by Jessica Schiff
figures by Rebecca Senft

For millennia, people have held to the notion that spending time in nature is healing and restorative. Hippocrates stated that “Nature itself is the best physician”. At some point in our lives, many of us have gone on an adventure into nature and left feeling rejuvenated and relaxed. However, it wasn’t until the past few decades that scientists really began to understand the complex relationships between the natural environment and our health, providing additional support for the need to protect nature and the planet. 

So, what are these health benefits?

Time spent in green spaces and nature is associated with a number of benefits for both mental and physical health, and the benefits don’t only come from intentionally taking time to hike through nature – benefits can be seen after only a few minutes of time spent in green spaces such as city parks!

Mental health benefits

There is strong evidence that spending time in nature is beneficial for various aspects of our mental well-being; providing stress relief, reduced anxiety, and improvement in one’s mood. A number of studies compared cortisol (a stress hormone) levels before and after spending time outdoors, ranging from exercising outdoors or gardening, to forest therapy/bathing (i.e.: walk or hike in a forest). They all saw a decrease in cortisol concentration suggesting that the time spent outdoors and in green space may reduce the feeling of stress. Furthermore, according to one study, the stress reduction benefits from spending time in nature were more striking for those who experience chronic stress.

Improvements in general mood and alleviation of symptoms of anxiety and depression have also been seen with increased contact with nature.  Walking in forests has been shown to result in a greater improvement in psychological wellbeing as compared to walking in an urban setting or performing routine, daily activities, such as commuting for work and household chores. Additionally, walking in nature, compared to an urban setting, saw decreases in rumination and anxiety. Understanding how nature influences mental health outcomes allows professionals to develop nature-based mental health interventions that can reduce stress and symptoms of anxiety and depression.

Physical health benefits

Not only does spending time in nature improve our mental wellbeing, but it also improves our physical well-being, such as immune health and blood pressure. Time spent in nature has been associated with increased expression of anti-cancer proteins and natural killer (NK) cells– immune cells that can kill tumor cells or cells infected with viruses. 

One study evaluated the immune effects of time spent in nature by comparing  a group of participants who spent three days and two nights in a forest and to the same group of participants spending a similar period of time in a city.  After the trip to the forest, the participants showed increased NK cell activity for more than 7 days, whereas no period of increased NK cell activity was seen after the time spent in the city. This suggests that exposure to the natural environment can contribute to enhanced immune function. Additionally, multiple studies demonstrate that time spent in forest environments (walking, sitting, and viewing landscapes) is associated with significantly lower blood pressure as compared to time spent in a non-forest environment doing the same activities. The effects of lowered blood pressure from nature exposure were greatest for people who already had elevated blood pressure. Other health benefits, such as decreased prevalence and mortality from cardiovascular disease have also been linked to nature exposure.

What are the mechanisms behind the benefits of time spent in nature?

There are currently several different hypotheses proposed to describe how nature might influence different aspects of human health. However, there are many gaps in the research that need to be addressed before we can identify a causal relationship. 

One hypothesis attributes the observed effects on immune function to breathing in phytoncides, volatile organic compounds that have antifungal and antibacterial properties that are released into the air by trees. Lab studies have indicated that phytoncides increase natural killer (NK) cell activity. Therefore, it is plausible that exposure to phytoncides in forests explains the enhanced NK cell function in people who spent time in forests. 

Additionally, it has been suggested that living in areas with more green space influences immune tolerance and could reduce risk of allergies through exposure to increased microbial biodiversity in the environment. Natural, more rural spaces have increased microbial biodiversity compared to more urban spaces. These environmental microbes are thought to influence the diverse community of the microbes that live on our skin and could influence the development of allergies.

Other pathways that could help to explain why we see the mental and physical health benefits from nature include increased physical activity, social cohesion, and decreased air pollution. Time spent outdoors and in green spaces is usually, but not always, connected with some form of  physical activity such as walking and therefore the positive physical and mental health effects of time spent outside may also be related to, or enhanced by, the benefits of exercise. However, there is research suggesting mental health benefits can arise just from looking at green spaces and thus not solely related to physical activity or exercise. Social cohesion, the amount of social connection, has been proposed as another pathway for some of the mental health benefits because many people engage in outdoor activities with other people, but has been less studied in the context of nature exposure. Scientists also hypothesize that green space can reduce air pollutant concentrations because green spaces usually have less vehicle traffic in, and around, the space resulting in less exhaust exposure.

So, why is it important to support preservation and conservation of nature?

Time in nature can help alleviate the symptoms for certain conditions and promote healthy activity and lifestyles that can reduce the risk for certain chronic diseases. Reflecting on the past year as the world grappled with the COVID-19 pandemic, many people began to recognize the importance of disconnecting and spending time in nature. As the naturalist and biologist E.O. Wilson said, “Just being surrounded by bountiful nature, rejuvenates and inspires us,” but how can we continue to benefit from nature if the natural environment is being destroyed?

The climate and environment are in crisis – primarily due to our activity and overexploitation of natural resources. Deforestation, land degradation, urbanization, and global climate change threaten the forests and natural spaces. According to the World Wildlife Fund, enough trees to cover 30 soccer fields were cut down every minute in the tropics in 2019. 

If we continue to lose forests at the rate we are currently, we will lose the health benefits associated with them. But we can all take efforts to prevent the loss of natural spaces! We can reduce our meat consumption and adopt a more plant-based diet, we can shop for locally produced food and products, encourage the planting of native plants in local parks, and we can make an effort to support local and international efforts to conserve and protect natural resources. Small changes can add up and make a difference!

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Jessica Schiff is a Master of Science student in Environmental Health at the Harvard T.H. Chan School of Public Health.

Rebecca Senft is a sixth-year Program in Neuroscience Ph.D. student at Harvard University who studies the circuitry and function of serotonin neurons in the mouse.

For More Information:

• To learn about Ecophysiology, check out this post about how nature benefits health published by the Yale Environment 360 (YE360).
• Check out this NPR podcast to learn about how green space in urban settings can improve health.
• REI Co-op has published a list of 95 ways to spend more time outside.