Green environments and cardiovascular health

Abstract

Several large epidemiological studies have found robust associations between greenness and the risk of cardiovascular disease (CVD). These studies report that close residential proximity to greenness is associated with a decrease in cardiovascular mortality as well as major adverse cardiovascular events. Although mechanisms underlying this link are not well understood, the beneficial health effects of greenness have been linked to its ability to relieve stress, decrease air pollution, and encourage physical activity. Greenness in residential neighborhoods could also increase access to healthy goods and services, as well as social interactions. Research into the health effects of greenness could provide new insights into the environmental determinants of CVD risk and could inform the development of actionable greenness-based strategies to prevent CVD and its clinical manifestations.

Introduction

Despite recent advances in treatment and management, cardiovascular disease (CVD) remains the leading cause of death worldwide, accounting for 17.9 million deaths per year [1]. Although some forms of CVD are due to inherited genetic disorders, its most common manifestations can be attributed to a variety of environmental exposures, lifestyle choices, and pathophysiological risk factors. Recent systematic attempts to evaluate the contribution of environmental factors to CVD risk have led to the development of the concept of the envirome. Unlike the often-utilized exposome, which is focused on physical exposures, the envirome encompasses all influences beyond genetics that affect health, disease risk, and longevity, and provides a nested framework through which to examine those influences [2]. In its simplest representation, the envirome could be thought to consist of discrete domains of the natural, social and personal environments. The natural domain of the envirome comprises natural, non-human elements, inclusive of ecology and geography. Nested within the natural environment, humans create distinct social environments by interacting with nature and with each other. The resulting social environment consists of built structures such as houses, roads, and public buildings, and is characterized by distinct social networks and cultures, as well as economic and civic organizations. From components of both the natural and social environments, individuals construct their own personal environments that are populated by personal circumstance, as well as by lifestyle choices related to nutrition, physical activity, smoking, and other health behaviors. There is extensive evidence to suggest that each domain of the envirome collectively and interdependently affects human health, disease risk, and aging. The concept of the envirome provides a useful framework for systemic identification of the influence of the environment on CVD.

Of the different domains of the human envirome, the natural environment is the most primary and primordial. Like all other life forms, humans depend on the natural environment for food, water, sunlight, and oxygen for survival and health. Throughout prehistory, human survival has depended also upon other living things in the natural environment such as plants, microbes, pests, parasites, and predators. Several of these continue to affect human health, even in modern societies. In addition, many physical features of the natural environment – the diurnal cycles of night and day, the rhythms of the seasons, the levels of sunlight exposure and the geographic features of altitude and latitude – have been found to exert a strong influence on cardiovascular health [3]. Importantly, emerging evidence suggests that the presence of grasses, shrubs, and trees in the natural environment could also influence human health [4].

A strong relationship between human health and vegetation is not surprising. Humans depend upon plants as an important food source and they use plant materials for making tools, medicines, and built structures. However, the notion that the presence of plants and vegetation in human environments could exert a beneficial effect on human health is relatively new. It has been appreciated for some time that humans have innate biophilic tendencies to seek connections with nature and non-human life [5], and that being in natural surroundings decreases mental stress and anxiety and improves attention and cognition [6]. However, only recently has empirical evidence emerged which suggests that living in green areas or in close proximity to vegetation could have salutary effects on chronic human health conditions, such as CVD [7].

Within the context of the envirome, humans are likely to be exposed to greenness in all natural, social, and personal domains of the environment (Fig. 1). In the natural domain, humans are exposed to forests, grasslands, brushes, mosses and wetlands, whereas parks, green public spaces, and neighborhood vegetation are a more frequent source of exposure in the social domain. In addition, people who cultivate gardens or maintain indoor plants live in close proximity to vegetation in their personal environments. There is evidence to suggest that exposure to and interactions with plants and vegetation in each of these domains could affect human health, particularly CVD risk.

Fig. 1.

Forms of greenness within each domain of the human envirome. These domains are distinct but are highly interactive. The social domain is nested within the natural domain and the personal domain is nested within the natural and social domains. Forms of greenness in each domain may influence cardiovascular health.

Interactions with vegetation in the natural domain during occupational or recreational visits have been linked to transient improvements in cardiovascular health. Data from individuals participating in “forest bathing” not only show improvements in positive emotions and decreases in hostility [8], but also favorable changes in cardiovascular function [9]. A recent meta-analysis of 20 trials involving 732 participants found that being in a forested area was associated with significant reductions in both systolic and diastolic blood pressure when compared with spending time a non-forested area [10]. Spending time in a forested area has also been associated with greater parasympathetic activity, lower sympathetic tone [11], higher levels of serum adiponectin [12], and a lower resting pulse rate [11]. The control setting for these studies is often an urban or residential environment, suggesting that parks or natural features within urban environments resembling forest settings could have a calming or a relaxing effect, which may be reflected in improved cardiovascular function. However, further research is required to determine how long these beneficial effects last after leaving the forested area and whether other, non-psychological, effects could account for the improvements in cardiovascular function in those visiting forested areas.

