Can House Plants Solve Indoor Air Quality Problems?
[Science is not always simple. This is a classic example of the intricacies
and problems of studies and conclusions. Practical Asthma]
Hal Levin firstname.lastname@example.org
Building Ecology Research Group
The idea of common plants solving IAQ problems is attractive. Most
people like having plants in their homes and offices and in the hotels,
stores, and public buildings they visit. However, important questions
exist as to whether plants can actually affect indoor air sufficiently
to warrant their use as air cleaners.
Nearly everyone has read or heard a press story about how common house
plants can affect IAQ. Many stories say spider plants or Boston ferns
remove formaldehdye. The Associated Landscape Contractors of America
(ALCA) and their promotional organization Plants for Clean Air Council
aggressively promote the idea through press releases, media briefings,
and other efforts.
Some scientists and interiorscapers (people who design and provide
plant environments in buildings) say that National Aeronautics and Space
Administration (NASA) research demonstrates the efficacy of plants as
indoor air cleaners. Critics and skeptics include high-ranking officials
of the EPA's Indoor Air Division. They say the research, if valid, indicates
the need for huge numbers of plants to remove indoor air contaminants
as effectively as normal air exchange in an energy-efficient house or
in a typical office building. In this article we discuss the research
promoting the use of plants, the limitations of the studies, and our
own thoughts on the subject.
Scientists funded by NASA say their research shows that plants clean
indoor air. These scientists and other vigorous advocates say that plants
have been cleaning the earth's atmosphere for millions of years. They
say that eventually their critics at EPA and elsewhere will realize
that using plants is the most reasonable method for indoor air pollution
NASA research tested plants' ability to clean indoor air for possible
use in space stations. Even before awareness of indoor air pollution
increased in the early 1980s, NASA had funded research on using plants
to biologically treat waste water. Biological waste water treatment
technology proved effective and is used at small- to medium-scale municipal
sewage treatment plants and to reclaim water for irrigation.
NASA is concerned about poor indoor air depositing gaseous contaminants
on critical electronic components inside spacecraft. NASA contractors
test for excessive emissions from both building materials and items
taken aboard spacecraft. They even test astronauts' space suits for
emissions. Chemicals depositing on spacecraft electronics can cause
short-circuiting, arcing, or bridging. The sensitivity of the electronic
components and the value of the space program missions have justified
carefully cataloguing thousands of materials and products from ball-point
pens, cameras and space suits to paints and gaskets. The testing has
been so extensive that NASA's emission data may prove applicable to
evaluating mundane indoor air pollution sources.
Dr. Bill C. Wolverton, since retired from NASA's Stennis Space Center
in Mississippi, carried out much of NASA's research. He had previously
studied the use of plants for waste water treatment. He researched the
effectiveness of plants in removing the common indoor air pollutants
benzene, trichloroethylene, and formaldehyde. Since leaving NASA, Wolverton
has continued to conduct research with funding from ALCA.
While at NASA, Wolverton and his colleagues placed over a dozen popular
indoor plants in sealed plexiglass chambers of 0.44 to 0.88 m3 (18.54
- 34.08ft3). In the early work he tested all three chemicals by injecting
them into the chamber to reach concentrations from 15 to 20 ppm. After
24 hours, chemical measurements were only fractions of the chemicals
measured in the chamber air immediately after injection.
Reported removal rates were from 10 to 70% of the initial concentrations.
In control tests without plants Wolverton reported that chamber leakage
could account for from 2.8 to 10% of the reduction in chemical concentration.
Then the researchers ran tests on the removal of benzene and trichloroethylene
at 0.1 - 0.4 ppm. These lower concentrations are slightly closer to
those measured in indoor air although still 100 to 1000 times higher
than typical indoor air concentrations. The reported removal rates ranged
from 9.2 to 89.8% and averaged 45.1% for the 15 plants tested. The researchers
reported that at low concentrations (<<0.15 ppm), pots containing
potting soil alone without a plant present removed 20.1% and 9.2% of
the measured initial benzene and TCE concentrations respectively. Removal
by leakage was reported at 5.3 and <1.0% for benzene and TCE respectively.
