–2017). Recruitment occurred Different aspects of mental health status were characterized using randomly from the municipal register of residents, while considering the three sources of information: (i) Perceived general mental health was age and sex structure of the 10 districts for representativeness purposes. assessed using the 12-item General Health Questionnaire (GHQ-12), Data collection was conducted through face-to-face interviews applying which is a multidimensional scale that assesses distinct aspects of the Computer-Assisted Personal Interviewing (CAPI) system [26] for the distress in the last 30 days (Sánchez-López and Dresch, 2008). Questions 4000 recruited participants on socio-demographic characteristics and were answered on a 4-point Likert scale (from 1 to 4) and were mental and physical health and well-being. The response rate was dichotomized (0/1) and summed, resulting in a score ranging from 0 to 33.5%. Non-participants were replaced by randomly selected in­ 12. A score of ≥3 indicated being at risk of psychiatric problems dividuals with identical characteristics: same district, sex and age. The (Goldberg, 1978). The GHQ-12 is shown to be a valid and reliable organisation of the Barcelona HIS was in agreement with the Law of 23/ screening tool for non-psychotic psychiatric problems (i.e., mental 1998 on Statistics of Catalonia regarding the fulfilment of statistical health status) in the Spanish population (Sánchez-López and Dresch, confidentiality Further details on the recruitment process and data 2008). (2) Data on the use of tranquilizer/sedatives and antidepressant collected are presented elsewhere (Bartoll et al., 2018; Bauwelinck et al., in the last two days was collected with two questions regarding the 2020). medication use (yes/no) and included those prescribed by a doctor, suggested by a pharmacist, or self-prescribed. (iii) Data on psychiatrist 2.2. Green space exposure variables or psychologist visits was obtained applying a question whether par­ ticipants had visited a psychiatrist or psychologist during the last year Residential addresses at the time of the study were geocoded and (yes/no). used for the assessment of residential green space exposure. The 3-30- 300 green space rule was assessed as follows: 2.4. Covariates - 3 trees from every home: Greenness of window view was assessed with a question on whether the participant was able to see green ele­ The covariates were chosen a priori based on previous studies ments from their home (none, some, quite some, very much). We com­ (Dadvand et al., 2016; Zijlema et al., 2015). Age, sex, marital status bined this information with information on the number of trees within (single, married, widow (er) divorced/separated), place of birth (within 15 m of the residence. The information on the number of trees was Spain, outside Spain), education level (no formal education/primary obtained from Institut Cartogràfic i Geològic de Catalunya (ICGC, 2014). education, secondary education, university), perceived income situation We created a variable “window view” with being able to see green space (very difficult/difficult, some difficulties, quite easy, (very) easy), (no vs some, quite some, very much) and at least three trees within 15 m employment status (employed, unemployed/pensioner/homemaker, (yes/no). Participants with no in both were classified as no window student, incapacitated for work), social class (5 groups based on occu­ view, while the rest (some, quite some, very much view and at least 3 pation, ranging from unskilled worker to company director with >10 trees) was classified having a window view. employees), and length of residence (in months) were collected within - 30 percent tree canopy cover in every neighbourhood: Although we the Barcelona Health Survey (Bartoll et al., 2018). Unemployment rate had access to tree canopy cover maps of the European Environment (% persons aged >16 years unemployed) at census tract level was used 2 M.J. Nieuwenhuijsen et al. Environmental Research 215 (2022) 114387 as an indicator for area-level socioeconomic status and was obtained Table 1 from the atlas of the Urban Vulnerability Index of Spain 2011) (Instituto Study