The presence of plants and vegetation in the social domain has been linked with improved cardiovascular health as well. For instance, in an analysis of the entire population of England, the rate of CVD mortality in individuals living in the least green areas was twice that of greenest areas [13]. Similarly, it was found that the odds of hospitalization for CVD were 37% lower in those living in areas of highly variable greenness [14], suggesting that even in urban environments, living next to greenspaces and plants could have beneficial cardiovascular effects. This notion is further supported by a study showing that the loss of 100 million trees to emerald ash borers in the northern United States was positively associated with mortality related to cardiovascular and lower-respiratory tract illnesses [15]. Remarkably, the study reported that the loss of trees corresponded to 15,080 additional cardiovascular deaths in the 15 states in the study areas, suggesting that urban vegetation exerts a strong and significant effect on cardiovascular health. A similar association between mortality and residential greenery was also observed in an analysis of 108,630 participants, showing that women who lived in areas of highest levels of greenness had 12% lower mortality rates [16]. Likewise, a 10-year longitudinal study of 1.2 million subjects living in Rome found that living in greener areas was associated with better health outcomes and significantly reduced levels of cardiovascular mortality [17]. In a 11-year study of an urban cohort of 1.3 million people living in Canada, it was found that higher level of greenness within 250 m of a person’s residence was strongly associated with reduced cardiovascular mortality relative to other cause-specific risks [18]. Taken together, the findings of these large epidemiological studies suggest that living in close proximity to greenness, even in urban environments, profoundly affects both cardiovascular and all-cause mortality.

The influence of greenness on CVD appears pervasive; affecting not only the risk of myocardial infarction, but ischemic heart disease, heart failure, and atrial fibrillation as well. In a study of 249,405 Medicare beneficiaries 65 years or older living in Miami-Dade county, the risk of these 4 diagnoses of CVD was found to be significantly lower in those living in greener areas [19]. The associations between CVD outcomes and greenness were significantly attenuated after adjusting for biological risk factors, suggesting that the effects of greenness may be partly mediated by CVD risk factors. Our recent work suggests that residential proximity to greenness is associated with lower urinary levels of epinephrine and metabolites of isoprostane, a product of lipid peroxidation [20]. Given the extensive evidence linking β-adrenergic activation and oxidative stress to CVD, it is tempting to speculate that one mechanism underlying the beneficial effects of greenness may be reduction in sympathetic activation and oxidative stress. However, further work is required to substantiate this relationship.

The effects of greenness on cardiovascular health could be mediated by several processes (Fig. 2). First, plants and vegetation in urban locations can remove air pollutants by absorbing particulate matter and toxic gases from the surrounding air. Ambient air pollutants, including fine particulate matter, adhere to the surface of leaves, depending on attributes such as surface area, waxiness, and leaf hair density [21]. Moreover, respiration through leaf stomata could remove pollutants smaller than PM_2.5, such as Volatile Organic Compounds (VOCs) and ultrafine particulate matter [22]. In addition to uptake by leaves, the physical structure of trees and other plants has been shown to reduce the amount of air pollutant emissions from sources that reach nearby populations by serving as a physical barrier that increases pollutant dispersion and inhibiting wind flow [23]. Air monitoring studies have shown that near-road vegetative barriers, especially tall and dense barriers under low-wind conditions, are effective in reducing downwind concentrations of hazardous particles, NO_x, and CO [24]. Increased reductions were observed at closer proximity to the barrier and under light wind conditions. Concordant with observations of exposures resulting from roadway traffic, previous work has linked proximity to roadways to CVD risk – an effect that might be reduced by roadside greenness [25].

Fig. 2.

The intermediate steps through which greenness may influence cardiovascular health. This influence of greenness is heavily mediated by both environmental and personal responses to the presence of greenness. These responses then modify personal-level risk factors for CVD.

The ability of plants to remove air pollution may be particularly relevant in the context of CVD. In most urban locations, ambient air pollution is a major CVD risk factor. Worldwide, pollution has been linked to over 9 million premature deaths with an estimated 4.2 million deaths from exposure to ambient particulate matter alone [26]. Given that nearly 80% of deaths from air pollution are due to CVD [27], it remains likely that some of the salutary effects of green vegetation, particularly in urban areas, may be related to the reduction of air pollution levels in those areas. Nonetheless, it is unclear whether plants can remove air pollutants at levels that significantly impact CVD risk and whether the beneficial effects of greenspaces could be solely attributed to their ability to remove air pollution due to their absorbing or barrier properties [28,29].

