ed for a single plant to clean a large space out, may actually be doing the heavy lift- CHALLENGES TO THE RESEARCH such as a home or office.” ing—for more on this see box, page 21). It But when it comes to our homes and office clearing the air about To expand upon these initial experi- wasn’t long before those now-ubiquitous spaces, do these lab and field studies tell us ments, NASA built a “closed ecological lists of best plants for improving indoor air anything definite? John Girman, former life support system” called the BioHome quality started popping up. director of the Indoor Environments Cen- at the Stennis Space Center. At 45 feet ter for Analysis and Studies at the Environ- long by 16 feet wide, it looked a lot like FIELD TESTING mental Protection Agency (EPA), says no. a space-age doublewide trailer. Inside, a While laboratory tests were an informative In 2009, while working with the EPA’s Indoor Plants kitchen, sleeping area, and bathroom were first step, they were never meant to model Indoor Environments Division, Girman flanked by a large plant room to test the the complexity of real homes and offic- coauthored the first critical review of the ability of various species to clean recycled es. “In science there is always a need for indoor air phytoremediation research. air and raw sewage in a closed loop. complementary studies in the real world Published in the Proceedings of Healthy HANCES ARE, you have at least plants and their associated microorgan- he attempts to isolate himself in tightly The BioHome allowed Wolverton and and in laboratory chambers,” says Mar- Buildings, the report was coauthored with a few houseplants, and you’ve isms,” Wolverton wrote in the final 1989 sealed buildings, away from this ecological his colleagues to conduct “real-world tests, garet Burchett, an adjunct professor at Tom Phillips, an air pollution specialist C probably heard that they can report, “it should be obvious that when system, problems will arise.” as opposed to a single plant in a small test the University of Technology in Sydney, in the Program of the clean your air. If not, a quick And indeed, problems did chamber.” They found that human occu- Australia, and a coauthor of one of the California Air Resources Board, and Hal online search will yield doz- arise: many common building pants of the BioHome, who initially report- few experimental field studies testing the Levin, a research architect and head of the ens of articles on the subject, materials, such as plastics and and many lists comparing particle board, were known specific plants on their abil- to release pollutants into the ity to filter airborne toxins. air. Some of these chemicals Before you breathe easy had been linked to health though, notice that these problems including chronic lists don’t always agree on headaches, asthma, and skin which species to grow and irritation. NASA scientists are vague at best about the began studying various plants science behind their claims. to see if they could reduce or What do we really know eliminate these toxins in con- about the air-cleaning abilities ditions simulating those in a Built in the late 1980s at NASA’s Stennis of indoor plants, and how can space station. Space Center in southwest Mississippi, the this inform your selections? BioHome, shown in the photo above and as As it turns out, the situation FAVORABLE RESULTS a schematic on the left, housed some of the is much more complex than In the earliest studies, a variety earliest experiments testing whether indoor growing a few houseplants of indoor plant species were plants could remove pollutants from the air and expecting them to make sealed, one at a time, in Plexi- and from sewage. a big difference. glas® chambers measuring be- tween 15 and 32 cubic feet. For Building Research Group, a Cali- IN THE BEGINNING a sense of scale, a 2013 Toy- fornia consulting firm based in Santa Cruz. One of the first investigations ota Prius has about 22 cubic Overall, the review took issue with the into the notion that indoor feet of trunk space. So, these small size of experimental test chambers, plants can clean the air was chambers were fairly com- the scarcity of field studies, the unreal- conducted by the National pact. After NASA scientists ed symptoms of exposure to air pollution, effect of indoor plants on air quality in istic treatment of pollutants, and other Aeronautics and Space Ad- injected high concentrations could comfortably live in the unit once the office buildings, published in 2007. “Field methodological issues. On the results of ministration (NASA) in the of , trichloroethylene, plant filtration system was in place. observations and sampling give us infor- Burchett’s 2007 field study, for instance, 1980s. At the time, NASA and —com- After NASA reported its intriguing mation on correlations between air/soil/ the critique pointed out that “variations was tasked with building an mon indoor air pollutants— findings, other labs began their own exper- water factors and plant behavior.” in ventilation may have been responsible international space station into the chambers, they found iments. Most of these studies were similar To test plants in the “real world,” the for any apparent pollutant reductions,” and long-term air quality inside the com- A number of common that if a potted plant was present, the air to Wolverton’s initial Plexiglas chamber Australian researchers conducted a series of and that individual pollutant “concen- pletely sealed environment was a concern. was significantly cleaner after 24 hours. experiments—a rotating roster of house- trials in three separate office buildings over trations did not appear to be reduced” in Bill Wolverton, who was a research houseplants are widely touted That sounds very promising, but as plant species placed in small test chambers a nine-week period. Similar to the lab-based the study. Furthermore, it noted that the scientist in the Science and Technology Wolverton, who is now an environmen- and exposed to one or two common indoor chamber studies, the researchers found that five-minute duration of the weekly air Laboratory at Stennis Space Center, Mis- as natural air purifiers, but tal consultant, points out, these results air pollutants. A glut of research published spaces with plants had significantly lower sampling was “insufficient to characterize sissippi, proposed using plants as a natural should not be taken out of context. “The between the late 1980s and early 2000s con- concentrations of air pollutants. indoor concentrations.” air filtration system in imitation of their does research bear this out? small Plexiglas chamber studies gave us firmed NASA’s findings: the concentration “We found that three plants per office One of the most glaring problems the role on Earth. “Since man’s existence on the ability to control all test parameters of pollutants significantly decreased over were enough always to reduce total [air review raised was the use of small, sealed Earth depends upon a life support system and to introduce a single chemical at a time in the presence of plants (and their pollution] below health risk levels,” says test chambers in laboratory studies. In

involving an intricate relationship with BY AMY GEORGIANNA MCDERMOTT time,” he explains. NASA “never intend- MARY YEE COURTESY OF NASA (2) associated soil microbes, which, as it turns Burchett. them, one plant takes up a larger relative

18 the American Gardener September / October 2015 19 volume than it would in a typical home or expose yourself to”—we have little idea how office. You would need a small forest of PLANTS VERSUS MICROBES a lot of these chemicals could influence hu- indoor plants—Girman estimates 680— Even from the earliest experiments, researchers knew plants weren’t alone in man health, let alone plant metabolism, or to replicate the results of the chamber those chamber studies; they were rooted in potting soil that was rich in , at what concentration they might become studies in a 1,500-square-foot home. fungi, and other microscopic organisms. Scientists saw air pollutant levels drop- dangerous. And second, chemicals can in- “There are many variables as to how ma- ping, but they didn’t know for sure how much could be attributed to the plants teract to create synergistic effects. ny plants would be needed to improve in- and how much to the . Considering that billions of possible door air quality,” Wolverton agrees. That’s So research groups began probing the question directly. In one 2006 study, chemical combinations can occur, it’s clear because people don’t live in sealed Plexiglas for instance, scientists found that soil could keep cleaning the air in a Plexiglas that researchers have barely scratched the houses or NASA-style BioHomes. For ex- chamber even after a plant had been uprooted and removed. Curiously, virgin surface so far. Based on the limited existing ample, the ventilation rate of your space potting mix didn’t have the same effect. studies, Girman’s review ultimately con- is just one variable that’s much harder to Because of these results and those from similar studies, Margaret Burchett, an cluded that “indoor plants have little, if any, control in the real world than in a lab set- adjunct professor at the University of Technology in Sydney, Australia, believes benefit for removing indoor air [pollution] ting. And in the very few field studies that that “the microorganisms of the potting mix are the primary agents of [pollutant] in residential and commercial buildings.” have tested the effects of indoor plants on air removal.” The plants “exude goodies to attract and multiply the bacteria,” she The official stance of the EPA is similarly