A Meta-Analysis of the Impact of Urban Blue Spaces on Residents' Mental Health

Abstract

The research on the impact of urban blue spaces on residents' mental health has attracted great attention from scholars internationally, and quantitative studies of the effects dominate the current academia. This study, on the basis of reviewing the theories of urban blue spaces and residents' mental health, conducted a meta-analysis of 47 key studies by systematically selecting and examining the literature from Web of Science, CNKI, and other databases. This paper analyzed the measuring indicators and research models among the literature and standardized the effect size of the research findings. The meta-analysis results include that: 1) the measurements of the characteristics of urban blue spaces are mainly conducted in space-based and individual-based dimensions; 2) residents' mental health is mainly measured from aspects of general mental health, positive psychology, and negative psychology; 3) the proximity of blue space has a significant positive effect in improving residents' general mental health and positive psychology; 4) the availability of blue space is significantly positively correlated with general mental health and positive psychology; 5) although there are studies confirming that factors such as blue space visibility, frequency of visit, and exposure types have an impact on mental health, the relevant studies are still limited; and 6) research on the effect of blue spaces on negative psychology is controversial, especially on mental disorders such as depression, and the findings among existing studies vary significantly. The results of this meta-analysis can provide guidelines for future research and the construction of healthy cities.

Graphical abstract

Keywords

Urban Blue Spaces / Mental Health / Meta Analysis / Environmental Exposure / Effect Size

Highlight

	● Conducts a meta-analysis of the impact of urban blue spaces on residents' mental health
	● Clarifies the measuring indicators and research models of the impact of urban blue spaces on residents' mental health
	● Quantifies the effect of urban blue spaces on residents' mental health in dimensions of proximity, availability, and visibility
	● The effect of urban blue spaces on negative psychology is controversial, especially their effect of depression and other psychological disorders, and the findings among existing studies vary significantly

1 Introduction

Rapid urbanization, while significantly boosting economic development, has also caused a number of environmental and social issues, posing threats to people's physical and mental health^[1]. Statistics indicate that the depression risk rate among Chinese residents reaches as high as 10.6%^[2]. Improving public mental health has become one of the United Nations' Sustainable Development Goals^[3][4], highlighting mental health as a global concern. Urban blue space refers to all surface water in urban environments^[5], including natural water bodies like coasts and lakes, as well as artificial water bodies like fountains and ponds^[6]. As a crucial part of the urban natural environment, approximately half of the global population lives within 3 km from blue spaces^[7]. These blue spaces not only mitigate the urban heat island effect and promote social activities, but also help alleviate psychological stress, thereby improving mental health^[8][9]. Therefore, exploring the influence of urban blue spaces on mental health is of great significance for future urban planning and design.

Scholars have increasingly focused on the relationship between urban blue spaces and mental health, though research results vary. Some studies suggest that residents living near urban blue spaces tend to have better mental health^[10], more effective stress recovery ^[11], and lower rates of antidepressant use^[12]. Residents who have regular visits to urban blue spaces report higher subjective well-being^[13], while those with more blue space around their residences^[14] or more views of blue space^[15] report more positive mental health. Compared with urban green spaces, some research indicates that blue spaces have a more pronounced health-promoting effect ^[16] because they can offer diverse water activities that provide psychological benefits^[17]. However, other studies have found no significant correlations between urban blue spaces and mental health indicators due to the variety of geographic environment, research design, and individual factors^[18]. Additionally, the effects of urban blue spaces on different types of mental health issues vary^[19]; for example, they can reduce anxiety through stress relief pathways but have less effect on depression and other mental disorders^[17][20][21]. Given these research disparities, a systematic review of relevant studies is necessary to outline existing theoretical frameworks and discuss research shortcomings and controversies.

