The Science

Why is nature beneficial?

What are its 'active ingredients'?

An Overview

Narratives dating back two millennia and across cultures describe natural settings as places of refuge and comfort in times of psychological distress. For centuries, writers, composers, painters and poets, have revered nature and appreciated its ability to inspire awe and wonder, serenity and peace, whilst philosophers, physicians, teachers and politicians have made regular reference to its healing and restorative qualities. However, surprisingly, it is only within the past forty years that researchers have begun to investigate the effects of spending time in, or looking at, nature with any degree of scientific rigour.


One of the earliest scientific studies was undertaken by the American researcher, Roger Ulrich, in the 1980s. He compared the recovery records of 23 matched pairs of surgical patients, with half of the patients in a room with a window looking out over trees, and the other half in an identical room, but with a window looking onto a brick wall. He found that those patients with the tree view spent less time in hospital post-operation, elicited fewer negative comments about their progress from nurses and took fewer moderate and strong analgesics, compared with patients who had a view of a brick wall ( Ulrich, 1984 ).


Since this pioneering experiment, researchers all over the world have used a variety of designs and methodologies to investigate the impact of nature on human health. Studies have been both lab-based and field-based, and scientists have collected data through self-report measures such as questionnaires, and physiological measures such as heart rate, blood pressure, cortisol levels and skin conductance. Advances in neuroscience have enabled researchers to make use of MRI (magnetic resonance imaging) scans and portable EEG (electroencephalogram) machines, to study brain activity, and specifically alpha wave activity, considered an indicator of ‘a wakefully-relaxed state’ (Hagerhall et al., 2008, 2015). Most recently, scientists have been investigating natural killer (NK) cells in the blood, which, as part of our innate immune system, attack cells infected with a given virus, and appear to increase in number after spending time in nature (Li et al., 2006, 2007).


This growing body of research has led the government, in its recently published ‘ 25 Year Environment Plan ’ (2018) to assert:


Spending time in the natural environment – as a resident or visitor – improves our mental health and feelings of wellbeing. It can reduce stress, fatigue, anxiety and depression. It can help boost immune systems, encourage physical activity and may reduce the risk of chronic diseases such as asthma. It can combat loneliness and binds communities together.


Hundreds of studies have been conducted amongst diverse groups, both clinical and non-clinical, and across cultures and age ranges (see ‘ Research Studies ’ for a selection of these with references), and have demonstrated that engaging with nature can:

 

  • reduce depression
  • reduce anxiety
  • reduce stress
  • reduce mental fatigue
  • reduce anger and aggression
  • reduce experiences of pain
  • improve mood
  • improve sleep
  • improve confidence and self-esteem
  • improve cognitive functioning
  • improve creativity
  • boost immune functioning
  • increase longevity
  • increase energy
  • reduce symptoms associated with ADHD, trauma, psychosis, addiction
  • improve quality of life for those suffering from Alzheimer’s and dementia
  • reduce cognitive decline in older adults
  • improve relationships
  • increase sense of gratitude, generosity and selflessness
  • increase patience, life satisfaction and motivation

 

Alongside investigating the positive effects of spending time in natural environments, scientists have been hypothesising as towhybeing in nature, or even just looking at nature, might be so beneficial to human beings. For an account of the most widely accepted theories, see the sections below.


References

DEFRA (2018). 25 Year Environment Plan . Retrieved from http://www.gov.uk/government/publications.

Hägerhäll, C.,Laike, T., Kuller, M., Marcheschi, E., Boydston, C., & Taylor, R. (2015). Human physiological benefits of viewing nature: EEG responses to exact and statistical fractal patterns. Nonlinear dynamics, psychology, and life sciences, 19, 1-12.

Ulrich, R.S. (1984). View through a window may influence recovery from surgery. Science, 224, 420-21.

Li, Q., Nakadai, A., Hiroki, M., Miyazaki, Y., Krensky, A., Kawada, T., & Morimoto, K. (2006) Phytoncides (wood essential oils) induce human natural killer cell activity. Immunopharmacology and Immunotoxicology, 28 (2).

Li, Q., Morimoto, K., Nakadai, A., Inagaki, H. et al. (2007). Forest bathing enhances human natural killer activity and expression of anti-cancer proteins. International Journal of Immunopathology and Pharmacology, 20 (2), 3-8.