The barrier and absorbing properties of plants could mitigate noise and light pollution as well. Areas with more greenness exhibit a more natural sound profile of animals and leaf noises, and thereby could contribute to perceived calmness and lower stress [30]. This could have a significant effect on CVD risk, because exposure even to low-level noise has been linked to diabetes, obesity, and insulin resistance, which are strong risk factors for CVD [3]. Likewise, light pollution, leading to sleep disturbances and disruption of circadian rhythms, could elevate CVD risk. A recent study in India has shown that biomarkers of early vascular aging such as blood pressure and flow-mediated dilation were more strongly associated with levels of nighttime ambient light in less green areas [31]. In our study of a cohort with mild to moderate CVD risk, we found that high levels of residential greenness were associated with a decrease in urinary epinephrine, but not norepinephrine levels [20]. Given the previous observation that selective changes in urinary epinephrine levels are indicative of sleep quality [32], it is possible that some of the beneficial cardiovascular effects of greenness may be mediated by decreases in noise and light pollution, leading to improvements in sleep quality. Further research is required to obtain more direct evidence for this link.

The hospitable environment created by urban greenspaces could also decrease CVD by providing a greater incentive to spend time outdoors. Indeed, some studies have linked greenspaces to physical activity by providing a setting for exercise and leisure activity, improving the walkability of environments, and increasing access to healthy goods and services through mediators such as lowered crime and reduced urban heat islands as well as improvements in perceptions of nearby outdoor environments [33]. A recent study from China found that the observed association between greenness and CVD could, in part, be explained by more physical activity among residents of greener areas [34]. Because physical inactivity is a leading risk factor for CVD [3], even moderate levels of increase in physical activity could account for the observed association between greenspaces and cardiovascular health. However, not all studies have found a positive association between neighborhood greenness and physical activity. This may be due to differences in perceived and objectively measured greenness of neighborhoods [4,35]. Therefore, further methodological refinements are required to reliably capture the attributes of greenspaces that affect physical activity and to identify specific features and arrangements of neighborhood greenness that promote physical activity in residents.

Greenspaces could also promote cardiovascular health by reducing stress, anxiety, and depression [36]. These psychosocial risk factors are known to promote atherosclerosis and adverse cardiac events [37]. Previous work has shown that experiencing green spaces in virtual reality outside of natural settings increases positive affect and decreases stress [38]. Similarly, visiting a forested setting, or forest bathing, has been shown to cause immediate improvements in positive emotions and leads to decreased hostility and aggression, independent of exercise or taking part in favorable activities. These effects were especially pronounced among individuals in the study experiencing the highest levels of chronic stress [8]. A large European study found that differences in mental well-being from socio-economic inequality were 40% narrower in individuals residing in greener areas [39], and a study of 10,0 0 0 individuals over 17 years in the United Kingdom reported significant decreases in mood disorder responses with increasing levels of greenness [40]. In addition, several observational studies suggest that exposure to natural environments is associated with a reduced risk of depression [41]. However, most such evidence is derived from cross-sectional study designs, which cannot establish causation. While few longitudinal evaluations support the results of cross-sectional studies, the results have not been consistent. To address causation, South and coworkers [42] conducted a randomized trial in which the intervention was a cleanup and landscaping of vacant lots in neighborhoods. They found that the intervention led to a significant decrease in self-reported feelings of depression and worthlessness. Despite rather superficial evaluation of mental health and limited follow up, the study contributes to the growing body of evidence suggesting that exposure to greenery may improve mental health and thereby indirectly benefit cardiovascular health.

Finally, direct effects of greenery on human health may derive from repeated challenges to the human immune system through long-term repeated exposure to the abundant microbial biodiversity found in ecologically rich natural environments. This idea, first presented by Rook in his “Old Friends” hypothesis, posits that the rise of inflammatory disorders may have origins in a failure to “educate” the developing immune system with a wide range of non-lethal microbes [43]. These microbes, which greatly outnumber our own human cells, have co-evolved with our mammalian physiology dating back to pre-paleolithic times. The hypothesis suggests that inflammatory disease could arise from a malfunctioning immune system which, lacking sufficient encounters with immunoregulation-inducing organisms, could lead to an exaggerated pro-inflammatory response, and ultimately produce inflammation in otherwise healthy tissues. For example, atherosclerosis is associated with chronically raised C-reactive protein and is comorbid with other chronic inflammatory disorders. In healthy humans, the immune response is regulated by robust networks of T cells, which are the product of exposure to a wide range of microbes. Absent that training, the hypothesis predicts, there is an increase in inappropriate immune attacks and chronic inflammation that promotes atherosclerosis [43]. The notion that a broader ecology is necessary for good health is a novel and promising perspective on the link between CVD and greenness exposure.