Although some reviews have examined the effects of urban blue spaces on mental health, most are qualitative studies and offer limited guidance for future research directions. In contrast, meta-analysis adopts systematic and rigorous literature retrieval and screening criteria, and selects analytical models based on literature heterogeneity to provide more reliable and accurate conclusions ^[22]. Given the variations in research design and sample size in environmental and health studies, meta-analysis has been increasingly adopted to mitigate individual bias on the overall results^[6][23][24]. For instance, Niamh Smith et al. conducted a meta-analysis of 25 studies, revealing the positive effects of blue spaces on residents' obesity rates and mental health^[6]; however, this study did not differentiate types of mental health, and the literature for the meta-analysis was the works published before August 2019. Thus, an updated meta-analysis is needed to more comprehensively and precisely elucidate the effects of urban blue spaces on residents' mental health.

As a response, building on the existing theories of urban blue spaces and mental health, this research screened and analyzed empirical studies published between 2000 and 2022 sourced from databases of WoS (Web of Science), Medline, CNKI (China National Knowledge Infrastructure), and Wanfang Data. The study aims to reveal: 1) the characteristics of urban blue spaces that affect residents' mental health, the types of mental health, and the corresponding measurement methods; and 2) the effect size of each characteristic of urban blue spaces on mental health. This study will provide scientific evidence for urban planning and design practices for improving residents' mental health.

2 Theoretical Foundation and Research Framework

Research on urban blue spaces and mental health, as an emerging branch in the field of environment and health, integrates theory and knowledge systems from disciplines such as health geography, landscape ecology, and environmental psychology. Despite the varied interests of different disciplines, they collectively provide theoretical support for understanding the influencing mechanisms of urban blue spaces on residents' mental health^[8].

Specifically, research in health geography is centered on the Social-ecological Theory, regarding urban blue spaces as crucial components of the built environment that can influence residents' mental health directly or indirectly^[25][26]. Such studies often operate on a macro scale (e.g., national, regional, city), using large-scale standardized survey data (with analytical unit of community), and explore the heterogeneity of urban blue spaces' effects on mental health across different community types.

Landscape ecology research, supported by the Ecosystem Services Theory, examines the relationship between urban blue spaces and residents' mental health from a supply-demand perspective, proposing evaluation models for the health benefits of natural environments^[27]. This theory considers urban blue spaces as an essential component of urban ecosystems, where blue spaces are the supplier of ecosystem services and residents are the demander. The effective matching of supplier and demander occurs through environmental exposure and contact ^[28], where residents engage with urban blue spaces via multi-sensory perception (e.g., visual, auditory) to promote mental health. Such studies typically focus on meso- and micro-scales (e.g., community, single or several sites of blue spaces), emphasizing the differential impacts of various types of urban blue spaces on mental health.

In environmental psychology, the Attention Restoration Theory and the Biophilia Hypothesis are the most widely discussed foundational theories. The former, proposed by American environmental psychologists Stephen and Rachel Kaplan, posits that natural environmental elements (including blue spaces) have eight restorative qualities—such as being away, extent, and mystery—that aid in the recovery of directed attention^[29][30]. And the latter, emphasizing humans' innate affinity for nature, holds that nature plays a significant role in healing and improving human mental and physical health ^[31]. Similar to that in landscape ecology, environmental psychology research also focuses on meso- and micro-scales and utilizes experimental methods to explore the differential impacts of urban blue space characteristics and exposure types on residents' mental health.

Despite the disparity of research emphases across disciplines, all of them stress the significant role of urban blue spaces in strengthening residents' mental well -being, by measuring spatial dimensions (e.g., proximity, availability, visibility) and individual dimensions (e.g., visiting characteristics, exposure types). The influence of urban blue spaces on mental health can be summarized into three pathways^[8][32]. 1) Regulatory pathway: urban blue spaces regulate microclimates (e.g., reducing temperature, increasing humidity), improving residents' psychological and emotional states^[33]; additionally, sounds from water bodies like streams and fountains can improve the soundscape, enhancing psychological comfort^[34]. 2) Promoting pathway: urban blue spaces can provide places for physical and recreational activities that promote mental health ^{[17][35]~[37]}, and also foster social interactions, enhancing a sense of belonging and neighborhood cohesion, which benefits mental health^[38]. 3) Restorative pathway: as a vital component of natural environments, urban blue spaces can relieve psychological stress and restore attention, thereby promoting mental health^[8] (Fig.1).