An Evolutionary Theory: The Biophilia Hypothesis

The concept of ‘biophilia’, from the Greek meaning ‘love of life and the living world’, was first made popular by the American biologist, E.O. Wilson in 1984. He believed that, since we evolved in nature, we have a biological need to connect with it. We love nature because we learned to love the things that helped us to survive. We feel comfortable in nature because that is where we have lived for most of our time on earth. In other words, we are genetically determined to love the natural world: it is in our DNA. Strong evidence for our ‘nature-primed neurons’ comes from the related concept of ‘biophobia’, which asserts that we have an innate fear or disgust of natural phenomena that would have been harmful to our survival, for example we detect a snake pattern more quickly than any other, and a gagging reflex is activated at the smell of rotting meat. Further supporting evidence for an evolutionary basis to our affinity with nature comes from studies showing our marked preference for ‘savannah’ style environments, open, grassy landscapes with small scattered trees (Balling & Falk, 1982; 2010), similar to those of East Africa where the human species evolved. Appleton proposed a ‘prospect-refuge’ theory (1996) to explain our preferences for these landscapes, suggesting we make aesthetic judgements according to the extent to which they represent the genetically important features of ‘prospect’, an open, unimpeded view, and ‘refuge’, an opportunity to hide from any hazard. Our attraction to the sound of running water has been similarly attributed to the evolutionary advantage of a nearby clean water source (Choker & Mene, 1992; Ulrich, 1993).

With limited scientific evidence to support his hypothesis, E. O. Wilson wrote of our biophilic connection,

‘Our existence depends on this propensity, our spirit is woven from it, hope rises from its current’ (1984)

However, after more than thirty years of scientific investigation, Dr Qing Li, Associate Professor at the Nippon Medical School in Tokyo and one of the world’s leading experts on the health benefits of natural environments, agrees:

‘We are “hard-wired” to affiliate with the natural world, and just as our health improves when we are in it, so our health suffers when we are disconnected from it’ (2018).

Today, many scientists from a wide variety of disciplines, including biologists, medical doctors, psychologists and neuroscientists, are beginning to recognise that this affinity for the natural world may be fundamental to our health, and growing evidence is suggesting that contact with nature may be as vital to our well-being as regular exercise and a healthy diet.

Over the past forty years, two dominant theories have arisen as to how nature brings about positive responses in humans: Ulrich’s Stress Reduction Theory (SRT; 1981) and the Kaplans’ Attention Restoration Theory (ART; 1989). Both theories, first proposed in the 1980s, and with a basis in evolutionary explanations, are still considered to have merit today. 

References
Balling, J. D., & Falk, J. H. (1982). Development of visual preferences for natural environments. Environment and Behavior, 14(5), 5-28.
Balling, J. D., & Falk, J. H. (2010). Evolutionary Influence on Human Landscape Preference. Environment and Behavior, 42, 479-493.
Choker, B. A., & Mene, S. A. (1992). An assessment of preference for landscapes in the developing world: Case study of Warri, Nigeria, and environs. Journal of Environmental Management, 34, 237-256.
Kaplan, R., & Kaplan, S. (1989). The experience of nature: A psychological perspective. New York: Cambridge University Press.
Li, Q. (2018). Shinrin-Yoku: The art and science of forest-bathing. London: Penguin Random House.
Ulrich, R. S. (1981). Natural versus urban scenes: Some psychophysiological effects. Environment and Behavior, 13, 523–556.
Ulrich, R. S. (1993). Biophilia, biophobia, and natural landscapes. In S. R. Kellert & E.O. Wilson (Eds.), The biophilia hypothesis (pp. 73-137). Washington, D.C.: Island Press.
Wilson, E. O. (1984). Biophilia. Cambridge, MA: Harvard University Press. 

Stress Reduction Theory (Ulrich, 1981)

Ulrich’s theory proposes that natural environments promote recovery from stress, while urban environments tend to hinder the same process. Being in an unthreatening natural environment or viewing natural elements, such as vegetation or water, immediately activates a positive emotional response, and a decrease in blood pressure and heart rate, indicators central to the stress response; sustained attention is evoked, which blocks negative thoughts and emotion.

