In a study in the journal Environmental Research, researchers compared blood samples of people participating in the . The researchers found that people living in greener neighborhoods showed immune patterns consistent with lower chronic inflammation, including lower levels of several circulating immune cells linked to inflammatory activity, such as monocytes, natural killer cells, B cells and some white blood cell types.

“The study also found evidence of a shift toward a more regulated immune profile, suggesting that nearby trees and vegetation may do more than improve quality of life – they may also influence the body’s immune system in ways that support cardiovascular health,” said Daniel Riggs, assistant professor in UofL’s and first author of the study. “While the study cannot prove cause and effect, the findings add to growing evidence that greener residential environments may be an important public health resource for reducing inflammation and promoting long-term health.”

The findings provide additional support for preliminary findings from the Green Heart Louisville Project showing that people living in neighborhoods where the number of trees and shrubs was more than doubled had lower levels of a blood biomarker of inflammation than those living outside the planted areas. Those living in the greened area had 13-20% lower levels of high-sensitivity C-reactive protein (hsCRP) than those living in the areas that did not receive additional trees and shrubs. Higher levels of hsCRP are strongly associated with a risk of cardiovascular disease. Higher CRP levels also indicate a higher risk of diabetes and certain cancers.

The recent study evaluated the immune function of Green Heart Louisville Project participants based on the greenness of their residential environment prior to the addition of trees for the project.

“These studies show that greener neighborhoods don’t just look better; they appear to educate our immune system in ways that may reduce chronic inflammation and disease risk,” said Aruni Bhatnagar, director of the Envirome Institute. “They align with the Envirome Institute’s mission of identifying environmental conditions that shape human health and translating those insights into strategies for improving the health of our communities.”

UofL in partnership with The Nature Conservancy, Washington University in St. Louis, Hyphae Design Laboratory and others to study whether and how living among more densely greened surroundings contributes to better heart health.

Louisville residents are well acquainted with the challenges caused by urban heat, such as higher utility bills and serious health problems that disproportionately affect residents with pre-existing health conditions.

The new study also involves Brian Stone, a professor and director of the Urban Climate Lab at Georgia Institute of Technology, whose previous work identified Louisville as having one of the fastest-growing urban heat islands in the nation.

An urban heat island exists when the built environment – buildings, sidewalks and paved roads – absorbs more heat than natural surfaces such as grass, trees and exposed soil, causing higher air temperatures. Those heat-absorbent surfaces also retain that heat during nighttime hours.

“Louisville has been at the forefront of studying urban heat for years, thanks to foundational work like ,” Stone said. “However, the climate continues to change and the city continues to grow. This new project provides an unprecedented, highly detailed look at current temperatures and the specific cooling power of different types of vegetation in 2025. It’s about giving Louisville the most up-to-date, actionable science to make truly impactful decisions, which is especially relevant as residents are facing extreme heat right now.”

The project will use advanced climate models to generate highly detailed, hourly temperature maps for June, July and August 2025. These maps will cover the entire Jefferson County area – nearly 398 square miles – at a remarkable resolution of 100 meters, offering an unparalleled view of where heat is most intense across Louisville’s urban core. Sensors installed by the Envirome Institute around Louisville will provide data to help enrich the model. The project, supported by the James Graham Brown Foundation, begins in July and is expected to be completed within three months.

Beyond traditional temperature readings, the study will also measure factors that provide a more complete picture of actual heat stress experienced by individuals.

A key focus of this research involves an in-depth analysis of how various tree canopies reduce temperatures. Researchers will investigate which areas benefit most from new tree plantings and how different tree species — and whether they are coniferous or deciduous — impact cooling differently.

Precise insights gained from the new project also will enhance and expedite the Envirome Institute’s research into health benefits of additional greenness, including the and the in downtown Louisville.

“At the Envirome Institute, our mission is to understand how the environment shapes our health. This new analysis comes at a time when we are all facing tremendous heat burden,” said Aruni Bhatnagar, executive director of the . “By precisely mapping urban temperatures and land cover like trees and other vegetation, we’re not just gathering data; we’re creating a powerful new tool. This research will allow us to strategically use urban greening as a direct intervention to improve public health in our downtown core and across every neighborhood in Louisville.”