3 Data and Methods

3.1 Data Sources

Sourcing from databases of CNKI and Wanfang Data for Chinese literature, and WoS and Medline for English literature, this meta-analysis constructed three thematic keyword inventories, "blue space, " "resident, " and "mental health" in both languages (Tab.1). Literature searches were conducted using these combined keywords, covering the works published from January 1, 2000, to December 31, 2022, among relevant fields such as "Urban Studies, " "Geography, " and "Environmental Sciences." Additionally, a further search was performed among the "highly cited papers" from WoS. In total, 5, 765 journal articles were collected (Fig.2).

Subsequently, a two-step screening process was employed to select the valid samples. The initial screening, using the Rayyan (a tool for systematic literature review) examined titles, abstracts, and keywords to remove duplicates and irrelevant studies, and obtained 481 articles (432 in English and 49 in Chinese). The second round screening was to select empirical studies through full-text reading: 1) selecting empirical studies that employed cross- section analysis, longitudinal analysis, randomized crossover trial or group experiment, and excluding qualitative studies; 2) selecting natural and constructed blue spaces in urban areas, and excluding studies which utilized blue spaces as an intermediary factor or mixed with green spaces; 3) selecting research that focused on urban regions, and excluding those studied microelements and microorganisms; 4) eliminating the literature without available full text. This process yielded 47 eligible articles^{[10]~[15][17][18][20][21][32]~[36][38]~[69]}, 45 in English and 2 in Chinese (Tab.2).

3.2 Research Methods

Meta-analysis typically includes both qualitative summarization and quantitative synthesis ^[6]. Qualitative summarization provides an overview of the included studies, encompassing research design, measurement indicators, sample characteristics, etc. Quantitative synthesis is conducted by combining effect sizes, such as standardized mean difference (SMD) and correlation coefficients, to represent the strength of the correlation between two variables. Among environmental health meta-analyses, SMD is often adopted as the indicator of effect size^[23][24]. In this paper, health effect size refers to the variables' impact on health, and the quantitative synthesis comprises two steps: 1) using Psychometrica, an effect size conversion calculator, the effect sizes of some included studies were converted to SMD; 2) using RevMan 5.4 software, the sample sizes and converted SMD values were aggregated—due to the heterogeneity of psychological health dimensions among study subjects, indicated by a high I² (I-squared, a statistical measure for heterogeneity in meta-analysis), a random effect model was used^[23][70]. Given the disparity of research design and the feasibility of effect size conversion, not all included studies were suitable for quantitative synthesis. Therefore, only the studies meeting the requirements underwent effect size aggregation, while the others were performed qualitative summarization.

4 Results Analysis

Statistical results (Fig.3) indicate that the number of publications has shown a fluctuating upward trend since 2010, with a peak period from 2018 to 2022, during which 74.5% of the total literature was published. Totally, 29 articles took adults as research subject, followed by 6 studies focusing on children and older adults each, 4 on younger adults, and 2 on all -aged groups. Geographically, 29 studies were conducted in Europe, 9 in Asia, 5 in North America, 2 in Oceania, and 2 were cross-continental.

4.1 Measurement and Models of Urban Blue Space Characteristics and Residents' Mental Health

4.1.1 Measurement of Urban Blue Space Characteristics

Among the included literature, the measurement of the characteristics of urban blue spaces was conducted from the space-based (i.e., residential or workplace) dimension or individual- based (i.e., respondent) dimension. The number of space-based and individual-based is 21 for each, while 5 studies combining the both.

(1) Measurement from space-based dimension

Space-based measurements typically use macro data (e.g., land use and transportation data) to determine residents' exposure levels to urban blue spaces. Such studies generally assume residents remain stationary throughout the day, and use the blue spaces within the buffers around their residence or workplace (often in a radius of 0.3 ~ 1.5 km) to represent the exposure level, indicated by proximity, availability, and visibility. Proximity refers to whether live in coastal areas^[63], the Euclidean distance from the population centroid of the analysis unit to the urban blue space^{[10][12][13][18][35][47][48][49][53][57][58][60]}; availability is determined by the presence of urban blue spaces within the given buffers^{[17][36][40][47][49][50]}, percentage of blue space in area^{[12][14][15][20][21][45][58][60]~[62]} or per capita blue space area^[58]; and visibility is usually measured by the visible rate of urban blue spaces from specific locations based on land use and elevation data^[46][48]. With advancements in artificial intelligence and big data, some researchers have started using street view images and deep learning techniques to measure the proportion of urban blue spaces within certain visible ranges^[15][54].