quality in office buildings, ventilation rate explains. “The microbes are capable of utilizing [pollutants] as food, even down bleak: “[T]here is currently no evidence was considered, but never quantifiably mea- to amazingly low concentrations.” that a reasonable number of houseplants However, there’s still no consensus among researchers on whether plants are will remove significant quantities of pollut- primarily responsible for removing pollutants, or whether bacteria, fungi, and ants in homes and offices and consistently Resources other decomposers are doing most of the work. —A.G.M. improve indoor air quality.” Critical Review: How Well Do House For Kays, the jury is still out. “In theory it Plants Perform as Indoor Air Cleaners? works, but the bottom line is there just isn’t by J. Girman, T. Phillips, and H. enough research that’s been done,” he says. Levin. Proceedings of Healthy Build- ings (Vol. 23, pp. 667–672, 2009). THE BOTTOM LINE http://nepis.epa.gov (publication So now what? Should you your number: 402-F-92-003). good-for-nothing spider plant? Are your How to Grow Fresh Air by B.C. Wolver- palms and philodendrons a waste of space? ton. Penguin Books, New York, NY, Definitely not! It’s likely that indoor 1997. plants (in sufficient numbers) do have a Plants: Why You Can’t Live Without positive effect on the air you breathe. We Them by B.C. Wolverton and Koz- just don’t yet know to what extent. aburo Takenaka. Roli Books, New Regardless, you can be sure that indoor Delhi, India, 2010. plants offer other positive benefits. More and more research indicates that indoor greenery sured. “If [polluted] outside air is constantly can improve mood and boost performance introduced, conditioned and distributed, it of creative tasks, and even accelerate the would overwhelm any benefit from plants or recovery of hospital patients. While these any other type of air filter,” says Wolverton. studies are ultimately just as limited as the Burchett agrees that ventilation can and does air quality research, they do dovetail with a overwhelm the benefits of indoor plants, larger sensibility that anyone who gardens particularly in “newer buildings with stron- will agree on: plants just make us happy. ger air conditioners,” she says. Red-margined dragon tree (Dracaena marginata) is often listed as an indoor plant that can So enjoy your indoor plants, and may- Then there’s the issue of the pollutants help remove airborne pollutants, based on results of studies NASA conducted in the 1980s. be even find room to grow a few more. As themselves. The lab studies introduced very Wolverton advises, “I always recommend high concentrations of airborne chemicals. en off by everything from carpeting and only test one or two common pollutants a variety of plants and as many as one can The change in concentration was then mea- furniture to paint and cleaning solvents. as “models” of reality. Approximating the reasonably maintain.” Appreciate them sured over time. But in reality, some pollut- Stanley Kays, professor emeritus in the De- real world through models is good standard for the beauty and sense of well-being they ants are emitted continuously in relatively partment of Horticulture at the University practice in science. Model organisms like bring to your space. And if you want to small quantities, not in high concentrations of Georgia, measured upwards of 180 dif- Arabidopsis plants and fruit flies have taught capitalize on their tentative potential to all at once. The review notes that the slow, ferent airborne compounds in a survey of us volumes about our own human genetics filter your air, don’t limit yourself to the continuous release of toxins into the air several houses in Athens, Georgia (for a list for example. But in the case of these air qual- top species on those popular lists because could eventually overwhelm a plant’s ability of common indoor air pollutants, click on ity experiments, a few proxy chemicals do it’s simply too soon to judge. to remove them in a way that sudden high the web special linked to this article on the not necessarily reflect reality. concentrations do not. AHS website at www.ahs.org). That’s a problem for two big reasons, Amy G. McDermott is an editorial intern Our dwellings also tend to be contam- Rather than replicating a mixture of all says Kays. First, “we don’t have very good More research is needed to determine how much of a difference indoor plants make to air for The American Gardener and founder

inated with a cocktail of chemicals, giv- those chemicals, most existing experiments information on the maladies that you can COURTESY OF FORREST AND KIM STARR ALEKSANDRA SZYWALA quality, but growing a diverse array like in this living wall most likely has some positive effects. of Hawkmoth magazine.

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