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An apple a day: How trees improve human health by Kathi Kemper

“An apple a day keeps the doctor away.” At least one tree product, the apple, is thought to be important for human health. Is there actual research to support the belief that trees promote human health? YES! Do we need more research into how trees affect health and how health professionals can effectively advocate for reforestation in our communities? YES!

 Overview

In a large Dutch study involving over 250,000 people, living near green space including trees was strongly associated with overall health; benefits were greatest for the most vulnerable, the poor, the elderly, and youth. 1 Living with trees reduced the risk of over a dozen diseases, most significantly for anxiety and depression. 2 An Australian study found that people who lived in a highly green neighborhood reported 40%–60% better odds of being healthy than those living in less green environments; living in a greener environment also seemed to promote other healthy lifestyle behaviors such as walking and having more social connections. 3 In a 2014 article in the Atlantic magazine, James Hamblin estimates that trees prevent $7 billion a year in health care costs just by filtering air pollution, not to mention the other health benefits. 4 A 2016 study from the Harvard School of Public Health that followed over 100,000 American nurses found that even after controlling for age, race, smoking, and income, those who lived in the greenest neighborhoods had a 12% lower risk of dying over the 8 years of the study than those in the least green neighborhoods. 5 Reduced mortality rates with residential green environments were confirmed in a national study in Switzerland involving over 4 million adults – the greener the home neighborhood, the lower the risk of death overall and particularly from heart and lung diseases, even after controlling by gender, education, job, income, neighborhood and air pollution. 6 Additional research is needed to better understand the specific mechanisms by which trees improve human health.

Here are over a dozen ways trees improve human health:

1 Trees offer nutritious food - -fruits, nuts, and healthy oils.

Trees are a source of many fresh fruits, rich in antioxidants, fiber (which modulates cholesterol and promotes intestinal health), energy, vitamins, and minerals. Fresh fruit is so healthy; most major medical organizations such as the American Heart Association and American Cancer Society recommend an intake of 2–3 servings daily to prevent heart disease and cancer.

 Some fruits, such as olives and coconuts, provide healthy forms of fat or oil that fight inflammation and promote optimal function of the heart, blood vessels, and brain. 7

 Trees are also the source of nuts. Tree nuts are great sources of healthy monounsaturated and polyunsaturated fat, fiber and plant sterols that help reduce cholesterol, B vitamins, Vitamin E, amino acids (such as arginine which helps keep blood vessels elastic and healthy), and minerals such as magnesium, calcium, iron, zinc, potassium, and selenium. Eating about 30 g of nuts daily can reduce the risk of type 2 diabetes by 30% and heart disease by 30%–50% and reduce the risk of death by heart disease by 20%. 8 Tree nuts are considered a key part of the healthy Mediterranean diet. Although a small percentage of people are allergic to certain kinds of nuts, over 90% of people are not allergic and do better with a diet rich in nuts. Additional research is needed to determine the broader health costs and benefits of tree nuts and fruits.

2 Trees are the original source of some of our most important medicines.

 One of the most famous examples is the bark of the white willow, which contains salicin, which can be converted salicylic acid, and then acetyl-salicylic acid or aspirin. 9 Another famous example is Peruvian cinchona bark, the source of quinine originally used to treat malaria. 10 The oil of eucalyptus tree leaves is used in aromatherapy and as a topical antimicrobial and antiseptic. 11 Some tree fruits, such as elderberries, are also used medicinally, to help fight viral infections. 12 The oil of the Australian tea tree has strong antibiotic and antifungal effects, and it is used in many non-prescription products to treat acne as well as more potent forms to treat serious infections like pneumonia. 13 Even after they die, decaying trees serve as hosts for several fungi used in traditional Asian medicines, such as Reishi (Ganoderma species), Agaricus brasiliensis, Trametes versicolor, shitake, maitake, and others. 14

Trees help improve the climate and environment and human health in several ways. 15

3 Trees remove air pollutants such as nitrogen dioxide, sulfur dioxide, ozone, and particulate matter, thereby significantly reducing cardiopulmonary deaths due to air pollution.

