In January, a 13-year-old patient named Kelssi came to Dr. Robert Zarr’s office at Unity Health Care, a community health center in Washington, D.C. Kelssi had struggled with her weight for as long as Zarr had known her and was now obese. But during this visit, she looked at Zarr and told him she was finally ready to do something about it.

Zarr told me that exhorting patients to “get more exercise” was too vague. Last year, he decided to start trying something different. He stopped asking his patients, “Do you move?” and began asking “Where do you move?” He discovered that many spent very little time outdoors, and he began prescribing time outside for conditions as wide-ranging as ADHD, high blood pressure, asthma, obesity, anxiety, diabetes, and depression.

With Kelssi, Zarr figured out that she could squeeze outdoor time into her commute. Kelssi lives in Northeast D.C. but goes to school in Northwest. Every day, she takes a bus to a train, and the commute lasts about an hour each way. Zarr and Kelssi decided she would get off the bus to school four stops earlier and take a walk through a park.

At Kelssi’s next visit, she was no longer obese; her weight was within the “overweight” range, a feat that Zarr described as no small victory. But perhaps even more strikingly, she seemed much happier and more positive. “It’s working,” she told Zarr.

Why don’t more doctors prescribe nature? They used to do so regularly. The practice gained popularity in the mid-19th century as cities rapidly expanded. In Your Brain on Nature, Eva Selhub and Alan C. Logan explain that sanitariums, or health resorts, gained popularity during the Industrial Revolution as concerns increased about overstimulation, noise, and smoke in cities. Doctors began prescribing their patients visits to more hospitable climes such as the Swiss Alps or the Adirondacks for afflictions from tuberculosis to mental health issues. A thriving industry of sanitariums sprang up with names like the Pines, Lake View, and River View.

The scientific breakthroughs of the 20th century put a premium on treatments that could be tested in a lab. “The half-page advertisements for the Glen Springs Sanitarium gave way to the full-page advertisement for the anti-anxiety drug meprobamate,” Selhub and Logan write. Today, the idea of a doctor telling you to spend time in a pleasant climate seems like something out of Masterpiece Theatre, a quaint tonic from a bygone era available only to those of the leisure class.

While it was difficult to prove the benefits of nature 100 years ago, today the intervention is supported by compelling evidence. A psychologist at Western Michigan University, Roger S. Ulrich, spearheaded some of the earliest studies. He found that students who viewed nature photos after being exposed to a stressful and demanding task reported increased feelings of affection, playfulness, friendliness, and elation. The group that viewed urban scenes reported feeling sadness.

Some of the most ambitious studies of our reactions to nature come out of Japan. The Japanese Society of Forest Medicine’s Shinrin yoku plan, an effort to promote health through “forest bathing” (short visits to forests), has been fertile ground for scientific inquiry. Yoshifumi Miyazaki of Chiba University found lower cortisol levels in those who took forest walks when compared with those who walked the same distance in a lab. Qing Li and his colleagues from Nippon Medical School found that visits to the forest (compared with urban trips) can have a long-lasting influence on immune system markers, increasing the activity of antiviral cells and intracellular anti-cancer proteins—and these changes remained significant for a full week after the visit.

Even the scent of nature may be beneficial: Chemicals secreted by trees, known as phytoncides, have been linked with improved immune defense as well as a reduction in anxiety and increase in pain threshold.

While researching a story on outdoor education, I came across studies showing that children’s ADHD symptoms decreased after they spent time outside. In 2011, research at the University of Illinois at Urbana-Champaign found that those who played regularly in outdoor settings with “lots of green” had milder ADHD symptoms than those who played indoors or in a built environment. Environmental psychologist Nancy Wells has found that views of nature boost attention.

Intuitively, it probably doesn’t seem surprising that kids who run around in the woods are less fidgety when they do have to pay attention. Or that the smell of a pine forest is so pleasant that it soothes anxiety. But even if the mechanisms aren’t entirely clear, a steadily growing stack of scientific evidence suggests that time in nature is really good for you. Why don’t we embrace this idea of healthy nature more fully? Perhaps popping a pill is less daunting than an overhaul of our daily routines.

Or maybe we don’t think that our environments matter. While writing an article about cognitive functioning in offices, I interviewed social ecologist Stephen Kellert, who told me, “We see ourselves as a-biological.” We manipulate our environment in ways to compensate for its deficiencies. We see pharmaceuticals, gyms, and standing and walking desks as a fair trade for the outdoors. And while they are certainly better than nothing, the research suggests that they may not be enough.

Zarr now leads the year-old program called DC Park Prescription, which matches people with parks. Zarr and his team constructed a database that maps 380 of D.C.’s parks and rates them in terms of safety, level of activity, cleanliness, and accessibility. The database is searchable by ZIP code and linked to patients’ electronic medical records. In a post on outdoors advocate Richard Louv’s website, Zarr explains that prescribing a park is much like prescribing an antibiotic. He diagnoses the problem and then determines which treatment has the least cost and risk.

According to Zarr, there is a lag time between research and action, and right now the effects of nature are “not enough on people’s radar.” A practitioner of what he preaches, Zarr spoke with me on the phone during his lunchtime walk around Meridian Hill Park in Northwest D.C. He talked animatedly about the progress his patients were making. Occasionally, he interrupted our conversation to pluck a mulberry from a tree and eat it. “You just notice things when you’re out walking around,” he said. He said that he has many other stories like Kelssi’s. So far, Zarr and his team have given out 600 prescriptions for nature. “It’s not a panacea,” he said, “but we’ve touched on something exciting.”