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.

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.”