By Colin Murray
Between the seemingly random start and stops of our Uber driver, I tried—with difficulty—to spot the top of the buildings encircling the vehicle. Toronto, with a population of nearly 3 million people, is Canada’s largest city. One could walk for miles amongst the concrete and blinding windows and not stumble upon a single plant, unless you find yourself in one of the many waiting rooms at the base of every tower. It’s no accident that waiting rooms, offices and apartments have plants. Subjectively, they can make a space feel welcoming, visually appealing, lively and perhaps fragrant. But what scientific evidence has tied indoor plants to mental health? What about the recent global initiative to retrofit our concrete jungles into… well, concrete jungles?
Scientists estimate that people spend up to 90% of their time indoors. Knowing this, it’s not a surprise that many of us go to great lengths to make our indoor environment as comfortable as possible. This has never been more true than in the context of the COVID-19 pandemic. Lockdowns and quarantines increased time spent indoors even further, creating a level of intimacy with our indoor environment not yet experienced by most. This drew not only ergonomic and aesthetic attention, but also scientific attention to our indoor environment, prompting studies about how indoor plants may affect mental health.
Although evidence is still scarce, almost all studies suggest that indoor greenery likely has a positive impact on mental health. In fact, even just viewing images of natural scenery can result in a more relaxing bodily state compared to viewing images of human-made landscapes, a reason why most software companies queue up a beautiful natural landscape image on your desktop. But what functions are performed by living plants?
Plants provide many different services that can reduce stress and promote mental health. They release oxygen, control temperature and humidity, increase the microbial diversity of our homes and perhaps most importantly, have the ability to purify our air.
For urban dwellers, air pollution is a regular part of day-to-day life. Many might see our homes as safe-havens, but indoor environments also have air quality issues that can directly influence our health from allergies to cancers. Indoor air pollutants include particulate matter (e.g. dust) and gasses like nitrogen dioxide or formaldehyde, which can be formed/released by building materials, furniture, cooking with gas stoves or even burning incense. These compounds have been found to cross from the blood into the brain, where they cause inflammation and toxic stress to the cells necessary for proper brain function. Partly due to this, long-term exposure to air pollutants—even in small concentrations—can increase the risk for depression, anxiety and psychological distress.
House plants can offer a decorative and cheap solution to help combat poor air quality in our homes. If you think back to your high school biology classes, you may remember that plants ‘breathe’ in carbon dioxide and ‘breathe’ out oxygen during the process of photosynthesis. During this ‘inhale’, carbon dioxide is accompanied by other gasses and particles in the air, and these often remain inside the leaf where it is converted into non-toxic compounds. The breakdown of pollutants by microorganisms in the soil or the ability for plant stems to hold on to these compounds are also ways house plants can remove damaging compounds from the air. For example, the golden pathos (Epipremnum aureum), a very common indoor plant, can remove all detectable levels of formaldehyde from the air in a space slightly larger than a double oven within 5 hours.
In this way, plants can effectively purify the air within our homes, preventing the inflammation and cellular stress that is tied to mental health issues. Additionally, cleaner air would also mitigate other health complications like respiratory illnesses that also contribute to mental health, since physical ailments are tightly linked to mental well-being. In line with this, improved mood and reduced psychological stress can help our body physically heal, another benefit offered by indoor plants.
Plants have the ability to improve mental well-being through various different mechanisms.
The best example showcasing this effect was a clinical trial that looked to see if indoor plants had an impact on the recovery of patients after a surgical procedure. Patients that received the same operation were either placed in a hospital room with or without indoor plants. Remarkably, the presence of plants had a positive effect on patient recovery, evidenced by lower systolic blood pressure, reduced pain and anxiety, and higher satisfaction with their hospital experience. Perhaps this comes from a feeling of connectedness to the natural world. A sort of living relationship that we aren’t really conscious of, ultimately improving our mood and health (a hypothesis currently more philosophical than scientific).
It is important to note though, that these studies are extremely difficult to control, meaning that there are multiple factors that can influence the outcome of the study that the researchers can’t always account for, making any conclusion difficult to tie directly to house plants. Because of this, scientists cannot definitively say how or why indoor plants may cause these beneficial effects. It is also unclear how many indoor plants are needed for us to actually reap the benefits of these leafy functions. Added on top of this, the stress of trying to keep a plethora of house plants alive may counteract these beneficial effects!
Although it is still uncertain how or why plants improve our physical and mental health, increasing evidence suggests that they have a real, tangible positive impact. So even though taking care of house plants can be frustrating, remember that the relationship is reciprocal and that the effort is rewarded. It is clear that indoor plants can have profound effects on our health, but what about our outdoor spaces? Can these benefits be harnessed on a larger scale?
Green cities are more or less exactly what they sound like—a push to retrofit our largest cities to incorporate green spaces like parks, street trees, community gardens and rooftop gardens. Singapore is a great example of how modern architecture can be combined with plant life to form a thriving green landscape. Besides the proposed benefits to wildlife, biodiversity, temperature moderation of buildings and air quality, green cities also have an impact on our mental health.
Singapore has made impressive strides in including plant life within the city. Credit.
Several studies have shown the positive association between time spent in green spaces and mental health outcomes. A recent 2021 study with data from 18 different countries concluded that frequent visits to green spaces was linked to positive well-being, reduced rates of mental distress and reduced the use of doctor-prescribed medication for depression.
These benefits may be directly related to interactions with green spaces for many of the same reasons as for indoor plants, or because of social interactions and/or exercise that often accompany these visits. Distinguishing between factors like these is difficult and no clear answer has yet been agreed upon for how green spaces positively influence our mental health. But this distinction, although important, is not absolutely necessary for policy makers, city planners and/or individuals on a day-to-day basis. Whether green spaces directly influence mental health in a positive way or whether they provide a means for increased exercise and social interactions—which also produces these beneficial outcomes—or whether it is a combination of the two is beside the point. What matters is that there is a clear link between access to these areas and improvements in mental health, which is reason enough for their incorporation into our cities.
As our Uber driver abruptly pulls over and stops, a cool breeze meets us as we clamber out of the vehicle. We have arrived at Allan Gardens. Toronto, as menacing and busy as it may be, has also made impressive strides in including some green amongst the grey. The breath-taking green houses at the centre of this park, free to the public, showcase flowering tropical plants, which are stoutly guarded by a number of pampered turtles. This park is evidence enough that green homes and cities have the ability to promote mental well-being. Whether it is something as simple as the addition of a house plant or as complex as retrofitting a bustling city centre, everyone should have easy access to green spaces to collectively improve our mental health.
If you’re interested, check out some of the sources I used in researching this answer! All sources used for this answer are strictly evidence-based.