Because humans developed and evolved in natural environments as opposed to urban settings, Ulrich proposed that engagement with such environments continues to be positively adaptive for modern humans. To display a stress response involving high levels of physical arousal in the face of an unthreatening natural setting would be maladaptive, since this process would cause fatigue and lead to chronic cardiovascular and endocrine responses that would adversely affect health. This is why such behaviour is not usual for humans (Ulrich, 1991). In contrast, according to Ulrich, a similar innate preparedness to respond positively to urban settings is not likely to have developed. Stimulating environments such as cities, with high levels of visual complexity, noise, intensity and movement, can affect people negatively by producing stressful and fatiguing levels of psychological and physiological arousal. Nature tends to be lower in intensity and less perceptually jumbled than many urban environments, and therefore has comparatively positive, stress-reducing effects on people.

References
Ulrich, R. S. (1981). Natural versus urban scenes: Some psychophysiological effects. Environment and Behavior, 13, 523–556.
Ulrich, R. S., Simons, R. F., Losito, B. D., Fiorito, E., Miles, M. A. & Zelson, M. (1991). Stress recovery during exposure to natural and urban environments. Journal of Environmental Psychology, 11, 201-30.

Attention Restoration Theory (Kaplan and Kaplan, 1989)

Kaplan and Kaplan’s theory proposes a more cognitive explanation for nature’s restorative characteristics. They suggest that there are two types of attention: directed or voluntary attention, used when a task requires deliberate and sustained attention, and involuntary attention, or ‘soft fascination’, which does not require an effort. After prolonged used of directed attention, we suffer ‘attention fatigue’, with reduced ability to perform cognitive tasks, and increased levels of mental fatigue. According to the Kaplans, involuntary, effortless attention or ‘soft fascination’ is capable of restoring our directed attention. They suggest that there are four processes at play that contribute to nature’s capacity to provide this restorative effect:

  • being away: natural environments provide opportunities to gain distance from routine activities and thoughts.
  • soft fascination: there are aspects of nature that hold human attention effortlessly, while providing the mind the opportunity to think about other things; clouds, sunsets, leaves in the wind, snow patterns are all natural phenomena that induce a ‘soft fascination’ in the viewer that in turn enables one’s directed attention to rest.
  • extent: nature provides an experience extended in scope and depth, in which one can become immersed, engaging the mind and gaining rest from concerns.
  • compatible: natural environments provide a setting that is well-matched to human needs and desires, allowing attention to rest and leading to a feeling of being in harmony with a greater whole.
References
Kaplan, R., & Kaplan, S. (1989). The experience of nature: A psychological perspective. New York: Cambridge University Press.

A Central Pathway: Enhanced Immune Functioning (Kuo, 2015)

In her recent review paper, Ming Kuo, Associate Professor at the University of Illinois, suggests that enhanced immune functioning may provide a satisfactory central pathway for nature’s positive impact, and identifies 21 pathways, comprising environmental factors , physiological and psychological states , and behaviours and conditions , that have empirically linked nature to mental and physical health outcomes.


Environmental factors (the ‘active ingredients’ of nature)

 

  • Phytoncides - antimicrobial volatile organic compounds excreted from plants and trees, which reduce blood pressure, alter autonomic activity, and boost immune functioning (Komori et al., 1995; Dayawansa et al., 2003; Li et al., 2006, 2009).
  • Mycrobacterium vaccae - a microorganism in soil that appears to boost immune functioning (Lowry et al., 2007).
  • Negative air ions - found in high concentrations in forested and mountainous areas, and near moving water (Li et al., 2010), and found to reduce depression (Terman et al., 1998; Goel et al., 2005).
  • Environmental biodiversity - proposed to play a key role in immune function via its effects on the microorganisms living on skin and in the gut.
  • Natural sights - window views and images of nature reduce sympathetic nervous activity (flight/fight system) and increase parasympathetic activity (rest/digest system), (e.g., Gladwell et al., 2012; Brown et al., 2013), restore attention (e.g., Berto, 2005; Kaplan & Kaplan, 1989), and promote healing from surgery (Ulrich, 1984); most recently, fractal patterns in nature (see below) have been identified as restorative.
  • Natural sounds - increase parasympathetic activation (Alvarsson et al., 2010); sympathetic and parasympathetic effects drive the immune system’s behaviour (Kenney and Ganta, 2014), with long-term health consequences.
  • Air pollution - associated with myocardial inflammation and respiratory conditions (Villarreal-Calderon et al., 2012): reduced air pollution has an inevitable positive health impact.
  • High temperatures - can cause heat exhaustion, heat-related aggression and violence, and respiratory distress due to heat-related smog formation (Anderson, 2001; Akbari, 2002; Tawatsupa et al., 2012), all reduced by nature’s cooling effect.
  • Violence - affects physical and mental health (Groves et al., 1993); vegetation filters pollutants from the air, dampens the urban heat island (Souch and Souch, 1993), and appears to reduce violence.

