With flowers in the spring, lush green leaves in the summer and changing colors in the fall, trees have long been appreciated for their natural beauty. A new report from the Nature Conservancy shows that they can also be appreciated for their ability to save lives.

Released at APHA’s Annual Meeting and Expo Tuesday, the “Planting Healthy Air” report finds that investing $4 per resident to plant trees in some of the world’s largest cities could lead to better health for humans. Trees reduce air pollution and cool urban streets, thereby providing health benefits to their human neighbors, said the report, which was created in partnership with the C40 Cities Climate Leadership Group.

Spending $100 million each year to plant trees around the world could lead to cooler cities for 77 million people, the report predicts. It would also reduce particulate matter pollution for 68 million people.

“In terms of dollars per life, it’s a fairly cost-effective strategy,” said Rob McDonald, lead scientist for global cities at the Nature Conservancy, at the meeting session.

By 2050, as much as 80 percent of the world’s population is expected to live in cities, McDonald told attendees. But many of those cities have unhealthy air. According to the World Health Organization, in cities that monitor air pollution, many people are regularly exposed to air quality levels that exceed agency limits. Poor air quality is linked to risks such as asthma, stroke, heart disease and lung cancer, WHO says.

Planting trees in urban areas can help reduce those risks, said the report, which identifies places where investing in tree planting can have a positive impact on lives.

One community that is planning to use trees to improve health is

Louisville, Kentucky. The Nature Conservancy, University of Louisville’s Department of Environmental Cardiology and other partners are planning a study in the city that will measure exactly how much tree planting can improve the health of residents.

Over a five-year period, the Green Heart Study will closely examine the health of 500 residents in two neighborhoods: one in which new trees are planted and one where they are not. Researchers are hoping to determine exactly how much green space can diminish cardiovascular risks.

When planting trees in communities, it’s important to determine which neighborhoods can most benefit from their health benefits, McDonald said. He recommended that urban forestry and public health workers collaborate on strategies for tree planting that will benefit community health.

“You have to have the trees where the people are,” he said.