A growing trend in medicine has doctors prescribing visits to parks for their patients. A pediatrician named Robert Zarr at Unity Health Care in Washington, D.C., has worked with the National Park Service and other institutions to create DC Park Rx, an initiative that helps health care providers prescribe activity in outdoor spaces to patients. And National Geographic recently highlighted the rise of this practice in Vermont, where doctors are now prescribing thousands of visits to state parks. In the last several years park prescription programs have spread nationwide, from Maine to California, South Dakota to New Mexico.

Proponents of these programs promote outdoor activity as a means of tackling chronic medical conditions like obesity, high blood pressure and type 2 diabetes. But park prescriptions also hold considerable promise for patients suffering from mental health issues.

A large body of evidence suggests that exposure to nature may promote mental well-being. A 2010 meta-analysis of 10 studies including over 1,200 participants found people who exercised in green environments demonstrated significant improvements in mood and self-esteem. A 2011 systematic review looked at 11 trials that compared indoor and outdoor activity, finding that exercise in natural settings was “associated with greater feelings of revitalization and positive engagement, decreases in tension, confusion, anger and depression, and increased energy.” Another recent review of studies found activity in natural environments correlated with reductions in negative emotions like sadness, anger and fatigue.

Some studies suggest interactions with parks and other natural settings may influence rates of mental illness in the population. For example, a study of over 160,000 people in South Korea found those living in areas with the lowest amount of parks and green spaces had 20 percent greater odds of depressive symptoms and 28 percent greater odds of suicide attempts compared with those living in areas with the highest amount of natural spaces, even after controlling for potential confounders like age, gender and monthly income.

Researchers are trying to better understand the neuroscience behind why exposure to parks and other natural settings might promote mental health. A study out of Stanford University, where I’m training as a resident physician in psychiatry, used brain imaging to examine participants who walked in urban or natural environments; the authors found those who went on a nature walk reported decreased rumination (that is, repetitive thoughts focused on negative aspects of the self) and had lower activity in the subgenual prefrontal cortex, a region of the brain associated with mental illness. These effects were not seen after urban walks.

Given these promising results, it’s not surprising to see burgeoning interest in park prescription programs and other mental health treatments designed around exposure to nature. Researchers are exploring whether outdoor activities like hiking or horticulture may help treat veterans suffering from post-traumatic stress disorder. Multiple studies suggest outdoor exercise may be helpful in treating depression. Nature-assisted treatments have been associated with improved outcomes for conditions like schizophrenia as well as decreased levels of health care consumption among people with mental health issues.

How park prescriptions work may vary depending on the health care provider and the patient’s location. In the model developed by DC Park Rx, clinicians can generate a prescription from a database that maps out local parks for patients with recommended activities and park ratings. A number of prescription programs rolled out by states now also waive park entrance fees for patients.

But park prescriptions aren’t panaceas just yet. Many patients, including those who are working multiple jobs, unemployed or homeless, may not be able to travel or to take the time to enjoy the benefits of activity in nearby parks. And although park prescriptions are popping up across the country, we still have little data to confirm whether these programs have a meaningful impact on patients’ health—mental health or otherwise.

Another concern raised by National Geographic’s coverage of park prescriptions is the potential misuse of these budding programs. In the article the chair of the Vermont Governor’s Council on Physical Fitness and Sports chastised some doctors for writing park prescriptions for their staffs instead of their patients. Another doctor at the University of Vermont Medical Center talked about how park prescriptions frequently end up stuffed into patients’ discharge paperwork without much discussion of what they’re for.

As a doctor specializing in mental health, I still see a lot of good that can come from these programs. Parks are all around us, and a fair amount of evidence suggests they can help patients suffering from mental health issues. And as Zarr of DC Park Rx has pointed out, visiting a park has fewer side effects and lower costs compared with some of the medications we give our patients.

During my residency training, I often see patients lying on stretchers in emergency department hallways, struggling to sleep in noisy intensive care units or sitting in windowless clinic rooms. Indeed, I wonder whether these environments say something about the kinds of treatment that we provide.

Last month, I slipped away from the hospital during a weekend to do some hiking in a local park. As I walked along a ridge, the sounds of my pager faded from my consciousness. I listened to the trees as the sun warmed my face. A sense of calm came over me.

That felt like good medicine.