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|Year : 2014
: 16 | Issue : 70 | Page
|Urban green spaces' effectiveness as a psychological buffer for the negative health impact of noise pollution: A systematic review
Angel Mario Dzhambov1, Donka Dimitrova Dimitrova2
1 Faculty of Medicine, Medical University of Plovdiv, Plovdiv, Bulgaria
2 Department of Health Management, Health Economics and Primary Care, Faculty of Public Health, Medical University of Plovdiv, Plovdiv, Bulgaria
Click here for correspondence address
|Date of Web Publication||20-Jun-2014|
Noise pollution is one of the four major pollutions in the world. Little evidence exists about the actual preventive benefits of psychological noise attenuation by urban green spaces, especially from the perspective of environmental medicine and, to the best of our knowledge, there is not a systematic analysis on this topic. The aim of this review was to systematically evaluate whether there is conclusive scientific evidence for the effectiveness of urban green spaces as a psychological buffer for the negative impact of noise pollution on human health and to promote an evidence-based approach toward this still growing environmental hazard. MEDLINE and EMBASE databases were searched for experimental and epidemiological studies published before June 04, 2013 in English and Spanish. Data was independently extracted in two step process by the authors. Due to the heterogeneity of the included studies qualitative assessment was performed. We found moderate evidence that the presence of vegetation can generally reduce the negative perception of noise (supported with an electroencephalogram test in one of the experimental studies; consistent with the data from two epidemiological studies; one experiment found no effect and one was inconclusive about the positive effect). This review fills a gap in the literature and could help researchers further clarify the proper implementation of urban green spaces as a psychological buffer in areas with population exposed to chronic noise pollution.
Keywords: Green spaces, noise pollution, psychological buffer
|How to cite this article:|
Dzhambov AM, Dimitrova DD. Urban green spaces' effectiveness as a psychological buffer for the negative health impact of noise pollution: A systematic review. Noise Health 2014;16:157-65
|How to cite this URL:|
Dzhambov AM, Dimitrova DD. Urban green spaces' effectiveness as a psychological buffer for the negative health impact of noise pollution: A systematic review. Noise Health [serial online] 2014 [cited 2018 Feb 19];16:157-65. Available from: http://www.noiseandhealth.org/text.asp?2014/16/70/157/134916
| Introduction|| |
Noise pollution is one of the four major pollutions in the world and approximately 80 million people in the European Union suffer from unacceptable noise levels (65 dB) and over 170 million are exposed to noise levels between 55 and 65 dB.  The interest towards environmental noise as a cause for economic losses is growing as it costs approximately 0.2-2% of the gross domestic product in the developed European countries. 
Noise levels above 55 dB are considered as important because they disturb the comfort of hearing. Chronic exposure to noise between 65 and 80 dB can cause damage to the hearing function. In addition, traffic noise - the most prominent and common type of urban noise pollution - causes physiological and cognitive alterations, sleep disturbances, psycho-social stress, etc. , Noise has a significant effect on quality of life even when it is not loud enough to cause medical or psychological symptoms. 
These are the reasons why local authorities are increasingly interested in reducing noise's effects on human health and well-being. Many working solutions are available such as green belts around highways, barriers along railways, green roof installations, double-skin facades, etc. However, these constructions are not only often considered esthetically displeasing by the general population due to blocking the view, but are also costly and provide few other obvious health benefits. On the other hand, if we can design and plan better quality green spaces, they might reduce the general negative reaction to environmental noise. This would be possible because as a unique environmental stressor noise's effects greatly depend on how people perceive it. According to Borsky, dissatisfaction with various components of the environment enhances noise annoyance. , This notion is clearly supported by the Report of the Environmental Protection Department in Hksar, which states that one of the factors affecting sleep disturbance and annoyance is "one's perception of the environment, namely, satisfaction with neighborhood environment." 
