The connection between trees, human health and well-being dates back millennia. The ancient Celts worshiped in sacred groves, believing the trees would protect them from physical and spiritual harm. In Hebrew and Christian scriptures a tree of life in the Garden of Eden imparted immortality. Potted conifers helped to cleanse the air inside tuberculosis sanatoriums of nineteenth century Europe.
In recent years, scientists studying urban forests have turned up links between exposure to green space and health benefits, including fewer deaths from heart disease and respiratory diseases, fewer hospitalizations, better infant birth weights and even less crime.
"We've had this intuitive understanding that nature is good for us. Now we're backing it up on an empirical level," said Geoffrey Donovan, a resource economist with the U.S. Forest Service Pacific Northwest Research Station in Oregon.
Donovan and others are digging into the underlying science to understand the relationship between nature and health, a step they say will help guide the design of healthier cities and suburbs.
"We've had this intuitive understanding that nature is good for us. Now we're backing it up on an empirical level."-Geoffrey Donovan, US Forest Service Early indications of health benefits
In 1984, University of Delaware researcher Roger Ulrich made the observation that gall bladder surgery patients stayed in the hospital for less time and took fewer painkillers when they could see trees out their hospital window than when their window faced a brick wall.
Ulrich's study was small -- just 46 patients -- and raised more questions than it answered. Yet it suggested for the first time scientifically that our perception of nature could potentially influence health outcomes.
That same year, American clinical psychologist Craig Brod coined the term "technostress" to describe the increasingly artificial elements of our built environment that appeared to be raising stress levels. Chronic stress can weaken the immune system. Some experts hypothesized that this kind of constant stress -- exacerbated by the urban environment -- was making people sick.
In Japan, Yoshifumi Miyazaki wondered whether the antidote could be as simple as a long walk in the woods. Miyazaki, a physiological anthropologist at Chiba University, is widely regarded in Japan as the father of forest therapy -- a preventive medicine approach aimed at preventing disease by exposing people to nature.
Over the last three decades, Miyazaki has led more than 60 studies investigating the physiological effects of being in a forested environment. His team has taken measurements including blood pressure readings and changes in heart rate. They've tested saliva samples for cortisol, a hormonal marker of stress. Overwhelmingly, they've found that when people spend time in a forest, their bodies act less stressed out.
Miyazaki hypothesizes that exposure to natural stimuli -- the sound of a woodpecker drumming away on a tree trunk or the smell of damp pine needles, for instance -- promotes physiological relaxation. He's shown it may help to lower blood pressure, stress hormone levels, sympathetic nervous system activity (think fight-or-flight response) and relieve depression and anxiety.
But how much time in the forest is enough? A group of Stanford researchers in 2015 showed that just a 50-minute walk in a park or forest could decrease anxiety and rumination (a psychology term that basically means dwelling on the negative thoughts caused by upsetting situations) compared to a 50-minute walk through an urban environment.
A New Environmental Exposure -- Greenness
What do those nature exposures mean when they add up over a lifetime?
Previous research suggested that neighborhood vegetation might reduce obesity, promote physical activity, and improve mental health and heart health. Yet most of these studies looked only at one point in time -- making it tricky to tease out whether living on a green block actually made people healthier or whether healthier people just chose to live in greener neighborhoods.
Adding to the problem, urban dwellers often pay a premium for access to green space. If wealthier people are more likely to live in greener areas and wealthier people also are more likely to have better health outcomes, maybe it's their wealth -- and not exposure to nature -- that's making them healthier.
James and his colleagues at Harvard set out to examine the association between greenness and mortality in a large, ongoing study of nurses living in mostly urban areas around the country. In gathering data repeatedly on the nurses over time (and the terminal nature of the chosen endpoint -- death) it was more likely that any association between greenness and mortality was actually due to the greenness and not some other factor.
And the fact that all study participants shared the same occupation -- nursing -- made it less likely that socioeconomics would confound their results.
In a 2016 study, the researchers reported that nurses with high levels of greenness surrounding their homes over the course of the eight-year study were about 12 percent less likely to die during that period than nurses living in the least green areas. The associations were strongest for respiratory, cancer, and kidney disease-related deaths.