The project will also analyze heat-related mortality at the neighborhood level in order to highlight areas where residents are most vulnerable to heat-related illnesses and where targeted interventions are most urgently needed to improve public health outcomes. This insight will help guide tree planting initiatives by and other partners, ensuring that efforts maximize cooling benefits.

“TreesLouisville has been working tirelessly to expand our urban canopy, utilizing data from previous assessments to guide our strategic planting,” said Cindi Sullivan, executive director of TreesLouisville. “This new study will provide us with an even sharper focus, pinpointing exactly where our efforts will have the greatest impact and helping us choose the right trees for the right places to achieve significant temperature reductions for our community. This is particularly urgent as we continue to push towards our city’s goal of every neighborhood having at least 30% canopy cover.”

Article is co-authored by Ted Smith.

The launched the first-of-its-kind project in 2018 in partnership with The Nature Conservancy, Washington University in St. Louis, Hyphae Design Laboratory and others to study whether and how living among more densely greened surroundings contributes to better heart health. The design of the study closely mirrors clinical trials which test whether medical treatments are effective. The team applied the treatment – the addition of large trees and shrubs – to some participants’ neighborhoods but not to others. They then compared residents’ health data to see how the addition of the trees affected their health.

“The Green Heart Louisville Project is an excellent example of how our university’s innovative and collaborative researchers are working to improve lives in our community and far beyond,” UofL President Kim Schatzel said. “Trees are beautiful, but these results show that the trees around us are also beneficial to individual and community health. Through this and many other projects, the Envirome Institute is improving health at the community level, not just for individuals, but for everyone living in a neighborhood.”

To understand the state of community’s health at the start of the study, researchers took blood, urine, hair and nail samples and documented health data from 745 people living in a four-square-mile area of south Louisville. The researchers also took detailed measurements of tree coverage and levels of air pollution in the area.

Following this baseline data collection, the Envirome Institute worked with The Nature Conservancy and a host of local partners and contractors to plant more than 8,000 large trees and shrubs in designated neighborhoods within the project area. Those living in the greened area were considered the treated population and the results obtained from this population were compared with residents of adjacent neighborhoods, where the project team did not plant any trees.

After the plantings, the research team reassessed residents’ health. They found that those living in the greened area had 13-20% lower levels of a biomarker of general inflammation, a measure called high-sensitivity C-reactive protein (hsCRP) than those living in the areas that did not receive any new trees or shrubs. Higher levels of hsCRP are strongly associated with a risk of cardiovascular disease and are an even stronger indicator of heart attack than cholesterol levels. Higher CRP levels also indicate a higher risk of diabetes and certain cancers.

A reduction of hsCRP by this percentage corresponds to nearly 10-15% reduction in the risk of heart attacks, cancer or dying from any disease.

“These results from the Green Heart Louisville Project indicate that trees contribute more to our lives than beauty and shade. They can improve the health of the people living around them,” said Aruni Bhatnagar, director of the Envirome Institute and UofL professor of medicine. “Although several previous studies have found an association between living in areas of high surrounding greenness and health, this is the first study to show that a deliberate increase in greenness in the neighborhood can improve health. With these results and additional studies that we hope to report soon, we are closer to understanding the impact of local tree cover on residents’ health. This finding will bolster the push to increase urban greenspaces.”

As more is known about the health impacts of increased tree cover, increased greening in cities may emerge as a key method to improve public health.

“Most of us intuitively understand that nature is good for our health. But scientific research testing, verifying and evaluating this connection is rare,” said Katharine Hayhoe, chief scientist of The Nature Conservancy. “These recent findings from the Green Heart Project build the scientific case for the powerful connections between the health of our planet and the health of all of us.”

Earlier in August, the Green Heart Louisville Project was awarded an additional $4.6 million in funding from the National Institute of Environmental Health Sciences to support continued research over the next five years.

These findings were presented by Daniel Riggs, UofL assistant professor of environmental medicine, at the in Santiago, Chile on Aug. 26.

STUDY DETAILS

This study is part of the Green Heart Louisville Project, ongoing research to discover how trees and shrubs affect people’s health.