Being an essential feature of residential quality, vegetation has been regarded as a cheaper and more natural material to physically reduce outdoor noise pollution in comparison to concrete, metal, plastic and other man-made materials.  The construction of green belts around highways and the incorporation of green spaces in the cities are not new concepts. There are three major ways for vegetation to reduce noise pollution: Diffraction and reflection of sound waves by plant elements; absorption of sound waves and transformation in mechanical vibrations of the plant elements; destructive interference of sound waves. There are some indirect effects of vegetation such as psycho-acoustical effects which are in the scope of environmental hygiene/medicine.  Vegetation influences both the physical properties of sounds and the ways in which people perceive, evaluate and respond to sound in different urban settings. 
Many noise attenuation studies conclude that some level of noise reduction on the physical level can be achieved through the use of roadside vegetation, ,,, or even green roof installations.  Some of them investigated the different types of vegetation that helped attenuate noise exposure,  while others focused on the phenomenon of "soundscape." ,,,
The ultimate goal of noise control is to promote relaxation, satisfaction and well-being in urban residents.  As noise reduction on acoustical level greatly depends on the characteristics of vegetation and the design of urban green spaces the exploitation of the psychological role of vegetation as a noise buffer is much preferable. Little body of evidence exists about the actual preventive benefits of noise attenuation by urban green spaces on psycho-acoustical level.
Even more, there is a lack of synthetic research on the subject. Borderline topics such as this present an excellent opportunity for an interdisciplinary approach to health promotion by bringing together two seemingly distinct research areas such as environmental hygiene and environmental psychology.
The aim of this paper was to answer the questions "Does interaction with urban vegetation modify the way people perceive environmental noise through buffering its psycho-acoustical effects on health?" and "Does the quality of existing evidence allow making feasible inferences?"
| Methods|| |
First, we performed a pilot study, analyzing the publications we found through PubMed and Google search using the keywords "noise" and "vegetation." This process provided an overview of the main features that had to be included in the review and allowed us to prepare a solid background.
We used a systematic review approach. Initial electronic literature search in MEDLINE and EMBASE databases was performed using PubMed and ScienceDirect search engines and the keywords "ruido", "plantas", "espacio verde", "parque", "urban", "green spaces", "vegetation", "greenery", "noise", "people" and "health" in different combinations when appropriate: PubMed - "noise + green spaces" (n = 10), "noise + vegetation" (n = 48), "urban + noise + park" (n = 20); ScienceDirect - "noise + people + psychology + vegetation + green spaces" (n = 220), "noise + people + health + vegetation + green spaces" (n = 913), "noise + people + vegetation + green spaces" (n = 1522), "noise + greenery" (n = 368), "espacio verde + ruido" (n = 60), "ruido + parque" n = (27), "ruido + plantas" (n = 85). The overlapping publications found amongst these results were considered only once. Searches were performed in the period between 25 May and 7 June 2013. Because the topic is generally underexplored no filters were applied. We contacted Assoc. Prof. Atanaska Aleksandrova, MD, PhD as an expert in this field in order to obtain publications in Bulgarian and Russian. We also hand searched the reference lists of the publications selected for quality assessment but we were unable to retrieve the full texts because of the limited access the Library Center of Medical University Plovdiv provided at that moment [Table 1].
The data was abstracted in two step process independently by the authors using methodology from Cochrane Handbook for Systematic Reviews of Interventions 4.2.6 and personal data extraction forms.  Any discrepancies were discussed and resolved with consensus. All experimental and epidemiological studies published before June 04, 2013 were analyzed with restriction to English and Spanish due to lack of sufficient founding for external translator. At the first step, 3273 results were retrieved. At the next step, we hand searched the titles and abstracts and selected the 24 studies that were in the general scope of our review. Their full texts were reviewed by the two authors. To determine the degree of agreement, the Kappa index of inter-observer agreement was calculated.  It was 0.6897. After evaluating the full texts, we selected five studies based on inclusion criteria: To deal with the noise buffering effect of urban vegetation via psychological mechanisms. Studies investigating the physics/acoustics of noise attenuation by vegetation, soundscape or psychological noise reduction not directly influenced by vegetation were excluded. We included five studies for the final quality assessment.