However, statically measuring residents' exposure to urban blue spaces sees limitations. The Uncertain Geographic Context Problem (UGCoP)^[71][72] indicates that results would vary depending on the different divisions of geographic units (e.g., community, census tract, postal code precinct)^[73]. Additionally, the Neighborhood Effect Averaging Problem (NEAP)^[74] highlights that static measurement methods cannot capture variations of individual visit characteristics, economic status, and exposure types, leading to discrepancies between measured and actual exposure levels.

(2) Measurement from individual-based dimension

Individual-based measurements often use survey data to assess residents' active visit to urban blue spaces, and can accurately evaluate exposure levels by examining individual behavioral characteristics and exposure types to urban blue spaces. The former focuses on the influence of individual behavioral characteristics including whether urban blue space is visible^[44], whether residents visit^{[13][41][55][67]}, frequency/number of visits^{[11][13][32][38][39][42][51][61][63][65][66][68]}, travel distance^{[11][32][51][66][68]}, duration of visit^{[11][32][39][66][68]}, and residents' activity types taken place within blue spaces^{[33][34][43][52][56][59][64]} on mental health. The latter explores the different impacts of different exposure types including indirect exposure (visible from home), incidental exposure (visible during commute), intentional exposure (purposeful visit)^[32], and virtual exposure (viewing photos of blue space^[69]) on mental health.

(3) Measurement combined space- and individual-based dimensions

The studies considering both space- and individual-based dimensions tracked urban residents' spatio-temporal behaviors through activity logs, calculate blue space coverage around homes and travel routes^[14][20], or survey residents' addresses and urban blue space visitation data to comprehensively assess individual exposure levels to urban blue spaces^[13][61][63].

4.1.2 Measurement of Residents' Mental Health

In the selected literature, the measurement of residents' mental health mainly focuses on three aspects: general mental health, positive psychology, and negative psychology (Tab.3). General mental health is often assessed using overall questionnaires to evaluate individuals' psychological status. Positive psychology involves self-reported well-being (including subjective happiness and life satisfaction), perceived restorativeness, and positive emotions; negative psychology includes negative emotions (e.g., stress, frustration), and mental disorders (e.g., generalized anxiety disorder, depression). Additionally, a few studies examine mental health with medical statistics (e.g., duration of hospital admission, incidence of depression disorder) and physiological indicators (e.g., blood pressure, heart rate).

4.1.3 Research Models

The research models in the selected literature mainly include observational studies and experimental studies. Observational studies generally use cross-section or longitudinal analyses to explore the effect of urban blue space characteristics on residents' mental health through models such as multiple linear regression^[11], Poisso regression^[48], structural equation modelling^[49], and mediation effect model^[57], and analyze their spatial heterogeneity. Cross-section analysis is easier to obtain data and more used to explore correlations, while longitudinal analysis is often used for in-depth investigation of influencing mechanisms, though more challenging in data collection.

Experimental studies typically employ randomized crossover trials^[33][34][59] or controlled experiments^[43][52][56] that require respondents to be exposed to urban blue spaces in virtual (e.g., viewing photos^[69]) or real experiences (e.g., surfing^[43], walking^[34]) to observe the mental health effects obtained. These studies can better control experimental conditions such as exposure duration and mode, resulting in a more accurate assessment of short-term effects of blue space on mental health. However, due to the difficulty of recruiting respondents, the results of such studies may be biased by the limited sample size.

4.2 Effect of Urban Blue Space Characteristics on Residents' Mental Health

4.2.1 Effect of Urban Blue Space Characteristics on Residents' Mental Health Through Space-based Measurement

In this meta-analysis, space-based studies measured the correlations between urban blue space characteristics and residents' mental health from aspects of proximity, availability, and visibility.