They found that the association between greenness and mortality appeared to be explained by women living in greener neighborhoods experiencing less depression, higher levels of social engagement, more physical activity and lower exposures to air pollutants than their peers living in less green neighborhoods.
A Natural Experiment
"If nature can make us feel better in the general sense, then we should be able to see measurable differences in human health," said Donovan, who studies the social and health benefits provided by urban trees.
Under normal circumstances, he said, studying how large-scale changes in foliage over time impact the health of communities would take ages. It could take a generation or more before newly planted trees form a mature urban tree canopy.
Yet nature set up the experimental conditions Donovan needed to study the relationship between trees and health outcomes. The loss of more than 100 million ash trees over the last decade and a half has drastically changed the landscape in many U.S. cities -- making them a perfect laboratory to study the relationship between tree cover and health.
"Exposure to vegetation can be very restorative, but design does matter."-William Sullivan, University of Illinois at Urbana-ChampaignThe culprit? A shiny green beetle named the emerald ash borer. The ash borer, native to Asia, first turned up in Detroit in 2002. It's been spreading across the Northeastern U.S. since, leaving behind a trail of dead ash trees.
Using the presence of the ash borer as an indicator for tree loss, Donovan and his colleagues showed an increase in deaths associated with the presence of the beetle. In counties across a 15-state area, Donovan attributed about 15,000 additional heart disease-related deaths and about 6,000 respiratory disease-related deaths to a loss of trees caused by the emerald ash borer. They published their results in 2013.
"The magnitude of the effect was really eye-opening," Donovan said.
New Tools to Quantify Effects
Studies such as Donavan's natural experiment with the emerald ash borer give experts confidence that nature really is affecting health -- that researchers haven't just stumbled upon a giant set of coincidences.
Yet more science is needed "to tell us the conditions under which nature will and will not improve health, and how to use nature to improve health," said Ming Kuo, director of the Landscape and Human Health Laboratory at the University of Illinois at Urbana-Champaign.
"Exposure to vegetation can be very restorative, but design does matter," said William Sullivan, a landscape architect also at the University of Illinois at Urbana-Champaign. Hacking your way through an overgrown lot, for instance, may not have the same calming or restorative effect as a casual stroll through a grove of trees or an urban park.
As landscape architects move toward creating more ecologically healthy landscapes that foster ecosystem services -- for instance flood mitigation or temperature regulation -- it's important to understand the human health implications too, Sullivan said. For instance, are you creating a reservoir for mosquitoes, ticks or other insects that could be carrying disease?
"We need information on how exposure to different forms of green space impact health, how much exposure people need, and what kind of designs -- arrangements of plants, types of plants -- are healthy for the environment and for people," he said.
"Planting trees can literally save the lives of people."-Satoshi Hirabayashi, The Davey Tree Expert Company, US Forest ServiceResearchers now are developing tools that may soon answer some of these questions. Satoshi Hirabayashi, an environmental engineer at The Davey Tree Expert Company and the US Forest Service in Syracuse, New York, studies how much air pollution is removed by different types of trees and then estimates how those reductions in air pollution benefit human health. Previous studies suggest as many as 135,000 US deaths annually can be attributed to ground level ozone and fine particulate matter. Trees absorb some of those airborne particles by trapping them on their leaves and bark while gaseous pollutants are taken in through the leaf stomata.
Hirabayashi and colleagues are developing a national database that will allow users to quantify the air quality and related human health benefits associated with any forested area anywhere in the US. "We will be able to show people what kind of air pollution removal is going on in their own backyard," he said.
So far, they've shown that tree type matters and that urban trees give more bang for the buck when it comes to health benefits. Evergreens do a better job of removing pollutants year-round than deciduous trees, which drop their leaves in the fall, Hirabayashi found. And while rural areas experience more total air pollution removal from trees than urban areas (due to more tree cover in rural areas), the effects of that air pollution removal on human health appear greatest in urban areas where the most people are concentrated.
Urban forest managers and city planners around the country have begun using this technology to better understand the health savings associated with city trees on both a community and backyard level using tools such as i-Tree Eco and i-Tree Design, according to Hirabayashi. These programs can estimate air quality and associated human health benefits anywhere in the US.
"Planting trees can literally save the lives of people," he said.