Foliage Not the Key
Because the researchers initially assumed that the plants removed the
chemicals by uptake through the leaves and photosynthetic processes,
they carefully measured leaf surface area. However, when researchers
removed the lower leaves or all leaves of some test plants, they found
that the percentage of the tested chemicals removed actually increased.
Although initially puzzled by this result, they later observed that
this "...occurred only when large amounts of foliage covered the potting
soil surface reducing contact between the soil and the air inside the
Researchers then removed only the lower leaves and the results showed
that soil surface exposure to the air was important. Further studies
investigated the role of soil microorganisms in the chemical removal
process. Soil bacteria measurements did not always correlate with increased
chemical removal, leading to the hypothesis that "other yet unidentified
biological factors may also be important." They did not say what those
factors might be.
The scientists identified several common soil bacteria isolates in
the root-soil zone. The researchers said they were "common soil microorganisms"
most of which are "known to be capable of biodegrading toxic chemicals
when activated by plant root growth." [During a recent phone conversation,
Wolverton told IAB that he has reviewed the extensive Australian and
Canadian literature on soil microorganisms. He believes the selection
of the right bacteria is the key to improving system efficiency.]
The belief that soil bacteria were important led to efforts to increase
air-soil contact. Researchers used fans to move air rapidly through
the soil, and they used activated carbon in conjunction with the plants
in some tests. According to the final report, these studies were not
part of the NASA-ALCA two-year study.
Air concentrations of 0.15 and 0.25 ppm of TCE and benzene respectively
were reduced to close to zero in two hours using an eight-inch activated
carbon filter system with a golden pothos plant. Concentrations of 36
ppm of both chemicals also dropped to nearly zero in two hours by the
The NASA report concluded that the charcoal-fan-plant combination was
"an essential part of an indoor air pollution control system with plants
to remove high concentrations of pollutants such as cigarette smoke
and organic solvents." The researchers concluded that the activated
carbon adsorbed the chemicals and held them until the "plant roots and
microorganisms can utilize them as a food source, therefore, bioregenerating
Philip Morey of Clayton Environmental Consultants [Morey is now with
Air Quality Sciences, Inc.] confirmed the potential efficacy of the
bacteria. Morey is a plant physiologist by training and is well known
for his studies of microorganism-related problems in buildings. He told
us that there are typically 1010 to 1012 mg of bacteria in a spoonful
of soil. The bacteria eat sloughed-off plant cells, thus creating a
species-specific symbiosis. Additionally, Morey said that because house
plants are generally wide-leafed they intercept much light. This makes
them suitable for low-light conditions.
Limitations of the NASA Plant Tests
We have to ask how well the tests run on plants help us understand
their actual performance in buildings. A number of conditions in the
NASA tests were not "real world," and this raises questions about the
applicability of the results. Because of this limitation, we can't yet
evaluate plants' efficacy as indoor air cleaners.
Dynamic chamber studies with air exchange rates and mixing resembling
real-world conditions would help significantly. The results could easily
be modeled to predict performance in real-world settings. The best test,
of course, would be to place the plants in typical rooms in homes and
office buildings. Then scientists could evaluate the actual impact of
plants on indoor air concentrations of organic chemicals.
Failed Field Study
To date, advocates have not reported the results from actual field
tests. One field study was begun and failed, according to a strong advocate
of the interiorscape approach to IAQ control. Stuart Snyder is the president
of Aqua/Trends of Boca Raton, Florida, a firm that sells irrigation
systems for interiorscapes. He offered his explanation as part of a
13-page letter to Robert Axelrad, Director of EPA's Indoor Air Division.