In the US, an estimated 4700 deaths per year are due to exposure to ground level ozone, while over 130,000 deaths per year can be attributed to fine particulate matter, including deaths due to bronchitis, heart attacks, asthma, and other cardiorespiratory problems. 15 The World Health Organization estimates that over 7 million deaths globally are due to urban air pollution annually. 4,15 Both evergreen and deciduous trees can decrease deaths by helping remove pollutants from the air. Dave Nowak and colleagues noted that while most of the cleaning occurred in rural areas with dense tree cover, most of the benefits occurred in urban areas with dense human habitation. 16 Researchers at Portland State University in Oregon estimated that kids avoided missing more than 7000 school days due to asthma annually thanks to trees’ effectiveness in reducing nitrogen dioxide levels; they estimated that increasing tree canopy by 5% could lower asthma exacerbations by 6%. 17

A natural experiment in the US has confirmed trees’ benefits for heart and lung health. The emerald ash borer has ravaged ash trees across the American Midwest. A study that monitored disease and death in 15 states from 1990 to 2007 concluded that the ash borer destruction of trees resulted in over 6000 excess deaths from lung disease and 15,000 deaths from heart disease. 4 Additional research is needed to determine the impact on community cardiovascular health by planting trees near hospitals, clinics, schools, public buildings and spaces, neighborhoods and businesses.

4 Trees help cool the air, reducing the risk of heat stroke.

 Local cooling is particularly important as climate change progresses, particularly in urban areas which tend to trap heat. Reductions in tree canopies are a major contribute for urban heat island effect; many cities are working to restore tree canopies to help keep cities comfortable and reduce the need for air conditioning. 18 On average, the temperature beneath trees is 2–4 degrees cooler than other shade. 19 Cooling from trees helps reduce the risk of heat stroke, particularly in the frail and elderly. Again, research is needed to determine how community treeplanting efforts affects morbidity and mortality from heat waves and the most cost-effective strategies for promoting reforestation.

5 Trees create oxygen, which is necessary for human life.

Trees depend on photosynthesis to live and grow. During photosynthesis, trees take in carbon dioxide and water, and using the green pigment chlorophyll, convert them into chemical energy and release oxygen as a byproduct. It is estimated that the amount of oxygen produced by an acre of trees annually equals the amount of oxygen consumed by 18 people. See www.growingairfoundation.org/facts.

6 Trees help filter and clean ground water

 Water from the soil is drawn up into the tree’s roots to its leaves where it is combined with carbon dioxide to make energy for the tree and oxygen for the rest of us; some is released as water vapor, increasing atmospheric humidity and stabilizing temperatures. Acting like sponges, trees collect and filter groundwater, slowly releasing it into underground aquifers, streams, and rivers, helping reduce runoff and maintaining water quality.

7 Trees sequester carbon dioxide, helping holding in check the rising levels of carbon dioxide globally and preventing a more rapid decline in climate.

 Agroforestry is the practice of growing trees to help capture and sequester atmospheric carbon dioxide, which can not only sequester significant amounts of carbon, but also serve as windbreaks to prevent soil erosion, provide food and feed for humans and livestock, pollen for bees, and sustainably harvested wood for construction of shelter. 20 Planting trees is a straightforward strategy to combat the adverse health effects of climate change due to increasing atmospheric carbon dioxide. 21,22 Additional research is needed to determine the impact on human health of mitigating climate change through agroforestry – how much can this practice reduce the increasing risk of rising sea levels, sea level acidification, decreased biodiversity, drought, storms, famine, conflict, infectious diseases, violence, and climate refugees?

8 Trees improve blood pressure and reduce stress.

Trees are so beneficial, in 1982 the Japanese Ministry of Agriculture, Forestry and Fisheries coined a term for the health effects of spending time in forests: shinrin-yoku or “forest bathing.” 23 Studies across Japan have shown that just 15–30 minutes of spending time walking or resting in a forest helps lower levels of stress hormones, high blood pressure, and heart rate. 23 A recent analysis of over 20 studies showed that forest bathing significantly improved high blood pressure. 24 A German study confirmed the benefits of trees on high blood pressure in children. 25 What are the population health benefits on blood pressure of reforestation?

9 Trees improve blood sugar in diabetics.

 In another study, forest bathing helped lower blood sugar levels in adults with diabetes. Blood sugar fell on average from 179 mg/dl to 108 mg/dl after a walk; repeated walks lowered average blood sugar over time, reducing Hemoglobin A1C levels significantly, too. 26 A German study confirmed that in adolescents, greater exposure to green environments with trees was associated with lower levels of insulin resistance; the researchers reported that the trees helped clear the air of nitrogen dioxide and particulate matter which increase the risk of insulin resistance. 27 What are the population health benefits on diabetes and blood sugar with reforestation?

 10 Trees help bolster immune function.

Trees create compounds called phytoncides, chemicals plants emit to protect themselves from bacteria, fungi, and insects. When humans breathe phytoncides, our bodies boost the number and activity of natural killer (NK) cells, white blood cells that protect us from viruses and tumors. 28,29 Forest bathing increases NK activity and promotes the expression of more anti-cancer proteins. 30 In a study of over 100,000 women, even after controlling for age, race, smoking, and income, those who lived in the greenest settings had 12% lower risk of dying than those in the least green settings; the greatest benefits were for lung disease and cancer. 5 What are the most effective and efficient strategies to encourage individuals and communities to plant more trees?