(1) Proximity

Meta-analysis results (Tab.4) indicate that the proximity of urban blue spaces is significantly positively correlated with residents' self-reported general mental health (SMD = 0.33, p = 0.0001) and positive psychology (SMD = 0.15, p = 0.006). Specifically, residents living near urban blue spaces (e.g., those close to coasts or rivers) generally have better self-reported mental health^[57], which is particularly significant among low-income groups^[10] and strengthens with longer residency^[18]. Studies also found that outdoor activities (e.g., walking in coastal area) and perceived restorative quality of the living environment^[49] are the affecting pathways of urban blue spaces on mental health. For example, a study in Belgium found that coastal residents experienced higher happiness during pandemic lockdowns than inland residents^[63].

However, studies on the effect of proximity to urban blue spaces on negative emotions show inconsistent results. Meta-analysis results suggest that the proximity to urban blue spaces does not significantly influence negative emotions (SMD = –0.33, p = 0.10), especially for children^[40][53]. This may be due to the complex and diverse factors influencing negative emotions, where blue space exposure, though helpful in alleviating anxiety and stress, may play a limited role, especially in treating mental disorders like depression. Additionally, different age groups may have varying attitudes and usage patterns of urban blue spaces, with adults possibly requiring more emotional solace and psychological recovery from natural landscapes, enhancing the positive effects of urban blue spaces on their mental health^[40]. Lastly, geographical variety can also impact exposure effects. For example, a study in the UK proved the positive effects of proximity to the coast on residents' mental health^[18], while in relatively small nations with islands^[53] or sparsely populated areas like Canada^[40], the effect of proximity is not significant.

(2) Availability

The availability of urban blue spaces is significantly positively correlated with residents' general mental health (SMD = 0.16, p < 0.0001) and positive psychology (SMD = 0.91, p < 0.0001) (Tab.5). In other words, residents living in areas with blue spaces nearby or more blue spaces have better self-assessed mental health and stronger happiness. Studies suggest that the availability of urban blue spaces may have a greater effect in enhancing residents' mental health through improved environmental perception and physical activities^[49][58], compared with the effect of green spaces^[45]. Additionally, studies have found that childhood residential environments would affect their mental health in adulthood. A study in Denmark found that a higher proportion of blue spaces within 1 km of childhood residences was associated with greater calmness in adulthood, though the effect on mental health was not statistically significant, the regression coefficient was positive^[62]. Consistent outcome has been validated with studies in China^[14][60], England^[10], Bulgaria^[50], etc. Besides, some studies examine the impact of urban blue spaces around residential areas and travel paths by considering residents' spatio-temporal behaviors. For example, a study in China found that urban blue spaces around daily travel paths (≤ 200 m) had a more significant impact on individual mental health^[14], while a study in the Netherlands did not find significant difference in health effects with urban blue spaces around residential areas and travel paths^[20]. Overall, the availability of urban blue spaces is confirmed to be beneficial for mental health, but the specific impact may vary due to geographical and individual behavioral variety.

Furthermore, the meta-analysis results suggest that the effect of the accessibility of urban blue spaces on psychological disorders such as depression is not significant (SMD = –0.33, p = 0.01), and the research findings vary. Although some studies have found positive effects of urban blue spaces in specific cases—for example, a study in Scotland found that elderly community members with high freshwater coverage had lower frequencies of antidepressant intake^[12]; a study in China revealed that the residents living within a 300-m buffer to blue spaces reported significantly less depression^[60]—most studies did not find significant effects. Examples included the studies about children in USA^[47] and New Zealand^[53], adults in the Netherlands^[20] and Spain^[36], and among cohort of Alzheimer and families in Spain^[17]. These findings suggest that while urban blue spaces may benefit certain groups, their effects on preventing or alleviating mental disorders may be limited, requiring further research to explore the influencing mechanisms and conditions.

