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).

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