We performed quality assessment with using modified versions of the previously adapted "Newcastle-Ottawa Quality Assessment Scale for cohort studies", , of "Consolidated Standards of Reporting Trials (CONSORT) 2010 checklist of information to include when reporting a randomized trial" and of "CONSORT checklist." , Two different scales were necessary because of discrepancies in study design. The modification of the "Newcastle-Ottawa Quality Assessment Scale for cohort studies" did not differ significantly from the mentioned adaptation. We simply assigned 1 star to "Interview" as a possible assessment of outcome due to the nature of the surveys analyzed. Our modification of "CONSORT 2010 checklist of information to include when reporting a randomized trial" and of "CONSORT checklist" awarded a maximum of 13 "Yes" per study for all of the following elements: Background; Aim; Methods; Intervention; Outcome; Statistical analysis. The definition that we gave to high quality was 12-13 "Yes," to medium quality-10-11 "Yes" and to low quality-9 "Yes" and below.
The quality of the cross-sectional studies was defined as follows: High (10-9 stars), moderate (7-8 stars), low (6 stars and below). Studies were independently checked by the authors and their evaluations were cross-matched and discussed. The Kappa index for the quality appraisal was 0.5833.
All research designs were examined and separated into two broad categories: Interventional studies and observational studies. Due to the clinical and methodological heterogeneity of the designs of the final five studies, we found it inappropriate to undertake quantitative meta-analysis. The latter would result in low statistical power and some analyses like funnel plot would not be reliable. Hohmann et al. cautioned against meta-analyzing and summarizing data from heterogeneous studies.  Therefore, a qualitative assessment of evidence level approach was adopted.
| Results|| |
Li et al. aimed to study the effects of neighborhood greenery as noise annoyance modifier. Gidlöf-Gunnarsson and Öhrström  examined whether the green-area availability moderated resident's noise responses and whether the potential effect of green-area availability varied depending on access or not to a quiet side of the dwelling. The characteristic of the studies are presented in [Table 2].
|Table 2: Characteristics of the observational studies: Population, inclusion criteria, design and outcome|
Click here to view
Li et al. selected randomly 992 participants from residential estates in Hong Kong. Before the full-scale survey they standardized the design and tools with a trial run. Gidlöf-Gunnarsson and Öhrström  used data from previously utilized questionnaire designed for investigating the effects of having access to a quiet side of dwelling on residents' health and well-being.
An inclusion criterion for their study was high road traffic noise exposures (60-69 dB). The remaining residents were excluded from further analyses. Because of financial constraint Li et al. controlled the range of values for the factors of interest and implemented the following inclusion criteria: Road traffic had to be the major noise source; participants' demographic profile had to be comparable and the greenery had to be in vicinity of the estates, had to be different for each of them and had to be perceived by some but not all of the residents.
In the study of Gidlöf-Gunnarsson and Öhrström,  out of the 500 residents 367 lived in dwellings with access to a quiet side and 133 lived in dwellings with no access to a quiet side. Gidlöf-Gunnarsson and Öhrström  compared the current sample with the excluded sample and found that the excluded sample (<60 dB) were significantly (P < 0.05) older (≥46 years of age = 54% vs. 41% for the excluded and current samples, respectively). There were no differences in sex and sensitivity to noise (P > 0.05). The authors did not expect that the differences between the current and excluded samples would affect the results of the study. Only 688 of the participants in Li et al.'s  study provided all the necessary personal data completing the questionnaire and were divided into three groups according to the type of greenery they could perceive from their home. We found no information about cross-checking between respondents and non-respondents.
Both studies were cross-sectional questionnaire surveys. Li et al. conducted theirs over the period from October 2008 to October 2009, while Gidlöf-Gunnarsson and Öhrström  , as stated before, used data from previously designed survey. After selection Li et al.  asked the participants to complete a specially designed questionnaire comprising two batteries: Socio-demographic profile, noise sensitivity, health status and access to green space views; and annoyance reactions to road traffic over the past year. The 500 participants in Gidlöf-Gunnarsson and Öhrström's  study answered a previously used questionnaire designed to assess adverse health effects of noise and perceptions of soundscapes. For their current study the authors used only the batteries "perceived availability to green areas", "longstanding illness", "sensitivity to noise" and "noise annoyance." To ensure that each group had sufficient sample size for further analyses Gidlöf-Gunnarsson and Öhrström's  examined the possibility to create only two green-area groups by comparing mean scores regarding long-term noise annoyance "at home" and "outdoors close to the dwelling."