The neighborhood

The project involved a four-square-mile area of South Louisville straddling the Watterson Expressway, including the Taylor-Berry, Jacobs, Hazelwood, Oakdale, Wilder Park and Beechmont neighborhoods. The project team planted trees and shrubs in a central portion of the project area that included Wyandotte Park, which served as the treated area. They planted no trees in the outer areas of the project area. [SEE MAP]

The people

Researchers enlisted 745 people living in the project area to participate in the project’s ongoing health study. At enrollment, participants were between 25 and 75 years old, 60% female, 77% white and 50% had household income below $50,000 per year. They live in the planted intervention area as well as outside it.

Researchers collected participants’ blood, urine, nail and hair samples and health information before and after planting the trees and shrubs in the “treatment” area.

The trees

Between 2019 and 2022, the Green Heart Project planted 8,425 evergreen trees and shrubs of 42 different species from five different families and 630 deciduous trees. Evergreens were pine, cypress, yew, holly and magnolia. Deciduous trees were mainly dogwood, oak and serviceberry.

Project team members and BrightView Landscape maintain the trees to promote their survival, growth and sustained impact.

The data

High-sensitivity C-reactive protein (hsCRP) is a blood test that measures general inflammation in the body. After the planting, people living in the areas where trees and shrubs were added had 13-20% lower hsCRP levels compared with those living in the areas that did not receive greening from the project.

Inflammation as measured by hsCRP , including stroke, heart attack and coronary artery disease. Previous studies have shown that hsCRP increases with age. The change in hsCRP seen in individuals living in the planted areas was similar to the difference between typical levels in a 42-year-old compared with a 33-year-old.

This difference also is similar to the difference between people who exercise regularly and those who do not. Finally, the decrease in hsCRP levels with planting corresponds to a nearly 10-15% reduction in the risk of heart attacks, cancer or dying from any disease.

Funding

A $3 million grant from the National Institute of Environmental Health Sciences supported the health evaluation of community participants and an additional $4.6 million was recently awarded for continuing this research. These grants bring the total investment by the institute to more than $8 million. The Nature Conservancy provided over $8.7 million in funding to support tree planting and maintenance, project management and other key project needs. The project has had an additional $3 million in support from local donors. Recently, $4.6 million in funding was awarded from the NIEHS for continued research over the next five years.

Collaborators and community partners

Collaborators for this project include the University of Louisville, Washington University in St. Louis, , , the United States Forest Service, and other partners.

Community organizations and resident involvement are vital to the success of this ongoing work to learn the health effects of urban greening. The team is grateful to the residents participating in the health study and those who have allowed tree planting on their property.

Watch a video about the research:

More:

View pictures from the Aug. 27 research results announcement on .

Watch the full Aug. 27 research results announcement on .

A new discovery at the University of Louisville may help scientists test the effects of exposure to green plants “greenness” on health. Pawel Lorkiewicz, associate professor of chemistry and environmental medicine at , measured a person’s exposure to trees and plants by testing their urine.

Limonene is a nontoxic substance emitted into the air by many trees and plants. Concentrated limonene has a lemony smell and is used in perfumes, foods, cleaning and other household products. When a person breathes concentrated limonene, the body breaks it down into metabolites which can be measured in their urine.

These metabolites also are in a person’s urine after they spend time in a forest, . This means the metabolites can be used to measure a person’s exposure to trees and plants. Comparing this exposure with health information may help researchers more accurately determine whether time spent in nature is beneficial for an individual’s health.

“Even though we may not smell limonene exactly, usually we can tell that we are surrounded by trees and greenness because of the smell. Parks and forests, or even meadows or larger green spaces have a characteristic smell because of plant volatile organic compounds such as limonene,” Lorkiewicz said.

For several years, Envirome Institute researchers have been studying the health effects of living near trees and greenness. As part of the , they planted trees and shrubs in specific neighborhoods, measuring changes in the residents’ health compared with those living in areas with fewer trees. But the researchers found it challenging to accurately measure an individual’s actual exposure to trees. There may be many trees where they live, but they spend much of their time working in tree-sparse locations or indoors, insulated from trees and plants.

So the team searched for a way to measure how much time an individual spent around greenness. Because many trees and plants release significant amounts of limonene into the air, they developed a method to test for limonene exposure using urine, which is easy to collect.

After testing the urine of people who smelled pure limonene from a vial, they compared those results with urine from people who spent time walking in a forest. They found the same metabolites in the urine of both groups, tagging three of them as reliable biomarkers of exposure to greenness.

