In his 2005 book, “Last Child in the Woods,” Richard Louv wrote about what he called nature-deficit disorder, and cited about 60 scientific studies looking at the benefits of nature and the problems that can come from being too isolated from the natural world. Today, Mr. Louv said, there are more than 700 studies (abstracts can be found on the website of the Children and Nature Network, of which Mr. Louv is the co-founder).

Dr. Robert Zarr, a pediatrician at Unity Health Care in Washington, D.C., was inspired by hearing Mr. Louv speak at an American Academy of Pediatrics meeting and then by reading his book. Dr. Zarr is the founder and medical director of Park Rx America, a nonprofit that encourages doctors to prescribe parks; for some states, the website allows a doctor to search for parks near a family’s home address, or with particular available amenities, and write a specific prescription, with the name of the park, the activity, the duration, the frequency.

“I do at least one a day or more, I really let the patient write it,” Dr. Zarr said. The two most common scenarios, he said, are overweight children and stressed-out, anxious and depressed teenagers. “Is there anything they want to do that’s enjoyable outside, that they’re willing to commit to?” For example, “you just told me you play soccer. How often do you go? Would you be willing to commit to going once a week or twice a week?”

The idea of prescribing parks is “probably more common sense than rocket science,” Dr. Zarr said, but it is also based in the expanding scientific literature that shows that spending time outdoors is good for physical and mental health.