Li et al. appraised annoyance reactions to road traffic using two scales recommended by ISO standard 15666 (2003) in order to countercheck the results. Noise levels at the estates were predicted using a validated calculation of road traffic noise method. In the study of Gidlöf-Gunnarsson and Öhrström's,  calculations of traffic noise levels were based on traffic input data and geometrical data of the field site.
Li et al.  divided the participants in their study in three groups according to the type of greenery they interacted with, which were homogenous by demographic characteristics. Gidlöf-Gunnarsson and Öhrström  formed two homogenous (P > 0.05) groups: Residents with "poorer" availability to green areas (n = 354) and residents with "better" availability to green areas (n = 146). To validate the construction of these two green-area groups, they examined recent aerial photographs over the study sites and their surroundings and approximately judged the distance between each of the sites and the nearest green area, confirming that their initial definition was correct.
Li et al. analyzed the questionnaire data with an ordered logit model with McFadden's ρ2 value of 0.14 while Gidlöf-Gunnarsson and Öhrström  performed a two-way multivariate analysis of variance (MANOVA) to examine the independent and joint effects of perceived availability to green areas and access to a quiet side of the dwelling on long-term noise annoyance.
Li et al. concluded that the perception of greenery, according to its type, can generally reduce noise annoyance at home. Gidlöf-Gunnarsson and Öhrström  found that residents with "better" access to green areas were significantly less noise annoyed due to road traffic both when being "at home" (P < 0.01) and "outdoors close to the dwelling" (P < 0.001). The results of the study suggested that availability to green areas could buffer the effects of chronic noise exposure on health and well-being.
One of the limitations stated by Li et al.  was that the sample was not truly representative of the average in the population due to financial restrictions. For that reason participants' estates were all located in relatively deprived areas of Hong Kong. Gidlöf-Gunnarsson and Öhrström  emphasized that the secondary utilization of data collected for the previous study might have reduced the variation in access to green areas. Furthermore, the cross-sectional nature of the survey did not permit to determine causal sequences between assessed variables.
Joynt and Kang  aimed to indicate to what extent preconceptions held about varying materials used in noise barrier design impacted on perceived noise reduction. They also aimed to determine whether auditory and visual intersensory interaction influenced respondents' perception of noise attenuation by the noise barriers. Maffei et al. analyzed the effect of several barriers on perceived loudness and noise annoyance at different levels and verified if some interaction among the factors at different levels could be considered to be statistically significant by means of ANOVA analysis. A study by Yang et al.  focused on the psychological effects (psychological noise reduction) of visual sensations from the natural environment and how psychological noise reduction by means of landscaping can achieve improvements in health benefits and psychological behavior. The characteristics of the studies are presented in [Table 3].
|Table 3: Characteristics of the interventional studies: Population, interventions and outcome|
Click here to view
Joynt and Kang  selected a random sample of respondents from the University of Sheffield population through advertising for volunteers. Each volunteer was compensated for their time. Due to restricted time availability and physical spaces, the sample was limited to that which could be assessed during a 2 day period. The opportunity to volunteer was not restricted to students although majority of the participants were students. Maffei et al. selected a sample of participants living next to a railway line as representative of the inhabitants of the area and invited them to participate in the experiment. Like Joynt and Kang,  Yang et al.  randomly selected students from Zhejiang Forestry University.
Neither of the studies defines inclusion or exclusion criteria. The inclusion criteria for Joynt and Kang  were not specified and most probably were solely the consent to participate. Maffei et al. did not describe the inclusion criteria as such but they preliminary asked the participants to report normal hearing and normal or corrected to normal vision. Yang et al.  did not mention inclusion or exclusion criteria.
The dropout rate of neither Joynt and Kang  nor Maffei et al.  or Yang et al.  was described.