(3) Visibility

Due to data availability constraints, only four studies explored the association between the visibility of urban blue spaces and residents' mental health. Although the meta-analysis did not cover these studies for effect size conversion reasons, they all found that the visibility of urban blue spaces not only helps residents' self-reported mental health but also has significant positive effects on negative psychology. Daniel Nutsford is one of the earliest scholars focusing on the health effects of the visibility of urban blue space. His study in New Zealand found that residents with ocean views had lower psychological stress even when controlling other factors such as proximity to urban blue spaces^[46]. Similar conclusions were validated in a study in Ireland, where residents with high visibility of coastal views from their homes had a lower risk of depression, and visibility had a more significant effect than proximity^[48]. Recently, Chinese scholars have also analyzed the effects of blue space visibility on residents' mental health based on street view images in Beijing^[54] and Guangzhou^[15], finding that a higher proportion of blue space area in the images can reduce the risk of depression in the elderly and improve mental health.

4.2.2 Impact of Urban Blue Space Characteristics on Residents' Mental Health Through Individual-based Measurement

In the selected literature, 25 individual-based studies explored the association between urban blue space characteristics and residents' mental health by examining individual behavioral characteristics (e.g., visit frequency, types of blue spaces visited, visiting distance) and exposure types. Due to the diverse measurement methods of these studies, meta-analysis is not feasible; instead, this paper qualitatively summarized these studies in both aspects.

(1) Individual behavioral characteristics

Scholars have found that individuals who frequently visit urban blue spaces (at least once a week) generally have higher subjective well-being^[13][32] and lower mental distress^[61] and inattention^[42]. Visiting urban blue spaces more frequently in childhood also positively influences mental health in adulthood^[65]. Different types of urban blue spaces lead to varied mental health effects. For instance, a study in England found that frequent visits (more than twice a month) to rivers, canals, and seas are positively correlated with better mental health, while frequent visits to lakes show no significant correlation^[68]. This may be due to the different spatial experiences and physical activities provided by various types of urban blue spaces. In addition to visit frequency, studies indicate that whether residents visit urban blue spaces^[55], the visibility of blue spaces, the likelihood of visiting blue spaces during the pandemic^[63], and the characteristics of blue spaces along recreational routes^[64] are important factors affecting mental health. In addition, although beaches, harbor areas, and other waterside areas can significantly promote restorative benefits^[39][41], blue spaces' associations with depressive symptoms and stress are debatable^[20][66], especially for the areas with a low amount of blue spaces where green spaces may overwhelm blue spaces in alleviating negative psychology^[51].

Experimental studies, compared with observational studies such as cross-sectional analysis, often evaluate the benefits of physical activities (e.g., walking^[59], surfing^[43]) in urban blue spaces on mental health. These studies compare changes in physiological indicators (e.g., pulse, blood pressure) before and after activities, revealing that physical activities in urban blue spaces can promote restorative experience^[34] and perception^[64], and psychological improvement^[33][52]. For example, a study in Spain found that members of the experimental group who walked in urban blue spaces reported significantly enhanced well-being and mood after the activity^[59]. This positive effect is not evident in the general population but also in individuals with severe psychological disorders. For instance, PTSD patients who participated in surfing sessions showed significant symptom relief^[43]. Similar findings have been validated in studies conducted in Spain^[33], USA^[34], and UK^[56]. Experimental studies also found that different types of urban blue spaces have varying effects on residents' restoration. For example, the restorative capability of beaches is about 30% higher than that of harbors^[69].

(2) Exposure types

Only a few studies have evaluated the impact of exposure types to urban blue spaces on residents' mental health. For instance, a study in Hong Kong, China found that intentional exposure to blue spaces was positively correlated with higher well-being, while indirect exposure (viewing from home) was positively correlated with better self-perceived physical health but had no significant association with mental health^[32]. A study in Guangzhou, China discovered that incidental exposure to blue spaces from travel had more significant mental health benefits than indirect exposure^[14]. Additionally, short-term intentional exposure to urban blue spaces positively impacts mental health. Studies in UK indicated that residents who recently and intentionally visited blue spaces like rivers and canals reported higher life satisfaction and better mental health^[38][67]. Another study in Spain compared the effects of intentional visits to urban parks and beaches on psychological restoration, finding that blue space visitors reported higher relaxation and attention restoration^[11]. These findings suggest that different exposure types to urban blue spaces have varying impacts on residents' mental health.