11 Exposure to trees and green neighborhoods is associated with fewer low birth weight births and decreases in inflammation and stress hormones.

In pregnant women in Massachusetts, living in a greener neighborhood has been associated with a reduction in the risk of having a low birthweight baby. 31 What is the population attributable risk of deforestation on low birth weight? And how might reforestation improve health outcomes more generally?

In older adults with chronic obstructive pulmonary disease (COPD), forest bathing led to reduced inflammation and stress hormones. 32 What are the mechanisms by which forests decrease stress and inflammation? What is an adequate or optimal “dose” or “exposure”? How might these effects translate into benefits for other types of inflammatory diseases?

12 Trees enhance recovery from surgery.

In a landmark study done in Pennsylvania in the 1970′s, patients who were getting their gall bladders removed in a standard surgical procedure were randomly assigned to recovery rooms with views of a natural setting including trees or not; those who recovered with natural views had significantly shorter hospital stays and were noted by nurses to be more pleasant and require less pain medication than patients who lacked the benefit of these views. 33 This study needs to be replicated in additional surgical settings.

13 Exposure to trees decreases stress, improving mental and physical health.

Forest bathing also reduces stress and improves mood and attention. 34 Research in middle-aged adults has shown that forest bathing is associated with significant reductions in tension, anxiety, anger, fatigue, depression, and confusion, and improvements in vigor. 35 A Harvard study published in 2019 followed nearly 40,000 middle-aged and older women over ten years and found that those who lived in the greenest neighborhoods had a 13% lower risk of depression compared with those living in the least green neighborhoods 36 These benefits have been confirmed in studies of children and adolescents – the greater the exposure to greenness, the lower the risk of depression, particularly in urban areas. 37,38 Other research shows that exposure to green environments containing trees improves self-esteem and mood in patients with mental health problems. 39 Trees buffer noise, thereby reducing stress. 40 They also provide habitat for songbirds, which can also improve mood. A study of over 300 school children found that levels of nearby nature moderated the impact of stressful life events; the more exposure to nature, the better the children’s overall psychological health. 41 In a study of adult New Yorkers, greater exposure to vegetation, particularly trees, was associated with overall less stress and better health. 42

While exposure to particulate matter and nitrogen dioxide air pollutants is associated with a markedly increased risk of attention problems like ADHD, exposure to forests and nature in general helps reduce the risk of children being diagnosed with ADHD and may serve as an effective complementary therapy for children and young adults with ADHD. 43,44 Green spaces with trees have restorative effects on mental health and emotional well-being in both adults and children. 45 Exposure to nature helps improve executive cognitive function and helps people cope with stress. 46,47 Green environments are linked to enhanced academic performance. 48 For more information about the cognitive and emotional benefits to children, see the Children and Nature Network (www.childrenandnature.org/).

14 Exposure to trees and well-maintained green spaces decreases violent crime

 In a large randomized controlled trial involving over 541 vacant lots in Philadelphia, residents who lived near lots that were cleaned up and planted with grass and trees experienced a 42% drop in depression, 37% decrease in crime, and a 7% decline in shootings compared with residents living near vacant lots that were not cleaned and planted. 49–51 A study in Portland also found that the more trees planted in underserved communities, the lower the rate of violent crime in the following years. 52 Because homicide is leading cause of death in young adults, additional research is needed to determine the optimal concentration of trees in a neighborhood to reduce stress and crime levels.

 Summary:

Overall, trees enhance human health and lengthen lifespan in over a dozen ways 5 :

 Creating oxygen Filtering and reducing toxic air pollutants Filtering and cleaning water and reducing damaging runoff Sequestering carbon to reduce the adverse health effects of climate change Locally lowering temperatures that can reduce the risk of heat stroke Growing nutritious food Providing natural medicine Lowering blood pressure Reducing high blood sugar in people with diabetes Reducing the risk of low birthweight babies Boosting immune function Improving surgical outcomes Improving mental health and reducing violent crime

 Despite the abundance of research on the health benefits of trees, few medical schools cover these data, and few encourage advocacy for reforestation as part of broader public health advocacy efforts. Additional research is needed to determine the mechanisms for the many medical benefits of trees. Research is also needed to determine optimum mix and density of trees, how to minimize adverse effects such as allergies (pollen), damage to power lines and structures from downed trees, etc. Insights are needed from behavioral health researchers to better understand the most effective strategies to encourage individuals, governments, and businesses to become engaged in reforestation both for specific health benefits and for the broader benefits to climate change. We encourage the submission of cutting edge research on these questions to Complementary Therapies in Medicine.

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