Everyone says trees are good for us. This scientist wants to prove it.

In 2018, Bhatnagar, a University of Louisville medical school professor, decided that he wanted to “do something” about air pollution in Louisville, which has repeatedly earned failing grades for air quality from the American Lung Association. His contribution, he decided, would be to find the connection between trees and better heart health using the gold standard for evidence: clinical trials.

“The idea is to learn to examine everything, no matter how obvious they may seem,” he says.

Bhatnagar is well aware of the massive forest of urban tree research available, but much of it involves observational health studies, in which scientists measure potential correlations between urban trees and residents’ health.

“What I thought was we really don’t know if trees are beneficial for health,” Bhatnagar said.

To get beyond that, he proposed the Green Heart Louisville initiative, which launched in 2018. Over time, contractors and volunteers have planted nearly 8,000 trees and shrubs in a cluster of lower-to-middle-income neighborhoods in southern Louisville and measured health data from nearly 500 residents.