 


Physiological and psychological states

 

  • Increased didehydroepiandrosterone (DHEA) with cardio-protective, anti-obesity and anti-diabetic properties (Bjornerem et al., 2004; Li et al., 2011).
  • Increased adiponectin , which protects against atherosclerosis, among other things (Li et al., 2011).
  • Increase in immune function (Orange & Ballas, 2006; Li et al., 2011), including reduction in inflammatory cytokines, implicated in diabetes, cardiovascular disease and depression, and increase in ‘natural killer’ cells (see below).
  • Normalised blood glucose - with multiple health benefits and in particular reducing risk of blindness, nerve damage, and kidney failure (Ohtsuka et al., 1998; Sheetz and King, 2002).
  • Relaxation or stress reduction - the experience of nature helps shift individuals toward a state of deep relaxation and parasympathetic activity, which improves sleep (El-Sheikh et al., 2013), boosts immune function in a number of ways (Kang et al., 2011), and counters the adverse effects of stress on energy metabolism, insulin secretion, and inflammatory pathways (Bhasin et al., 2013).
  • Awe - linked to lower levels of inflammatory cytokines (Shiota et al., 2007).
  • Vitality – enhanced vitality experienced in nature (Ryan et al., 2010) predicts resistance to infection (Cohen et al., 2006) and lowered risk of mortality (Penninx et al., 2000).
  • Attention restoration - see ART (Kaplan & Kaplan, 1989) above; also suggested that it may bolster impulse control, reduce risky health behaviours such as smoking, overeating, and drug or alcohol abuse (Wagner and Heatherton, 2010).

 


Behaviours and conditions

 

  • Physical activity
  • Obesity
  • Sleep
  • Social ties

 

See Kuo’s paper, ‘How might contact with nature promote human health? Promising mechanisms and a possible central pathway’(2015), for further detailed explanations of each of these mechanisms.


Whilst Kuo acknowledges that each of these factors is likely to contribute to nature’s impact on health to some extent and under some circumstances, she suggests ‘enhanced immune functioning’ as a potential central pathway that may account both for the size of nature’s impacts on health and for its specific health outcomes. She cites the work of Dr Li, which has demonstrated that time spent in nature has substantial beneficial effects on the immune system, for example, two 2-hour forest walks on consecutive days increased the number and activity of anti-cancer NK cells by 50% and 56% respectively, and activity remained significantly boosted even a month after returning to urban life — 23% higher than before the walks (Li, 2010). Moreover, extended time in a forest decreased inflammatory cytokines implicated in chronic disease by roughly one-half (Mao et al., 2012), with urban walks having no such effect.


Kuo also looked at the specific health benefits linked to contact with nature. She identified evidence for a favourable impact of nature on the following: acute urinary tract infections, anxiety disorder, ADHD, birth outcomes, cancer, cardiovascular disease, depression, diabetes mellitus, healing from surgery, infectious disease of the intestinal canal, musculoskeletal complaints, medically unexplained physical symptoms (MUPS), migraines, upper respiratory tract infections, respiratory disease, and vertigo. On reviewing the literature on immune functioning and each of these specific outcomes, she concluded that the evidence indicated that enhanced immune functioning may be able to account, wholly or partially, for all 18 of these outcomes. Since the immune system has multiple roles, including warding off infectious disease, protecting the body from bacterial, parasitic, fungal, and viral infections, assisting in wound healing, destroying tumour cells, and governing inflammation, it is possible to link enhanced immune functioning, as a result of contact with nature, with a positive outcome in each of the above.


