By Erin Marissa Russell
As someone who loves plants and is enthusiastic about gardening, you’ve probably heard people say that houseplants promote a healthy environment by cleaning the air, reducing toxins and pollutants. In fact, if you’ve heard the claim once, you’ve probably heard it a thousand times. We’ve even covered it in the past! Despite how frequently people proclaim that plants clean the air, the reality is this statement just isn’t quite true—at least, not to the extent that we previously thought. (To be honest, you’d be better off simply opening a window to purify the air in your home.)
The Myth: NASA Clean Air Study Reported Certain Plants Can Purify the Air
The idea that houseplants are capable of cleaning the air got started with a 1989 study called Interior Landscape Plants for Air Pollution Abatement, which is usually referred to as the NASA Clean Air Study for short. NASA researchers (co-funded by the trade group Associated Landscape Contractors of America) measured the ability of certain plants to purify the air, reducing volatile organic compounds (VOCs) such as benzene, formaldehyde, and trichloroethylene. These compounds show up in fumes from automobile exhaust, but everyday objects like the drywall and paint we use in our homes emit VOCs as well, as do cosmetics such as nail polish or shampoo, fumes created by cooking, air fresheners, and a variety of other scented items.
VOCs may have some connection to sick building syndrome and other illnesses exacerbated by the “canned air” inside of buildings. People exposed to these compounds can face problems that range from a tickle in the throat to cancer of the esophagus, so cutting back on VOCs in the air is a major health concern. Unlike many pollutants, VOCs can’t be simply removed from the air or lessened using a finely graded filter. Their presence is especially problematic in sealed areas that recycle a limited amount of air, such as a space station—so you can see why NASA was working to find a solution.
The plants NASA tested include aloe vera (Aloe barbadensis miller), bamboo palm (Chamaedorea seifrizii), banana (Musa oriana), Chinese evergreen (Aglaonema “Silver Queen”), dragon plant (Dracaena deremensis “Warneckii” ), dragon tree (Dracaena marginata), English ivy (Hedera helix), Gerbera daisy (Gerbera jamesonii), golden pothos (Scindapsus aureus), green spider plant (Chlorophytum elatum), heart leaf philodendron (Philodendron oxycardium), Janet Craig (Dracaena deremensis “Janet Craig”), lacy tree philodendron (Philodendron selloum), mass cane (Dracaena massangeana), mother-in-law’s tongue (Sansevieria trifasciata “Laurentii”) peace lily (Spathiphyllum “Mauna Loa”), and pot chrysanthemum (Chrysanthemum morifolium).
Once the study was complete, NASA trumpeted its victorious findings to the media and the public. According to the results, the study report said, “If man is to move into closed environments, on Earth or in space, he must take along nature’s life support system” in the form of plants. The NASA researchers added that to effectively purify the air, a person would need at least one houseplant per 100 square feet.
Since that fateful press release in 1989, news agencies and bloggers have taken the idea that plants scrub the air of harmful VOCs and run with it. Countless lists of plant varieties recommended as the “best plants to purify the air in your home” cite NASA researchers as their source. So why, you’re probably wondering, is the claim that houseplants will make the air in your home cleaner untrue?
The Confusion: Why You Can’t Count on Houseplants to Substantially Purify the Air
NASA knows their stuff, as an organization that uses fire and metal to blast human voyagers past the sound barrier and into an alien realm should. An institution that sends adventurers into an expanse beyond the boundaries of our maps—and void of the oxygen and gravity we’re accustomed to—should be one you can trust to have their facts straight. So as you may have already guessed, the misconceptions surrounding whether plants can clean the air isn’t a result of any inaccuracy or error in NASA’s original experiment.
The NASA study never intended to evaluate how well plants would decrease VOCs in the air under domestic conditions like those ordinary houseplants would encounter. Instead, NASA’s tests investigated the influence of plants on air in small, sealed chambers to approximate the environment of a spacecraft. Observing how plants affect air quality under these conditions allowed NASA to analyze whether plants would help clean a spacecraft’s oxygen supply on a spacecraft if they traveled to space with the astronauts. Although NASA’s research was never meant to examine how plants affect the air in a normal home, many writers and gardeners have applied the study’s findings to their home gardens.