In addition to the social domain, humans are also exposed to and interact with plants in their personal domains. Many individuals cultivate personal gardens and such activity could have direct cardiovascular benefits. More directly, it has been suggested that indoor plants can remove indoor ozone and VOCs by direct absorption or by indirect biotransformation by micro-organisms in soil [44]. In addition to their potential air purifying properties, indoor plants could also have beneficial psychological effects. It has been reported that the presence of plants indoors is associated with improved task performance and reduced stress [45], which may have indirect cardiovascular benefits, but the impact of indoor plants on cardiovascular health remains to be studied.

Perspective

Although a large body of evidence supports the notion that living in areas of high vegetation improves cardiovascular function and decreases CVD risk, much work remains to be done to confidently shore up the link. More research is required to delineate the contribution of air pollution reduction, light and noise pollution remediation, physical activity, stress reduction, and immune response conditioning, to the overall cardiovascular benefits of living in areas of high vegetation. More work is also required to construct a reliable dose–response curve – what is the level of greenness that is required for optimal promotion of cardiovascular health? Preliminary estimates suggest that 10 additional trees per block is equivalent to increasing the income of a family by $20,000 [46], but whether this is the maximal benefit of greenness remains unclear. Estimates of the dose–response relationship between nature exposure and stress biomarkers indicate a 21% decrease in salivary cortisol and 28% drop in salivary amylase per hour of nature experience [47]. Evaluation of such dose–response relationships between CVD risk factors and nature exposure will be important in designing appropriate interventions in the future. Moreover, it will also be important to know the location of greenspaces with respect to a person’s residence that exerts maximal health benefit. In some studies, street trees, rather than trees in individual yards, have been found to be more beneficial, [48] but additional evidence is required to understand why and how.

An important limitation to current research in the field is the lack of consistency between studies in the use of greenness metrics and spatial scale. The most common metric of greenness used in CVD studies is the Normalized Difference Vegetation Index (NDVI), which is objectively quantified by satellite-based sensors used to measure chlorophyll activity by the ratio of red and infrared sunlight reflectance at the ground level. NDVI is particularly useful due to its universal availability, high spatial resolution, and consistency in objectively measured greenness. However, there are several limitations to this approach, primarily in regard to its applicability to relevant greenness exposure, its effects on health outcomes, and on mediators of exposure. For instance, NDVI may indicate overall chlorophyll activity in a given area, but cannot precisely indicate leaf surface area or characteristics, including species makeup, height, shade, perceptions of quality, overlapping layers of vegetation, or other attributes of greenness that may influence CVD risk factors. Other metrics of greenness, such as land use, tree cover, and distance to parks or green space have also been used in some cardiovascular studies. However, these metrics suffer from similar limitations, and often lack objective and consistent quantification and objective comparability between areas.

Calculations of pixel-level NDVI and other greenness metrics are often aggregated into spatial buffers or administrative areas. However, the usage of these areas varies widely, with images used to assign NDVI values that range in scale from 30 m resolution (900 m²) pixels to spatial buffer areas of over 1 km (1000,000 m²). Administrative units range from the size of city blocks to entire metropolitan regions. Most studies that report significant associations between NDVI within buffer areas and cardiovascular outcomes have used spatial buffers of participant location between 200 m and 500 m [16–18,20], still a 6-fold difference in total buffer area. Greenness within these 200–500 m radii is likely to influence walkability and neighborhood perceptions. At radii of less than 300 m, influences of greenness on exposures to air pollution, heat, noise, and light are more likely. However, there is relatively little reported investigation of potential associations between greenness and cardiovascular health at radii, or equivalent spatial units, of less than 200 m. It remains largely unknown whether a single plant or differentials in the structural layout of plants within yards or other small areas may influence cardiovascular health. Further complicating matters, nearly all studies only examine greenness around participant residential location, with large unaccounted portions of time and exposure to non-residential areas. Clearly, significant methodological refinements are needed before the effects of greenness on health could be investigated in greater depth.

Nonetheless, despite significant research gaps, this area of inquiry remains promising. Given the high incidence of CVD worldwide and the alarming statistics from developed countries showing only minor recent improvements in CVD incidence [1], it is imperative that we explore all possible avenues for its prevention and management. Even if greenness decreases CVD risk by a few percent, its public health benefits are likely to be significant, given the high incidence of the disease. Therefore, a more firm link between CVD and greenspaces in each domain of the envirome could provide not only a new way of preventing CVD and its lethal manifestations, but also help in developing a blueprint for healthier homes and cities in future.