References

Kuo M. (2015). How might contact with nature promote human health? Promising mechanisms and a possible central pathway.Frontiers in Psychology, 6.

Phytoncides

These are naturally occurring chemical compounds secreted by plants and trees to protect themselves from ‘a threat’ such as bacteria, insects and funghi. When a plant detects a threat of this kind, its immune system increases the production of phytoncides to control the growth of the infection. Our bodies appear to respond positively to some of these phytoncides, which, unsurprisingly, given we evolved amongst trees breathing in these substances, seem to work in concert with our own immune systems. 

Phytoncides are also part of the communication pathway between trees, and their concentration in the air depends on the temperature, with greater concentrations in warmer weather. Evergreens such as pine trees, cedars, spruces and conifers are the largest producers of phytoncides. Terpenes are the main components of phytoncides and are what we smell when we are in natural environments. For example, D-limonene smells lemony, alpha-pinene smells fresh and piney, and beta-pinene smells herby. 

Dr Qing Li and his team at the Nippon Medical School in Tokyo have conducted a number of experiments investigating the effects of phytoncides. In one experiment, they incubated natural killer (NK) cells with phytoncides for 5-7 days, and found that at the end of this period, both NK cell activity and the presence of anti-cancer proteins had increased (Li et al., 2009). In a follow-up experiment investigating the impact of phytoncides on immune function in people, 13 healthy men stayed in a hotel in Tokyo for three nights and hinoki stem oil was diffused into their rooms whilst they slept. Results showed significant increases in the numbers of NK cells (by 20%) and NK activity, enhanced activity of anti-cancer proteins, significant decreased levels of stress hormones, increased hours of sleep and decreased scores of anxiety and fatigue, whilst the control group saw no changes.

A study by the department of psychiatry in Japan, involving 12 participants who suffered from depression, showed that the phytoncide D-limone was more effective than anti-depressants in lifting mood and ensuring emotional well-being in patients with mental disorders (Komori et al., 1995).

Geosmin (the phytoncide that causes the smell of earth after rain) comes from soil organisms, particularly the streptomyces bacteria that are key to many antibiotics. We are alert to this rich smell in incredibly small quantities, a sensitivity likely to have developed as it led to water sources, which may also be an explanation as to why its presence helps put us at ease. 

References
Li, Q., Kobayashi, M., Wakayama, Y., Inagaki, H., Katsumata, M., Hirata, Y., … Miyazaki, Y. (2009). Effect of Phytoncide from Trees on Human Natural Killer Cell Function. International Journal of Immunopathology and Pharmacology, 951–959.
Komori, T., Fujiwara, R., Tanida, M., Nomura, J., & Yokoyama, M. (1995). Effects of Citrus Fragrance on Immune Function and Depressive States. Neuroimmunomodulation, 2, 174-80.

Fractals

Fractals are self-similar patterns that can be found throughout the natural world, but are uncommon in human-made structures (Mandelbrot, 1983). They are sometimes referred to as ‘nature’s roughness’ and examples include snowflakes, plant leaves, tree branches, forests, ocean waves, river systems, coastlines, clouds and galaxy clusters. 

Richard Taylor, a nanoparticle physicist, conducted experiments to measure people’s physiological response to viewing images with fractal geometries. He measured skin conductance and found people recovered from stress 60% better when viewing computer images with a mathematical fractal dimension (D) of between 1.3 and 1.5. Several experiments have confirmed that people generally prefer images with this low to mid-range D, most commonly found in natural phenomena. Taylor and Hägerhäll, an environmental psychologist, used EEG to measure people’s brain waves, and discovered that viewing computer-generated natural fractals increased alpha wave activity, an indicator of a wakefully relaxed state and internalised attention (Hägerhäll et al., 2008, 2015). 