“It was very exciting. That’s what constitutes a biomarker, something was released by trees in real life, metabolized by our bodies and found in our urine, not just the result of smelling a pure chemical in a lab,” Lorkiewicz said. “So, we objectively can tell when someone is not working in an office at a desk, but they actually go outside and are surrounded by greenness.”

Keith said the biomarker for greenness exposure is a breakthrough for the Green Heart Project, the Trager MicroForest project in downtown Louisville and other studies.

“We have been working with the overall premise that trees affect health in a positive way. Our mission in the Green Heart Project is to understand in some ways how they do that,” she said. “We have turned to the idea of using the human as the biomonitor of greenness in the human. We hope to use this to see changes in people’s overall exposure to greenness as we plant trees and determine whether those changes correlate to changes in health.”

To assess people’s exposure to trees and plants, Keith combines the limonene biomarker test with satellite images showing tree density where people live and surveys of their time spent outdoors and compares the exposure information with health data such as blood pressure and heart rate.

In additional research, Keith and her team are using the biomarker to compare the health effects of limonene and other compounds emitted by plants on the body.

Earlier in February, community partner and UofL neighbor, Dave Barker led participants through a maple tapping workshop, installing taps on several trees in the center of campus. Barker instructed participants on the best practices for tree tapping without causing harm to the trees. He explained that sap flows are much more accessible to tap after temperatures swing from below freezing at night to above freezing during the day, creating an internal pressure that carries sap through the tree. After these conditions are met, sap may be seen leaking through natural scars. The only equipment then needed to harvest sap is a drill to make a one-inch hole, a tap to hammer in, tubing and a covered bucket. Once the sap is harvested, it’s then boiled down to remove the water and create syrup.

Justin Mog, assistant to the provost for sustainability initiatives says tree tapping is a sustainable way to create local maple syrup and sugary products while cutting personal emissions. Combining maple tapping with other gardening and farming practices can make an important impact in reducing individual environmental impacts.

“I think the most important thing we get out of this project is a better understanding of sustainability, which is all about tapping into the local resources right under our noses that we tend to ignore,” Mog said. “As we learned at the start of the COVID-19 pandemic, what makes modern life so precarious and unsustainable is an over-reliance on extractive economies that pull in resources from all over the place. Rather than spending a bunch of money and burning a bunch of fossil fuels to ship in syrup from Vermont, I’d much rather see us make our own and reconnect with the abundance that nature provides for us right here in Louisville.”

To make a gallon of maple syrup about 30–60 gallons of sap is needed. Assuming weather conditions support the flow of sap, UofL Sustainability hopes to produce a few gallons of syrup from Belknap campus trees. While the workshop focused on tapping maple trees, several other tree species can be tapped for sap to make syrup. Birch, hickory, sycamore and other types of trees can be used to create unique syrups with different flavor profiles from maple syrup.

The annual maple tapping workshop is part of . The Garden Commons is a collectively managed space open to all students, staff, faculty and community members. Participants who assist in the garden’s projects are welcome to share in the harvest and take their share of organic produce.

The Garden Commons hosts several workshops and events to get volunteers engaged in recognizing the accessible abundance of gardening and foraging. Upcoming events include workshops on orchard care, berry foraging and community gardening in abandoned urban spaces. Garden Commons intern and UofL student Savannah Dowell organized this year’s maple tapping workshop.

“I want everyone who participates in the Garden Commons to walk away with the understanding and confidence that they, too, can grow, forage or learn anything their heart desires. The world of sustainable urban agriculture can be overwhelming, but don’t become debilitated by the plethora of possibilities,” Dowell said. “Whether you start with a mass-scale maple tapping project or a windowsill herb garden – just start!”

Over the coming weeks, Cardinals can help empty sap buckets into collection barrels at the North Information Center and behind Gottschalk Hall by .  

Taste UofL’s maple syrup and honey at this year’s on Friday, April 19, from noon – 1 p.m. at the Cultural & Equity Center 176 (Belknap Village North).

View pictures on .

