Depression is a critical public health issue. It is a common but serious affective disorder characterized by persistent feelings of sadness, low energy level, poor or increased appetite, changes in sleeping pattern, hopelessness, or guilt, and it may ultimately lead to suicide [1]. In addition, depression is a leading cause of disability and illness worldwide, with an estimated 300 million people affected globally [1]. In the U.S., an estimated 8.1% of adults aged 20 years or older have depression in any given 2-week period, with women and non-Hispanic black adults more likely than other groups to report depression, 10.4% and 9.2%, respectively [2].

Depression is the most prevalent mental health condition in late life [3, 4], affecting approximately 8% to 16% of U.S. adults aged 65 years or older [3, 5, 6]. Many studies have reported that individual characteristics may influence the risk of developing depressive symptoms later in life [7-9], such as female gender, previous depressive episodes, cognitive and functional decline, stressful life events, terminal or chronic illness, lower income level, retirement, bereavement, and living alone [4, 7-9].

Depression has been associated with a number of adverse outcomes, including reduced quality of life, increased morbidity and mortality, and suicide [9-11]. Accepted treatments, including the use of antidepressants, have been shown to be less effective for late-life depression. Considering the unique set of factors that influence depression onset in later life and the difficulty in treating it using standard approaches, this age group may be at differential risk for depression development and progression [12, 13].

There is an increased global interest in understanding the influence of neighborhood characteristics on health, including mental health, and more specifically, depression [9, 10, 14-20]. Built environment (BE) can be described as the physical components of where we live and work [21], such as green spaces and parks, availability of sidewalks, and access to destinations. BE has been linked to mental health status [6-8, 14-17, 22, 23] and is believed to interact with individual-level characteristics in different ways [4, 6, 15, 20]. For example, research has demonstrated that a lack of BE resources, such as recreational parks, green spaces, sidewalks, and accessible services, may act as a stressor [4, 6, 15, 20]. Conversely, the availability of these infrastructures in a community has been shown to ameliorate personal maladaptive responses, improve social connections and a sense of community, and, thus, promote physical and mental health [6].

The influence of BE on the mental and physical health of people in later life is similarly gaining attention [4]. Older adults may be more susceptible to environmental factors than other adults for several reasons [4, 24]. Older adults may experience decreased physical mobility; combined with retired status, they may be increasingly dependent on their local communities [4]. Additionally, older adults may experience impaired cognitive and physical function, which can reduce adaptive skills and reduced social support (possibly due to loss of loved ones or children relocating out of the area), which may result in increased engagement with and reliance on their neighborhoods, especially the built attributes [4, 7].

Several review studies have focused on the relationship between neighborhood characteristics and depression. A prior review assessing the link between neighborhood characteristics (including social and BE) and depression found that only BE was consistently associated with reduced depression, even after adjusting for sociodemographic and social attributes [15]. However, BE was not consistently measured across the studies included in the review [15]. A review of 11 cross-sectional studies evaluated the relationship between urban BE and psychological distress in a general population of individuals aged 16 to 75 years [20]. The findings demonstrated a relationship between urban BE and psychological distress, which was defined as including depression, anxiety, depression, and psychological stress [20]. However, results were not broken out by age group, and so it is difficult to say whether the relationship was held among the older adult participants.

Depression in later life is a critical public health problem. Late-life depression is associated with increased risks of metabolic and cardiovascular diseases, increased rates of morbidity and mortality, and suicide [3, 9-11]. Furthermore, depression in older age is more difficult to treat because older adults may resist seeking treatment and tend to respond poorly to antidepressant medications [12, 13]. As a result, efforts to better understand the factors associated with depression onset later in life may spur interventions to prevent depression among older adults.

In 2018, the World Health Organization (WHO) launched the “WHO Global Network for Age-friendly Cities and Communities” initiative to promote healthy aging in cities around the world through the improvement of built and social environments [25]. Given that people are expected to live longer due to ongoing advances in medical knowledge and technology, the WHO projects that the number of people worldwide aged 60 years and older will double by 2050 [26]. As people live longer and the population of older adults increases, the demands on health care systems and services are expected to increase substantially [27]. The economic burden of chronic health conditions such as depression is projected to increase equivalently. A comprehensive review of the influence of urban and rural BE on depression in later years could improve the understanding of the implications of this relationship as the population continues to age.

Prior to our review, we conducted a search of the PubMed database. There have been relatively few reviews of BE and depression [20]. Much of the literature, in general, focused on urban BE. Given that rural areas by definition have less built infrastructure and prior studies have established a favorable association between growing BE resources and reduced mental health difficulties, it appears that the absence of BE resources may have an impact on rural inhabitants' mental health.This may be even more critical for older rural residents, who may have reduced mobility but may not have as many options for neighborhood engagement as urban residents. Thus, older rural residents may be at higher risk of depression due to lesser BE infrastructure and resources [28]. To our knowledge, however, no previous review of the literature on the influence of rural environments on depression in older adults has been undertaken [29].