They have also used MRi scans to identify which areas of the brain are activated when viewing fractals: in addition to expected areas involved with high-level visual processing (the ventrolateral cortex) and spatial long-term memory (the dorsolateral cortex), they discovered that viewing fractals activated the parahippocampus, an area involved with regulating emotions and highly active when listening to music. Taylor and Hägerhäll claim this is because our visual system understands fractals. They argue that just as other human systems, such as our lungs, capillaries and neurons, are branched into fractals, so too is the movement of the eye’s retina. They used an eye-tracking machine to measure where people’s pupils were focusing, and discovered people first scanned the big elements in the scene and then the smaller ones in a fractal pattern with a mid-range D. Scientists have found that this is our quickest, most effective way to recognise objects, which our brains need to do when faced with new visual information. When a scene is too complex, such as a busy urban street, it is more difficult to take in and can lead to discomfort, whether conscious or unconscious. Conversely, when faced with common natural features we evolved alongside, such as raindrops on water, our visual cortex is in harmony. Taylor asserts that the visual system is hard-wired to understand fractals, with a physiological congruence when the fractal structure of the eye matches that of the fractal image being viewed, and suggests that this fluent visual processing is a natural stress reducer. 

References
Mandelbrot, B. & Wheeler, J. (1983). The Fractal Geometry of Nature. American Journal of Physics, 51, 286. 
Hägerhäll, C., Laike, T., Kuller, M., Marcheschi, E., Boydston, C., & Taylor, R. (2015). Human physiological benefits of viewing nature: EEG responses to exact and statistical fractal patterns. Nonlinear dynamics, psychology, and life sciences, 19, 1-12.

Sunlight

The benefits of increased exposure to sunlight, both for mental and physical health have been recognised by doctors for centuries, with Hippocrates routinely prescribing ‘sunbaths’ as treatment for a variety of maladies as far back as 400 BC. The Roman philosopher Aulus Cornelius Celsus (25 BC to 50 AD) recommended that sufferers of melancholy live in spaces full of light; and in 1863 Florence Nightingale appealed to hospital designers to include wards that were brightly lit by natural sunlight. In 1984, ‘Seasonal Affective Disorder’ (SAD) was recognised as a condition which causes lethargy and sadness during the winter months when people spend more time indoors and are exposed to less natural light. With our modern technology-based lifestyles, there are now growing concerns that depressive symptoms linked to a deficit in sunlight exposure may no longer be limited to the winter months. Whilst for many people the health benefits of spending more time outside are obvious, researchers have been using scientific methods to investigate the positive impact on mental health and wellbeing of increased exposure to natural light (aan het Rot, Moskowitz & Young, 2006; Genuis, 2006; Walch et al., 2005). 
 
There appear to be a number of factors involved in the positive role sunlight plays in promoting and maintaining mental wellbeing.  
 
Serotonin 

Serotonin is a neurotransmitter, commonly associated with feelings of wellbeing and happiness, although its biological function is in fact much more complex, with links to numerous physiological processes, including appetite and digestion, sleep, memory, cognition, reward and learning. Low serotonin levels  are often attributed to a number of mental health issues, including anxiety, depression, panic attacks, insomnia, eating disorders, alcohol abuse, low  self-esteem, and obsessive thoughts and behaviours.  Conversely, higher levels of serotonin correlate with better mood and feelings of satisfaction and calmness. Many antidepressants work by boosting levels of serotonin among brain neurons, and sunlight has been found to have a similar effect. 

Studies have measured levels of brain chemicals flowing directly out of the brain and found that people have higher serotonin levels on bright sunny days than they do on cloudy ones, and that effect remains irrespective of temperature.  
An Australian study involving 101 healthy men found that levels of serotonin in their brains increased in direct relationship to their exposure to sunlight. Catheters placed in the internal jugular veins of the participants allowed assessments to be done as they were exposed to varying degrees of sunlight. The study found that “the rate of production of serotonin by the brain was directly related to the prevailing duration of bright sunlight, and rose rapidly with increased luminosity” (Lambert, G., Reid, C., Kaye, D., Jennings, G., & Esler, M., 2002). 
 