sample characteristics of the Barcelona Health Survey 2016 (n = 3145). Nacional de Estadística, 2015). Population characteristics n (%)/mean (SD) 2.5. Statistical analysis Age (years) 49.2 (18.8) Sex Females 1640 (52.2) Descriptive statistics were used to characterize the study sample. Males 1505 (47.9) Analyses were conducted using mixed effects logistic regression models Marital status Single 1147 (36.5) with random intercepts for districts and robust standard errors to take Married 1566 (49.8) into account clustering within districts (Cerin, 2011). Models were Widow (er) 191 (6.1) Separated or divorced 241 (7.6) adjusted for age, sex, marital status, place of birth, education level, Place of birth In Barcelona/Catalonia/Spain 2399 (76.3) perceived income situation, employment status, social class, length of Outside Spain 746 (23.7) residence and area-level unemployment rate. We used the missing in­ Education No formal education/primary 1113 (35.4) dicator method to account for missing covariate data. We also tested for Secondary 852 (27.1) University 1180 (37.5) effect modification by sex, age and social economic status. Analyses Perceived income situation Very difficult/difficult 447 (14.2) were undertaken using STATA version 14.2 (StataCorp, 2015). Somewhat difficult 727 (23.1) Somewhat easy 1117 (35.5) 3. Results (very) Easy 854 (27.2) Social class Company director/manager 695 (22.1) with >10 employees; university- In total 3145 participants were included in the study. They were on level profession average 49 years old, 52% were female, 37.5% had university education, Company director/manager 313 (9.9) and 76% were born in Spain (Table 1). Eighteen percent reported poor with <10 employees; artist; mental health, with 8.3% percent reporting visits to the psychologist or sports profesional Administrative worker; self- 761 (24.2) psychiatrist over the last year and 9.4% reporting the use of tranquil­ employed izers/sedatives and 8.1% the use of antidepressants in the last two days Manual worker 943 (30.0) (Table 1). Just over 43% had a green window view and at least 3 trees Unskilled worker 433 (13.8) within a 15-m buffer, 62.1% had a major green space within 300 m, and Employment status Employed 1721 (54.7) 8.7% lived in an area with sufficient surrounding greenness. Nearly Unemployed/pensioner/ 1107 (35.2) homemaker 22.4% did not have any of these and 4.7% had all three and met the 3- Student 203 (6.5) 30-300 rule for green space (Table 1). Incapacitated for work 97 (3.1) A home window view with at least 3 trees within 15 m was not Missing 17 (0.5) associated with any of the outcomes. Having a sufficient level of sur­ Length of residence (months) 239 (216) Area-level unemployment rate (%) 21.7 (8.7) rounding greenness (=>0.3) was associated with a lower risk for all the 3–30–300 rule Green space indicators outcomes and this was statistically significant for poor mental health Green window view (home) Home view no (less than 3 trees 1786 (56.8) (Odds ratio (OR) = 0.60, 95% confidence interval (CI): 0.40–0.90), combined tree cover at least 3 or 3 or more trees) and Home combined medication use in the last 2 days (OR = 0.60, 95% CI: trees in 15 m buffer (%) view some, quite some and very 0.38–0.97), visit to the psychiatrist over the last 12 months (OR = 0.22, much and less than 3 trees Home view some, quite some 1359 (43.2) 95% CI: 0.08–0.62), and visit to the psychiatrist or psychologist over the and very much and more than 3 last 12 months (OR = 0.37, 95% CI: 0.19–0.73) (Table 2). Although trees there was some suggestion for a reduced risk with distance to a major 30% Tree cover/Residential <30% 2870 (91.3) green space for all the outcomes, none of the associations were statis­ surrounding greenness 500 m tically significant (Table 2). buffer (NDVI >0.3) >30% 275 (8.7) An increase in the 3-30-300 green space score was generally asso­ Distance to green space <300 m 1952 (62.1) ciated with a lower risk for all the outcomes, but only for visits to the >300 m 1193 (37.9) psychiatrist or psychologist combined over the last 12 months (OR = Score any criteria tree cover at None (reference) 703 (22.4%) 0.31 95%CI 0.11, 0.91) was this statistically significant when the 3-30- least 3 trees & green window view (home), more 30% 300 rule was fully met. residential surrounding We found no evidence for a statistically significant effect modifica­ greenness (NDVI>0.3), less tion by sex, age or social-economic status (stratified analyses results in 300 m distance to green space Supplement Tables S1, S2, S3). 