Study designs and interventions
The studies of Joynt and Kang  and Maffei et al. had similar designs. Yang et al.  also conducted a lab experiment with a general design like of the other experimental studies but did not include barrier noise attenuation assessments. Joynt and Kang  preliminary recorded audio-visual data using a video camcorder in locations throughout the United Kingdom and then synchronized the audio and visual sequences, while Maffei et al. recorded audio and video of a train passing at constant speed of 70 km/h and edited it with special software. Yang et al.  took videos of a busy road (Nanshan Road, Hangzhou) and the vegetation next to the road. In the study of Joynt and Kang  the barriers were selected in order to represent some standard style types commercially available: Concrete, timber, metal, transparent acrylic and a hedgerow of deciduous vegetation, while Maffei et al. used concrete, flaming colored and plant type barrier. Joynt and Kang  projected five sequential films to the participants on a large screen and the audio sequence was played on four large speakers on either side and behind the screen. Three tests were performed: First, the predetermined assumptions about barrier attenuation were analyzed with only video playing; second, the perception of noise attenuation by the five standard barrier types with a constant noise stimulus was investigated; finally, the esthetic qualities of each barrier were determined. All three test results were measured with a questionnaire. On the other hand, Maffei et al. assessed the noise-related aspects of barriers with different visual characteristics in an immersive virtual reality laboratory test. The subjects were seated in the middle of an anechoic chamber. A couple of loudspeakers and one subwoofer were calibrated by an artificial dummy head, in order to reproduce the same auditory conditions: The same sound levels and the best fitting of the spectrum at the listener's ear. At the beginning each participant had to fill in a questionnaire. Maffei et al. played six scenarios according to noise and barrier type and at the end of each the participants answered a questionnaire. Yang et al.  used videos played through video glasses and recorded sounds like Maffei et al. Their participants also completed a questionnaire prior to beginning of the experiment. Like the two other questionnaires theirs asked about personal characteristics, attitude and noise exposure history of the participants.
Outcome measures and criteria
Joynt and Kang  analyzed the preconceptions without audio stimulus, the perceptions with audio and visual stimulus, the influence of preconceptions on the perceptions of each barrier's performance and esthetic influences. The authors measured participants' responses with specially designed questionnaires and performed a series of statistical comparisons including one-way repeated measure ANOVA. Maffei et al. analyzed the barrier type concerning the visibility of the noise source through the screen, the visual aspect of the barrier concerning some esthetic issues and the noise level at the receiver concerning the acoustic performance of the barrier and the magnitude of the sound source. They also used ANOVA and self-report questionnaire. Yang et al.  analyzed the subjective emotional evaluation of noise exposure, the perceived noise reduction provided by landscape plants and, unlike the other two studies, chose to register quantitative emotional responses using electroencephalogram (EEG) in addition to the questionnaire evaluation. This was the only study that objectified the outcome.
In Joynt and Kang's  study transparent and deciduous vegetation barriers, judged most esthetically pleasing, were judged as the least effective at attenuating noise. In the scenarios with green barrier Maffei et al. found that the participants seemed to arouse less noise annoyance. Nevertheless, this result was not very conclusive. Yang et al.  conclude that both objective and subjective methodologies employed indicated that plants can induce psychological noise reduction.
Only Yang et al.  clearly stated that a limitation of their study was the inclusion only of students at Zhejiang Forestry University, which constitutes a biased sample of subjects. The citizens that suffer the most from noisy urban environments are those that are dwelling outside of the university and range from children to the elderly, particularly those who enjoy recreational activities in street parks and those living close to main roads. Joynt and Kang 35] indirectly mentioned that in future studies the sample should be more representative of the studied population.
| Discussion|| |
The quality of the observational studies was assessed according to the modified checklist for cross-sectional studies [Table 4].
The study of Li et al. scored 7 stars out of 10 according to our modification of the "Newcastle-Ottawa Quality Assessment Scale" and hence, we defined it as of moderate quality. First, information about the comparability between people who completed the questionnaire and the non-respondents was not reported. Second, as stated by the authors, their study was not representative and we found no information that the sample size was justified by a preliminary method for determining the necessary number of participants.
Gidlöf-Gunnarsson and Öhrström's  study scored the maximum of 10 stars and was classified as high quality. We unanimously established this given the justification of the sample size, the described comparison between included and excluded participants and the assessment of the outcome through record linkage, all in contrast to the study of Li et al.
The quality of the interventional studies was assessed according to the modified checklist for interventional studies [Table 5].