Vitamin D 

Optimum levels of vitamin D are important for overall health and wellbeing (Gillie, 2005). Although vitamin D is found in certain foods, humans get most of their Vitamin D from exposure to sunlight. However, the World Health Organisation estimates that over a billion people are currently deficient in vitamin D (approximately 50% of all Americans) as a result of our modern indoor lives, and considers vitamin D deficiency as a major public health problem worldwide across all age groups. Vitamin D deficiency is linked to a number of health issues, including tiredness, bone and back pain and depression.  
Although the amount of sunlight necessary varies according to age, skin type, and diet, the WHO recommends getting 10–30 minutes of sunlight, several times per week in order to maintain healthy blood levels. 
  
Ultraviolet light 

Natural light also contains a spectrum of light wavelengths. Although access to sunlight brings the risk of exposure to dangerous levels of ultraviolet (UV) light, recent research also suggests that UV-induced release of nitric oxide from skin may have certain health benefits, including lowering the incidence of hypertension and cardiovascular disease (Liu et al., 2014).  
There’s also evidence that UV light can push melanocytes—the cells that produce dark pigment in skin—to release endorphins, a feel-good chemical (Fell et al., 2104). 
 

References 
aan het Rot, M., Moskowitz, D., Young, S. (2006). Exposure to bright light is associated with positive social interaction and good mood over short time periods: a naturalistic study in mildly seasonal people.  Journal of Psychiatric Research.   
Fell, G., Robinson, K., Clifford, J., Woolf, D. & Fisher, E. (2014). Skin β-Endorphin Mediates Addiction to UV Light. Cell, 157(7), 1527-1534. 
Genuis, S. J. (2006). Keeping your sunny side up. How sunlight affects health and well-being.  Canadian Family Physician, 52(4), 422-3, 429-31. 
Gillie, O. (2005). Sunbathing is needed for optimum health in the British Isles. Health Research Forum. 
Lambert, G., Reid, C., Kaye, D., Jennings, G., & Esler, M. (2002). Effect of sunlight and season on serotonin turnover in the brain. 
The Lancet, 360,1840-42. 
Liu, D., Fernandez, B., Hamilton, A., Lang, N., Gallagher, J., Newby, D., Feelisch, M., & Weller, R. (2104). UVA irradiation of human skin vasodilates arterial vasculature and lowers blood pressure independently of nitric oxide synthase. Journal of Investigative Dermatology, 134(7), 1839-46. 
Walch, J., Rabin, B., Day R, Williams, J., Choi, K., & Kang, J. (2005). The effect of sunlight on postoperative analgesic medication use: a prospective study of patients undergoing spinal surgery. Psychosomatic Medicine, 67, 156-63. 

Nature Sounds

Urban noise is considered one of the greatest pollutants of modern city living, with the World Health Organisation attributing thousands of deaths every year in Europe to heart attack and stroke caused by high levels of background noise. Numerous studies have demonstrated the links between noise pollutants and stress: in a German study of 2000 men, environmental noise over 50 decibels was associated with a 20% increase in hypertension; in a sample of nearly a million people living near Bonn airport, women exposed to noise over a 46 decibels were twice as likely to be on medication for hypertension as those exposed to noise under 46 decibels; in a huge study amongst several thousand school children in UK, Spain and the Netherlands living near large airports, there were significant impacts of noise levels in reading comprehension, memory and hyperactivity (Clark et al., 2005); and a study before and after the opening of the international airport in Munich showed nearly a doubling of stress hormones epinephrine and norepinephrine in children near the airport after flights began, and a significant increase in blood pressure compared with the children living further from the airport.  

Natural surroundings often, although not always, represent respite from noise pollution, and is therefore experienced as peaceful, restorative and stress-reducing. A number of studies have demonstrated reductions in cortisol levels, and increases in heart-rate variability (HRV) when stressed participants are exposed to nature sounds (Alvarsson et al., 2010). The three most soothing sounds to humans have been found to be wind, water and birdsong (Nilsson, 2006). Psychological studies using birdsong have consistently shown improvements in mood and mental alertness, and its qualities perceived as both distracting and restorative (Radcliffe, et al. 2013). People’s preferences for the sounds of nature may also have an evolutionary basis: we associate birds singing in the morning with alertness and safety, and running water with a clean fresh water source. 