In the first study, UofL researchers Aruni Bhatnagar, professor of medicine and director of the Envirome Institute, Ray Yeager and Daniel Riggs, with Carver Coleman and Arden Pope of Brigham Young University and others, analyzed retrospective data from more than 5.5 million cancer patients and survivors from 2003 to 2016. They found that cancer patients in greener counties lived longer than those in counties with less greenness. The protective effects of greenness against cancer mortality were seen with both males and females and individuals of all ages, but were more pronounced at urban locations. Individuals with cancers that were highly survivable – such as breast, prostate and skin cancer – benefitted more from greenness than those with cancers with low survival rates such as brain, esophageal, liver, lung and pancreatic cancer.   

The study, “,” published in the journal Environment International, evaluated associations between greenness and fine particulate matter in the air on causes of death in a large group of cancer patients and survivors in the United States.

“This is the largest study of the relationship between greenness and cancer mortality, and it provides clear evidence that living in green areas is beneficial to cancer patients,” said Bhatnagar “However, we do not yet understand why greenness protects against cancer mortality. Much more remains to be done to see whether greenness benefits cancer patients by lowering mental stress and anxiety or by reducing the levels of air pollutants or whether some other mechanisms are at play.”

The study also found that cardiopulmonary disease, but not cancer, was associated with higher levels of fine particulate air pollution. Mortality from cardiopulmonary causes – stroke, heart attack or COPD – was not affected by county greenness, except in rural locations.  

Greenness and depression

A second study, led by Kandi Walker and Joy Hart, professors in the UofL Department of Communication, Bhatnagar and other researchers in the Envirome Institute, found that people’s satisfaction with levels of greenness in their neighborhood was associated with lower levels of self-reported depressive symptoms.  

The researchers surveyed participants about their perceptions of greenness in their neighborhoods and their mental health symptoms and found that those who were more satisfied with the greenness surrounding their homes also reported lower depression symptoms on a health survey, the Patient Health Questionnaire-9.

The 44.5% of survey participants who reported they were satisfied with the level of greenness in their neighborhood collectively scored better on the health questionnaire for depression.

“Given the pervasiveness of depression in the U.S. population, any changes that can reduce depression are essential, particularly those that can affect a large number of people,” Hart said. “The findings of this study suggest that greening interventions that focus on greenness satisfaction may be a strategy to reduce depression in urban populations.”

The study, “,” was published in July in the journal Population Medicine.

South Louisville study participants needed

Participants in the greening satisfaction study were part of the Health, Environment and Action in Louisville (HEAL) study of the Great Heart Project, which is an ongoing assessment of the effects of neighborhood greenness on individual health. Additional participants are needed for the HEAL Study by the end of September. More information is available on the website.

“Together, these studies add weight to the importance of trees, shrubs and other vegetation in urban areas in benefitting and nurturing human health. Accumulation of such evidence strengthens the case that increasing greenness in urban locations can decrease the high rates of non-communicable diseases in cities,” Bhatnagar said. “And since greenness improves health in general, it may also be effective against infectious disease.”

The designation means UofL has met or exceeded the foundation’s standards of care for the more than 2,000 trees that cover the 309-acre Belknap Campus.

“This honor represents a decade of dedication to the idea of trees as vital urban infrastructure – for shade and energy savings, food and wildlife habitat, storm water infiltration and soil stabilization, beauty and mental health, and clean air and carbon sequestration. We look forward to an even more leafy campus in the coming decade,” said Justin Mog, assistant to the provost for sustainability initiatives.

The university was first recognized in 2010 for its comprehensive tree-care plan that guides tree selection, planting, maintenance and education. represent more than 130 species, including many native to the region. Among them are ash, dogwood, tulip poplar (the Kentucky state tree), oak, persimmon, magnolia and elm, including a rare American elm. Visitors to Belknap Campus can take that highlights some of these treasures.

The Arbor Day Foundation has helped campuses throughout the country plant thousands of trees, and Tree Campus USA colleges and universities invested more than $51 million in campus forest management last year. The nonprofit foundation is a conservation and education organization with the mission to inspire people to plant, nurture and celebrate trees.

To read UofL’s 12-page tree care plan, .

The Nature Conservancy is overseeing the planting of approximately 8,000 trees and shrubs in the designated neighborhoods. Researchers with the University of Louisville’s Christina Lee Brown Envirome Institute will compare data about the health of people living in the neighborhoods and air pollution levels collected before the plantings with data collected two years after the vegetation is planted. The results will provide insight into how the additional vegetation affects the health of people in the community.