5 Conclusions and Discussion

This paper, as a meta-analysis, focuses on the effect of urban blue space characteristics on residents' mental health, systematically reviewing the measurement indicators, research models, and mental health effects among relevant literature, with main findings as follows. First, urban blue space characteristics can be measured from space-based dimension (i.e., proximity, availability, visibility) and individual-based dimension (e.g., individual visit characteristics, exposure types). Second, the impact of urban blue space characteristics on mental health is mainly measured by general mental health, positive psychology (self-reported well-being, perceived restorativeness, and positive emotions), and negative psychology (negative emotions and mental disorder). Third, the effect of urban blue space characteristics on residents' mental health varies significantly across studies, particularly concerning psychological disorders such as depression and stress disorders. Specifically, the proximity to urban blue spaces positively affects general mental health and positive psychology, while the accessibility of blue spaces is significantly positively correlated with general mental health and positive psychology; although factors like visibility, visit frequency, and exposure types influence mental health, related research is still limited, necessitating further exploration in the future.

Existing research on the health benefits of natural environments often focuses on green spaces. This study's findings reveal that blue spaces are equally valuable for residents' mental health. By quantifying the effects of urban blue space characteristics on general mental health and positive psychology, this meta-analysis supports the view that blue spaces can positively influence mental health. In terms of the effects of green spaces on mental health, researchers found that exposure to green spaces effectively alleviates negative psychology such as anxiety and depression^[23]. This study's results show that the proximity to urban blue spaces has a similar effect size to green spaces in alleviating anxiety, while the impact of proximity and accessibility on depression and anxiety remains controversial. Blue spaces are important components in most cities—the median distance of urban residents to freshwater bodies is only 3.1 km^[7]—thus, analyzing the health effects of urban blue spaces and leveraging their health-promoting capabilities are significant for the construction of healthy cities.

Although existing research has yielded substantial results, future studies should focus on the following aspects.

1) Increasing empirical research in developing countries and inland cities. The health benefits of urban blue spaces vary significantly in different geographical contexts. Most current studies are conducted in developed countries in Europe and America, while fewer studies in developing countries—limited empirical research in China also focuses on coastal cities like Guangzhou and Hong Kong, with insufficient attention to inland cities. In the future, empirical studies should be conducted in more areas, especially in developing countries and inland cities, to enrich the evidences on the mental health benefits of urban blue spaces.

2) Exploring refined measurement of urban blue space and mental health indicators. Current measurements of the characteristics of urban blue spaces are mostly based on large-scale land use data or remote sensing data, ignoring the impact of numerous small-scale urban blue spaces on residents' mental health. Ponds and lakes, for example, are the most commonly found water bodies on the earth, which are crucial for freshwater biodiversity and playing a significant role in ecosystem services^[75]. Moreover, more experimental research should be performed with physiological mental health data (e.g., heart rate, blood pressure) to refine the methods and systems of measurement indicators.

3) Expanding research on the effect of urban blue space characteristics on mental health. First, current studies mainly focus on a single type of blue space (e.g., oceans or inland blue spaces), and future studies need to explore the differences of varied inland blue space types (e.g., rivers, lakes, ponds) in health effects. Second, most studies examine the impact of quantitative indicators of blue spaces (e.g., proximity, availability) on mental health, and future research should also explore the impact of qualitative factors of blue spaces on mental health. Third, evaluations on urban blue space characteristics are often conducted from either space-based or individual-based dimensions, and future studies should combine travel data to comprehensively analyze the differences of mental health effects of blue space characteristics.

4) Deepening the exploration of influencing mechanisms. Current research rarely explores the influencing mechanisms of urban blue spaces on residents' mental health, especially the unique mechanisms of blue spaces own. Future research should put more efforts in investigating the effects of factors such as sound environment, physical activity, and social interaction.

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