1/3 criteria 1445 (45.9%) 2/3 criteria 850 (27%) 4. Discussion 3/3 criteria 147 (4.7%) Health outcomes To our knowledge, this is the first study evaluating the health ben­ Mental health (GHQ-12) Good 2578 (82.0) efits of the 3-30-300 green space rule, in which we used residential Poor 567 (18.0) Psychiatrist visit in the last 12 Yes 166 (5.3) surrounding green space as a surrogate for tree cover canopy. We months observed that in a population-representative sample of Barcelona in­ No 2976 (94.7) habitants only 4.7% met all the three criteria of this rule. Residential Psychiatrist or Psychologist visit surrounding green space, but not tree window view or access to major in the last 12 months green space, was statistically significantly associated with better mental Yes 259 (8.3) No 2882 (91.7) health, less medication use, and fewer psychologist or psychiatrist visits. Antidepressant use in the last Yes 254 (8.1) Similarly, meeting the full surrogate 3-30-300 green space rule was two days associated with better mental health, less medication use, and fewer No 2891 (91.9) psychologist or psychiatrist visits, but only for the latter combined was Tranquilizer/sedative use in the Yes 296 (9.4) last two days this statistically significant. No 2849 (90.6) This study evaluated the effect of the 3-30-300 green space rule on 3 M.J. Nieuwenhuijsen et al. Environmental Research 215 (2022) 114387 Table 2 Associations between green space attributes and poor mental health in the Barcelona Health Survey 2016 (n = 3145). OR (95% CI) Poor mental health Tranquilizer/ Antidepressant use Medication Psychiatrist visit Psychiatrist or (General Health sedative use in the in the last two days combined in the last 12 Psychologist visit in Questionnaire) last two days months the last 12 months Home view (some, quite some and very 0.88 (0.72, 1.09) 1.11 (0.85, 1.46) 1.14 (0.86, 1.51) 1.054 1.33 (0.95,1.86) 1.12 (0.85,1.47) much) and more than 3 trees within 15 m (0.83,1.34) Residential tree cover >30% within 500 0.60 (0.40, 0.90) 0.66 (0.39, 1.12) 0.71 (0.41, 1.22) 0.60 (0.38, 0.22 (0.08, 0.62) 0.37 (0.19, 0.73) m with NDVI >0.3 as surrogate 0.97) Distance to green space <300 m 1.05 (0.85, 1.31) 0.89 (0.67, 1.17) 0.84 (0.63, 1.12) 0.83 (0.65, 0.84 (0.60, 1.18) 0.97 (0.74, 1.28) 1.06) Score any criteria green None view (home) & 3 or (reference) more trees, more 30%, less 300 m distance 1/3 criteria 1.08 (0.83,1.40) 1.01 (0.71, 1.43) 0.75 (0.52, 1.07) 0.85 1.06 (0.69, 1.63) 1.10 (0.77,1.55) (0.63,1.15) 2/3 criteria 0.87 (0.64,1.17) 0.92 (0.62, 1.36) 0.96 (0.65, 1.42) 0.87 (0.62, 1.11 (0.69, 1.79) 1.13 (0.77, 1.67) 1.21) 3/3 criteria 0.77 (0.44,1.36) 0.85 (0.42, 1.74) 0.54 (0.24, 1.23) 0.57 (0.29, 0.24 (0.05, 1.05) 0.31 (0.11, 0.91) 1.11) health, specifically mental health. Previous evidence suggested that 30 ideal. The correlation between the tree canopy cover map and NDVI was percent tree canopy cover in the neighbourhood was beneficial for strong (r = 0.88) but it was driven by some extreme values and the tree health, but for access to a major green space, the evidence was more canopy cover map did not perform well at the lower end. An NDVI value mixed (Gascon et al., 2015; Triguero-Mas et al., 2015; Labib et al., 2020; of 0.3 generally probably represents less than 30% tree canopy cover, Astell-Burt et al., 2021; Astell-Burt and Feng, 2020a; Astell-Burt et al. but only with this measure we found statistically significant associations 2020b; Callaghan et al., 2021). Also in this study, we found that with mental health outcomes and it was most likely driving the associ­ particularly residential surrounding greenness as a surrogate for 30% ations we saw between the 3-30-300 indicator and mental health out­ tree canopy cover was associated with better mental health outcomes. comes. We urgently need a high quality tree