The primary aim of this study is to build on previous work and conduct a comprehensive review of the scientific literature on the relationship between BE and depression among older adults. We hypothesize that older people living in built environments with greater infrastructure and resources will have reduced rates of depression and/or report fewer depressive symptoms than other older adults. The secondary aim of this study is to review the scientific literature on the relationship between BE in urban vs. rural settings and depression among older adults. We hypothesize that, given the reduced built and infrastructure resources inherent in rural settings, older rural residents will have higher rates of depression and/or report a greater number of depressive symptoms than their urban counterparts. However, there may not be an adequate number of studies to assess the influence of rural vs. urban BE on depressive symptoms.

To our knowledge, this is the first comprehensive review to examine whether person-centered and objectively measured community physical infrastructure are associated with depression among older adults. All the studies included in this review reported an association between depression and at least one BE characteristic. We found disparities in the urban and rural BE exposures but similar effects of the adverse built environment on depressive symptoms in older adults, for instance, lack of green spaces [22, 23, 31], higher density of public transport [34], and deficient rural built infrastructure [28, 37], were all linked to poor mental health. Our review adds to the literature that an adverse physical environment may impact mental health negatively, especially among older adults.

Comparing our review to other studies, we observed that nearly half of the studies in this review reported that the prevalence of depression was higher among women than the men [3, 23, 27, 28, 31-33]. This finding lends support to previous studies, which suggested that women are more likely to report depressive symptoms than men [38, 39]. In addition, we also found that objectively measured (using NDVI and GIS instruments) availability and amount of green spaces within accessible distance to the users was consistently associated with positive mental health [22, 23, 31, 32]. Again, this concurs with previous studies, demonstrating the benefits of greenness on depression [40-43]. Studies have suggested that parks and greenery may reduce the risk for depression by improving mood, mitigating stress [44, 45], promoting social interaction [46, 47], and physical activity [48-50].

Apart from the green spaces, all the other BE attributes, including land use mix, walkability, public transport, and access to services/destinations, had inconsistent effects on depression among older adults. For example, previous studies have examined the association between neighborhood walkability and physical activity, and the results have suggested that more walkable neighborhoods promote active living and general health [3, 19]. However, the findings of this review indicate that neighborhood walkability and access to services or destination could impact the mental health of older adults positively or negatively. The evidence from this review suggests that the influence of the physical environment on mental health is far more complex and may be subject to multiple mechanisms of effect.

Prior studies suggested a dose-related response to neighborhood environment [33, 51]. Older adults may be more sensitive and reactive to the quality of their community offerings due to their increased reliance on their immediate environment because of reduced physical functions, mobility, and relative adaptive responses. They tend to spend longer time, compared to young adults, within their communities. In addition, previous studies have suggested that walkable neighborhoods promote walking and physical activity and general health [3, 19]. Nonetheless, we found an inconsistent association between walkable neighborhoods and depression among older adults. Despite living in a walkable neighborhood, high traffic density, crime, litter, poor quality pedestrian infrastructure may serve as psychological stressors and produce detrimental mental health effects [35]. Likewise, accessibility/availability of stores and public transport may not produce the desired positive health outcome if they are too many and concentrated in a location for similar reasons mentioned above. Despite cultural differences and the heterogeneity of the locations of the studies, we observed that all the studies showed an association between built infrastructure and depression after controlling for individual confounding factors.

We acknowledge that our review has strengths and limitations. We focused on geographically diverse studies investigating the associations between built environment attributes and depression. The majority of the studies had large sample sizes with sufficient statistical power to examine associations and used comparable and objective instruments in evaluating exposures and outcomes variables. However, the prevalent studies were cross-sectional, and we could not ascertain causality or temporality. As a result, the associations reported may be prone to reverse causation [22]. Additionally, scanty studies focused on rural BE, making it challenging to examine rural-urban differences. Finally, although all the studies selected for this review included people aged 50 or older and controlled for individual variables including age, only 8/13 studies focused entirely on older adults. Future research focusing on older adults and investigating longitudinal interactions between BE and depression in urban and rural communities are needed.

In conclusion, this review shows an association between built environment and depression among older adults, and our findings shed more light on the complexity of this relationship. We recognize the challenges of creating an entirely healthy environment. However, we recommend that health policymakers develop interventions to promote healthy aging by strategically developing carefully-planned neighborhoods and modifying existing communities. Neighborhood designs should offer quality and accessible infrastructures, including green spaces, pedestrian infrastructure, community services, and improved community safety.