According to our modification of "CONSORT 2010 checklist of information to include when reporting a randomized trial" and of "CONSORT checklist", the studies of Joynt and Kang  and Yang et al.  scored 10 out of 13 "Yes" and were classified as of moderate quality. They did not specify inclusion/exclusion criteria nor described the dropout rate. Joynt and Kang  also stated that due to restricted time availability and physical spaces, the sample was limited. The study of Maffei et al. also scored 10 out of 13 "Yes." They neither described the dropout rate nor inclusion/exclusion criteria. Their sample was representative unlike that of the other two studies but it was not randomly selected.
We have to point out a possible bias. The studies of Joynt and Kang  and Yang et al.  included participants regardless of noise exposure from two Universities. The other three studies, on the other hand, purposely assessed the buffering effects of vegetation in individuals exposed to chronic noise pollution: In the studies of Gidlöf-Gunnarsson and Öhrström's  and Li et al.  noise was above 60 dB and the participants in the experiment of Maffei et al. lived close to a railway line.
We present some of the limitations of our review. Probably the most prominent limitation is the limited number of studies which, on one hand, does not allow us to make firm inferences, but on the other only reiterates the necessity for further research. It was beyond the scope of the review to locate unpublished research or grey literature. Moreover, due to the limited full text access that our University provided, we were also unable to retrieve the studies from the hand searching of reference lists. However, having reviewed the abstracts of these articles and having in mind that none of them was published in the last 10 years, it is our strong believe that their non-inclusion in the full text review does not markedly impair the quality of our research, as they would have been eliminated anyway. We also had initial disagreement about the validity of the procedure for noise pollution measurement described by Gidlöf-Gunnarsson and Öhrström's  , which did not influence the final quality appraisal; we also disagreed on the quality of the study of Joynt and Kang,  the one that found vegetation ineffective as a noise buffer: Donka Dimitrova evaluated it of low quality because of inadequate recruitment [Table 5]. Last, we were unable to hire a translator in order to extend our search to foreign languages other than English, Spanish and Russian.
| Conclusions|| |
Due to the disparities between the studies such as inclusion and exclusion criteria, population characteristics, design, etc., firm conclusions cannot be reached regarding the effects of green spaces as a psychological buffer for noise pollution. We found consistent evidence among the observational studies about the attenuation effects of vegetation: The two cross-sectional (one high quality and one moderate quality) studies found that green areas could buffer the effects of chronic noise exposure on annoyance responses. On the other hand, the experimental studies presented conflicting evidence: Joynt and Kang  concluded that vegetation did not effectively reduce perception of noise, Maffei et al. observed some effect but their results were inconclusive and Yang et al.  objectively proved that the buffering effect of vegetation was significant.
Consistent with the epidemiological data we suggest that the presence of vegetation can generally reduce the negative perception of noise which was strongly supported with an EEG test in one of the experimental studies. In fact, only the study of Joynt and Kang  found vegetation ineffective and, as stated before, we had initial disagreement on one element of its design that would have lowered its quality to "low."
Environmental esthetics is an important feature of its quality, whose effects should be quantified. It is plausible that esthetics or more generally speaking the visual aspects of the environment and its elements can modify noise sensitivity, annoyance and hence health outcomes. Stress levels, for example, are lower when people interact with visually pleasing landscapes which makes noise more tolerable.  The time spent in green spaces, their proximity, physical and geomorphological characteristics should be measured.
The evidence summarized in this systematic review, however, needs to be interpreted with caution because of the fact that only five relevant studies were identified and because of their quality (multiple moderate quality and only one high quality study).
As we are unaware of a previous systematic review on the topic, we believe that ours fills a gap in the literature and presents a critical evaluation that could help researchers to further illuminate the adequate implementation of urban green spaces as a psychological buffer in areas with population exposed to chronic noise pollution. Further experimental research is required using validated tools (questionnaires and noise exposure assessment equipment) and standardized designs.
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Angel Mario Dzhambov
MMSc student, Faculty of Medicine, Medical University of Plovdiv, No. 15-A, "Vasil Aprilov" Blvd., 4002 Plovdiv
Source of Support: None, Conflict of Interest: None
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]
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|[Pubmed] | [DOI]|