References
Alvarsson, J., Wiens, S., & Nilsson, M. (2010). Stress recovery during exposure to nature sound and environmental noise. International Journal of Environmental Research and Public Health, 7,1036-46. 
Kaltenbach, M., Maschke, C., & Klinke, R. (2008). Health Consequences of Aircraft Noise. Deutsches Ärzteblatt international, 105(31-32).
Clark, C., Martin, R., van Kempen, E., Alfred, T., Head, J., Davies, H. W., Haines, M., M. Barrio, I., L Matheson, M., & Stansfeld, S. A. (2005). Exposure-effect relations between aircraft and road traffic noise exposure at school and reading comprehension: the ranch project. American Journal of Epidemiology, 163(1), 27-37. 
Nilsson, M., & Berglund, B. (2006). Soundscape quality in suburban green areas and city parks. Acta Acustica United with Acustica, 92( 6), 903-911.
Ratcliffe, E., Gatersleben, B., & Sowden, P. (2013). Bird sounds and their contributions to perceived attention restoration and stress recovery. Journal of Environmental Psychology, 36, 221-228.

Soil bacteria – mycrobacterium vaccae

There is growing evidence to suggest that breathing in, playing in, digging in, and even eating dirt may be good for our health. 
Mycrobacterium vaccae is the microorganism in soil that has been found to affect humans in a number of positive ways. 

In 2004, Mary O’Brien, an oncologist at the Royal Marsden, injected the soil bacteria in lung cancer patients, hoping it might help her patients’ immune systems beat back the cancer in their lungs. Although it was not successful in fighting the cancer, it ‘significantly improved patient quality of life’: patients were happier, expressed more vitality, and better cognitive functioning. 

In 2007, Lowry and his team at Bristol University, injected M. vaccae into mice and subjected them to a series of stress tests. Those injected with the bacteria showed far less stressed behaviour than the control group, acting “as if on antidepressants’. They found that the bacteria activated groups of neurons in the mouse brains responsible for producing serotonin (a neurotransmitter that, when impaired, can cause depression). They also noted that the neurons that were activated were also known to be related to immune response, suggesting a link between the immune system and emotional health. Further experiments injecting and even just feeding mice M. vaccae have shown mice racing through mazes far faster than controls, suggesting the bacteria gave a significant brain boost as well as elevating their mood. 

Scientists are beginning to understand that the immune system and the brain are closely connected, and are finding more and more evidence that depression and other mental health conditions are associated with prolonged inflammation (a clear indication of immune system dysfunction). Having previously believed that immune responses and brain activity were functions of separate systems, scientists now understand that at least half the brain cells are not nerve cells, but are immune- like cells (‘glial cells’) that communicate closely with our central nervous systems.

In 2016, Lowry tested how effective M. vaccae was in reducing anxiety in mice, by creating stressful maze situations and watching the differences in behaviour in those injected with the bacteria and those not injected. The results showed that the mice injected with M. vaccae readily explored open parts of the maze, compared with the controls who spent more time in the closed parts, forgoing the potential reward of exploring open spaces. The bacteria also reduced the colon inflammation typically seen in stressed-out mice. 
Another experiment placed mice with a dominant alpha-male mouse, usually creating discomfort, but found that those injected with the M. vaccae seemed unaffected and showed 50% less of the flight/freeze behaviours than the controls and remained less submissive for weeks after treatment. Lowry and his team concluded that this was a more proactive response to stress, rather than passive, and hope it might help in treating PTSD in humans, known as a passive response to stress. 

References
O'Brien, M.E., Anderson, H., Kaukel, E., O'Byrne, K., Pawlicki, M., Von Pawel, J., Reck, M. (2004). SRL172 (killed Mycobacterium vaccae) in addition to standard chemotherapy improves quality of life without affecting survival, in patients with advanced non-small-cell lung cancer: phase III results. Annals of Oncology, 15(6):906-14.
Lowry, C. A., Hollis, J. H., de Vries, A., Pan, B., Brunet, L. R., Hunt, J. R., Paton, J. F., van Kampen, E., Knight, D. M., Evans, A. K., Rook, G. A., … Lightman, S. L. (2007). Identification of an immune-responsive mesolimbocortical serotonergic system: potential role in regulation of emotional behavior. Neuroscience, 146(2), 756-72.
University of Colorado at Boulder. (2016, May 16). Immunization with bacteria promotes stress resilience, coping behaviors in mice. ScienceDaily.