canopy cover map for Access to a major green space did not show an association with mental Barcelona, although it may not help much with analyses presented here health outcomes, which for the latter was also found in a similar study in though because it may not change the distribution of people with suf­ the area (Triguero-Mas et al., 2015). However, access to a major green ficient tree canopy cover as there is relatively little in Barcelona. space, like a park may have other health related benefits such as physical Future studies should be conducted in cities with more tree canopy activity (Bancroft et al., 2015). We did not find an association with the cover than Barcelona as the lack of green space, particularly sufficient green window view which was surprising, given the number of recent tree canopy cover limits the ability to evaluate the 30 (%) aspect of the studies that showed associations between green views from windows 3-30-300 rule. Of course, the question is how feasible a 30% tree canopy showed and lower depressive symptoms and better mental health, cover is, particularly in compact cities which have other advantages particularly during the COVID19 pandemic (Dzhambov et al., 2021; such as higher walkability and destination proximity. Higher tree can­ Ribeiro et al., 2021; Soga et al., 2021; Spano et al., 2021). opy may be associated with sprawl, reduced walkability and larger The 3-30-300 green space rule focuses on the presence of green distances to destinations. Furthermore, we combined the self-reported space, but some studies have suggested that it is not (only) the presence, number of trees with a GIS based measure of the availability trees but the actual visits to nature that are important for better health and around the residence, which may give a good sense of what people well-being and suggested at least 2 h per week of visits (White et al., perceive and what actually is there, but is also a bit subjective and not 2019, Kruize et al., 2019). We did not evaluate the use of and visits to always feasible. Future studies possible could use deep learning methods green space, but this may be an important factor. Also we did not include to extract street view images to estimate the trees visible from windows. information on quality of green space which also may be an important We did not find an association with access to a major green space within factor for obtaining benefits (van Dillen et al., 2012; Kruize et al., 2019; 300 m and it may be that access is only important and relevant if actual Knobel et al., 2021). visits take place and this should be included in further studies. We found The strength of this study is the fairly large sample size and its an association with an NDVI of 0.3 and although NDVI is a very crude population representativeness, the detailed exposure measures such as measure, future studies should continue evaluating this, as an easy the combination of window view and actual trees and a detailed char­ measure, although the absolute NDVI value may differ by city based on acterization of mental health indicators, including perceived general for example the season and type of green space. mental health, medication use, and visits to psychiatrists and psychol­ What is obvious from the study is that there is relatively little green ogists. But it has also a number of limitations including the cross- space in Barcelona with only a small percentage of people meeting the 3- sectional nature of the study, the lack of large amount of tree canopy 30-300 green space rule, even though it has beneficial effects on mental cover in Barcelona and the lack of high-quality data on tree canopy health. There is an urgent need to provide more green space to the cit­ cover for which we had to use a surrounding greenness indicator (i.e. izens. We may need to dig up asphalt and plant more trees, which will NDVI) instead. The 3-30-300 green space rule does not specify quality or not only improve health but also reduce heat island effects and use of green space, but information on this may have strengthened the contribute to CO2 sequestration (Nieuwenhuijsen et al., 2017a). 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