“Our primary objective is to understand how increasing greenness affects the risk of heart disease within the community,” said Aruni Bhatnagar, PhD, director of the at the University of Louisville, who oversees the project. “Heart disease remains the leading cause of death throughout the world. We can treat heart disease with stents and statins, but we cannot prevent the disease. If we can show that putting in more trees will lower the incidence of the disease, even by a fraction of what it is, that would be a major achievement.”

The Nature Conservancy is planting the trees and shrubs in designated residents’ yards at no charge to property owners.

“We’re excited to enter the next phase of the Green Heart project,” said David Phemister, state director for in Kentucky. “Tree plantings at this scale require strong partnerships and deep engagement with the community. Green Heart is not just a science experiment – it is a deep investment to bring more nature to neighborhoods in the heart of Louisville.”

The residents’ health is being assessed by UofL researchers through the Health, Environment and Action in Louisville (HEAL) Study. The HEAL researchers obtained health information from approximately 700 participating residents of the Taylor-Berry, Jacobs, Hazelwood, Oakdale, Wilder Park and Beechmont neighborhoods before the planting began. They collected information such as blood pressure, cholesterol, heart health and other health indicators. The same participants will be evaluated again in two years to determine if these health indicators have changed.

“We wanted Louisville to become maybe one of the first cities in the country that makes its decisions through the lens of all forms of health,” said Christina Lee Brown, benefactor of the Envirome Institute. “That is the Envirome’s objective. This project is a magnificent example of healing our community from the roots up.”

In addition to the Envirome Institute and The Nature Conservancy, the project is funded by the National Institutes of Health and is supported by partnerships with Metro Louisville, Louisville Grows, Washington University in St. Louis, Hyphae Design Laboratory and the United States Forest Service. The total cost of the five-year project is more than $15 million.

A UofL video about this project is available on .

The trees join the more than 2,500 others on Belknap that bring shade, beauty and a sense of solitude and peace to our urban landscape. They are also a vital part of the city of Louisville’s tree canopy.

Earlier this year, the named the University of Louisville a Tree Campus USA for the eighth year. The designation means UofL successfully met five standards aimed at promoting healthy trees and student involvement.

At the Arbor Day celebration, a small group of students along with forestry experts and staff planted a native Shumard oak as well as a sawtooth oak on the lawn in front of University Pointe.

Bridget Abernathy, urban forestry coordinator with the Kentucky Division of Forestry, helped guide the planters, offering tips for digging a hole of the proper shape, height and width. She was especially adamant that the planters avoid the “mulch volcano” often seen in landscaping — mulch piled high against the tree trunk and sloping outward. The mulch should be in a very thin circle around the trunk and slope inward, allowing water to flow toward the tree, she said.

In addition, the UofL Sustainability Council launched its “Arbor Day to Earth Day” to create a fund that would allow to invest the money using socially responsible investing methods, then use the proceeds to fund philanthropic projects. The goal is to raise $5,000, an amount that would then be matched by Just Money Advisors.

Aruni Bhatnagar, PhD, director of the at UofL, will discuss the research, the , at the next Beer with a Scientist event on March 14.

“No one knows whether and to what extent trees and neighborhood greenery affect human health and why,” Bhatnagar said. “This work will tell us how to design a neighborhood that supports human health and whether an increase in the urban greenspaces and vegetation could enhance physical and mental health by decreasing the levels of ambient air pollution.”

The Green Heart Project is a collaboration of UofL, The Nature Conservancy, Hyphae Design Laboratory, the Institute for Healthy Air Water and Soil, the U.S. Forest Service and the City of Louisville. The goal of the project is to assess how residential greenness and neighborhood greenspaces affect the health of our communities by decreasing the levels of pollution and promoting physical activity and social cohesion.

The talk begins at 8 p.m. on Wednesday, March 14, at Against the Grain Brewery, 401 E. Main St. in Louisville. A 30-minute presentation will be followed by an informal Q&A session.

Admission is free. Purchase of beer, other beverages or menu items is not required but is encouraged.

At the next Beer with a Scientist, April 18, Deborah Yoder-Himes, PhD, will discuss super bacteria, antibiotic resistance and why everything is labeled “anti-bacterial.”