The misinterpretation is understandable, especially considering the phrasing used in certain sections of the NASA report. The excerpt we quoted earlier merely makes the point that plants improve air quality by reducing VOCs in “closed environments, on Earth or in space.” Without clarification of the exact conditions NASA used to test plants, a reader can easily interpret “closed environments, on Earth or in space” as including homes and apartments. In fact, the phrasing is vague enough to imply just about any environment we would describe as “indoors”.
One study after another has replicated NASA’s results, confirming that plants can lower VOC levels to improve the quality of the air in small, sealed chambers. In the reports describing these experiments, the original authors sometimes propose that because plants have been observed purifying the air in small, sealed chambers, perhaps they would also be able to clean and improve the quality of air in houses, apartments, workshops, offices, classrooms, or other large areas that are not hermetically sealed.
The mix-up came at the next step in this game of scientific telephone, when writers who covered the original reports from science journals for a wider audience began to compose their own articles. Many writers have oversimplified what these studies actually revealed, jumping to conclusions that the original research does not support. Whereas the original authors speculated that indoor air purification may be a possibility, an idea suggested by their findings that provides a topic future researchers have the opportunity to study, mainstream articles tend to present this speculation as gospel truth.
It’s unclear whether the writers who covered these studies misunderstood the facts themselves or simply would benefit from working with an editor to clarify how they present their points. Either way, many writers tended to cut the nuanced language and careful attention to detail of the original reports, language the researchers meticulously crafted to accurately reflect the conditions and results of their study, in favor of a style that was more conversational and engaging. The problem is that in doing so, writers played fast and loose with the facts, perhaps fueled by compensation systems that base pay on how many views an article gets or how often readers share it on social media.
Air purification studies on plants in small chambers also tend to introduce one VOC into the chamber, which is a controlled environment. In our homes and other indoor spaces, the many household items that emit VOCs would be releasing a continuous supply of various different compounds into the air. In other words, our research tends to measure a plant’s ability to purify air that contains a single VOC, while plants in real life encounter a whole slew of different VOCs simultaneously.
Also, researchers normally added a VOC to the air in a test environment via a single dose, a one-time shot. In the real world, VOCs are continually released into the air, not just once but over and over again. Our homes and offices also tend to have much more square footage than the chambers from these studies, and using a smaller space gives the plant being tested an edge from the start. The smaller volume of air being purified seems to boost the plant’s purification abilities because there is simply less air (and therefore a smaller amount of VOCs) for them to purify.
Some studies also failed to account for sorption, which is VOC attachment to the surface of objects in the chamber, so in those cases a portion of the VOC reduction researchers attributed to plants was actually thanks to sorption. There have been some studies to examine the ability of plants to cleanse air of VOCs in larger, unsealed indoor spaces like homes. Unfortunately, data collected in these studies was derived using inaccurate measuring equipment, and none of these studies accounted for VOCs that decreased due to air exchange.
As it turns out, the most efficient way to substantially reduce the amount of VOCs in indoor spaces like homes and offices is to allow the building ventilation to move air outdoors in a process called outdoor-to-indoor exchange. Air flows from indoor to outdoor zones as a result of cracks or other openings that let outside air into the building, natural ventilation when windows or doors are open, and mechanical ventilation, which relies on a system like climate control that is designed to allow the temperature and airflow in the building to be controlled.
The Truth: You’d Need a Nursery Full of Plants to Cleanse the Air in a Small Home
By this point, we should be clear on the fact that the NASA study was not exactly inaccurate—it’s better described as “commonly misinterpreted.” Because NASA’s test conditions do not accurately represent the typical home or apartment, we can’t apply their findings to our indoor gardens. So what is the truth about plants and air purification in spaces like homes or offices?
Bryan Cumming and Michael Waring crunched the numbers to find out, if a person wanted to use houseplants to improve air quality in their home, how many plants they would need for effective air purification. In Cumming and Waring’s report, titled “Potted Plants Do Not Improve Indoor Air Quality: A Review and Analysis of Reported VOC Removal Efficiencies” in the Journal of Exposure Science and Environmental Epidemiology, the authors provide a clear-cut answer. A home with 1500 square feet of space would require 680 plants to be growing inside in order for the plants to effectively purify the air.
Even the most eager gardening enthusiast won’t have a collection of 680 houseplants. (And if they did, they’d have long ago run out of places indoors where they could put those plants.) That means that although a more normally sized collection of indoor plants has some effect on the air in a home, the effect is so minimal that it might as well not exist.
All Is Not Lost: There Are Real Benefits of Indoor Plants
Hopefully, you’re not left kicking yourself for collecting those houseplants after finding out that they won’t really improve your home’s air quality. There are plenty of other reasons to enjoy an indoor garden, even if only to beautify your home and experience the joy of caring for your plants. To console indoor gardeners who’ve just learned the truth about just how many houseplants they would need to purify the air in their homes, here’s a list of bona fide benefits of keeping plants indoors.
Spending time with houseplants soothes both physical and mental stress as well as anxiety,
A study from the Journal of Physiological Anthropology reported that when young adults used their senses to interact with houseplants in ways like touching their foliage or smelling a blossom, they noticed a reduction in both mental stress and physical signs of bodily stress. Microbes in garden soil can also boost your mood due to the cytokines they release, although taking a nature walk outdoors will provide the most exposure to these natural antidepressants. People who kept flowers also said they generally felt happier than before they added plants to their homes.
Plants in your environment come along with all kinds of benefits to offer your physical health.
It’s been proven time and time again that keeping plants around can make you physically healthier in all sorts of ways. Experts saw benefits in the brain’s electrical activity, lowered muscle tension, and healthier heart activity. Patients who were recovering from surgery healed better and were happier with their treatment when an ornamental plant was placed in their hospital rooms than patients in rooms that didn’t have plants. The patients also said they felt less stressed and formed more positive impressions of their caregivers. Healing time is also reduced when patients have flowering plants in their rooms.
Adding a plant to your bedroom may result in improved quality of sleep and the ability to sleep for longer periods.
Scholars from Beihang University in Beijing, which is sometimes also called Beijing University of Aeronautics and Astronautics, published a study that found volunteers in isolated conditions had better sleep when strawberry and cilantro (or coriander) plants in their environments. Please note that the subjects of this study were under conditions intended to simulate missions to deep space or the deep sea, so the effects they experienced may apply to people’s everyday lives, but that was not proven by this study.
Bring a plant to work and experience so many improvements you may just get a raise.
Researchers from Sweden determined that simply being able to see plants from an office window felt fewer effects of stress than workers whose view was limited to a concrete wall. A company called IEQ Indoor Plants completed a study with help from a Norwegian oil company called Statoil, a horticultural company called Greenteam, and medical professionals that showed workers who had access to a window box full of flowers experiencing improved health in 12 different areas as compared to their colleagues who lacked a botanical view. The symptoms that workers said had improved most when plants were added to the environment are hoarseness and dryness in the throat along with coughing. A report by the Texas A&M Agricultural Extension also pointed out that working in a room with ornamental plants would boost memory and ability to concentrate, resulting in more accurate work and projects of higher quality.
Even though plants don’t clean VOCs from the air, they will do some cleanup of the mold, bacteria, mildew, or dust in their surroundings.
In rooms that held plants, researchers found 50 to 60 percent fewer bacteria and spores of mold than in rooms that lacked plant life. Their findings suggest that plants with large leaves are the most effective, as they have more surface area to contribute to the process of transpiration that is responsible for these benefits. The cleansing was especially notable when the plant’s large leaves were also pointy and sharp, as with an aloe vera plant. The bacteria and mold are reduced because of a phytochemical substance plants secrete into the air. And according to a study the American College of Allergy, Asthma & Immunology had presented to them, English ivy is particularly good at purifying mold in the environment.
Plants even make you a more compassionate person with more fulfilling social relationships.
This one is a bit surprising because the connection between plants and the benefits they provide isn’t as intuitive as some of the others we’ve discussed. Regardless, people who are often around ornamental plants have been proven to be more helpful to others, feel increased concern and empathy for the plight of their friends and colleagues, and generally report more advanced social relationships. The researchers surmise that spending time caring for plants makes a person more likely to also care about the people in their lives.
If you were relying on indoor plants to purify the air in your home, it’s time to consider other options. As we discussed above in the section on building ventilation, simply opening a door or window will let the airflow in your home replenish stale air with a fresh breeze. For those with air cleansing needs that building ventilation won’t address, perhaps it’s time to let your houseplants off the hook and consider buying an air purification system instead.