Responding to what he calls EPA's criticism of the NASA work, Snyder
wrote, "In many ways small systems are better able to isolate factors,
and more clearly define mechanisms at work... Larger environments are
too subject to conflicting variables. Real life, field studies with
their complex dynamics are also valuable, and should be implemented
at later stages of research they are however, more difficult
to accurately stage and evaluate."
Snyder continued, "Scaled up studies must be made at some point. Associated
Landscape Contractors of America have already attempted a controlled
study in an office building. It failed as a study because of these difficulties."
The office-building study was done for over a year under realistic conditions
and with as much control as can be achieved in a field study, There
was no indication that the presence of plants had any measurable effect.
HBI Inc., which conducted the study, reported virtually no effect of
plants on the VOC concentrations.
John Girman's Critique
John R. Girman (Chief of the Analysis Branch at EPA's Indoor Air Division)
has prepared a memo that details some shortcomings of the NASA study's
claims for the efficacy of plants. The memo was included in correspondence
between Axelrad and Snyder. Girman's memo responds to some of the technical
issues presented by Snyder and other advocates of IAQ control with house
plants. The memo's title is "Comment on the Use of Plants as a Means
to Control Indoor Air Pollution," (undated.) Girman analyzes the notion
that NASA research shows plants are effective at removing indoor air
pollutants at realistic concentrations and time frames. He calculates
that at the most favorable conditions, it would take 680 plants in a
typical house to achieve the same pollutant removal rate Wolverton and
his colleagues reported they achieved in the test chamber.
Girman, a chemist by training, is a thoughtful, experienced, and knowledgeable
indoor air researcher who brings important technical insights to EPA's
Indoor Air Division. Because the interest in NASA's research is so large,
we present Girman's memo in its entirety.
Comment on the Use of Plants as a Means to Control Indoor Air Pollution
by John Girman
"Several issues must be addressed before the use of plants can be considered
to be an effective means to control indoor air pollution. It is certainly
true that plants remove carbon dioxide from the air. It is also well
known that plants can remove other pollutants from water and this forms
the basis for many pollution control methods. However, the ability of
plants to control air pollution, particularly indoors, is less well
established. Even ignoring the debate about what specific processes
are important in the removal of airborne pollutants by plants, e.g.,
photosynthesis in leaves, deposition on foliage, microorganisms in roots
or soil, etc., and accepting the validity of the laboratory experiments
that Wolverton has conducted, there are still basic concerns about the
effectiveness of controlling indoor air pollution with plants."
"For example, if a particular plant can remove 90% of a specific pollutant
in 24h in a closed chamber (which appears to be one of the better test
results), then the pollutant concentration at the conclusion of the
test is only 10% of the initial concentration. [The highest removal
rate reported by Wolverton in the NASA study was 89.9% of the initial
concentration after 24 hours.] The equation C = C0e-kt determines the
concentration in the chamber, where
C = concentration of the pollutant at time t,
C0 = the initial concentration of the pollutant,
k = the first order pollutant removal rate constant, and t = the time
in hours since the beginning of the test.
Rearranging the equation, we obtain -(1/t)ln(C/C0) = k.
Since for our example, t = 24 h and C/C0 = 0.10, k or the pollutant
removal rate is 0.096 h-1. Determining the pollutant removal rate constant
in this manner is useful for two reasons: (1) it allows comparison of
a pollutant removal process with the most common pollutant removal rate
of the plant to environments other than just a test chamber."
"The pollutant removal rate of a plant in the test chamber (with appropriate
considerations of scale) can be compared with ventilation rates (the
most common pollutant removal process) of typical environments. Office
buildings have ventilation rates ranging from about 0.5 h-1 (or half
an air change per hour) to about 2 h-1. A typical residence may have
a ventilation rate of about 0.75h-1 and a tight house may have a ventilation
rate of 0.25 h-1. Thus, even ignoring scale up considerations for the
moment, the pollutant removal rate of plants in chambers, 0.096 h-1,
is much lower than typical low ventilation rates found in residences
"However, scale-up considerations are also important. It appears that
the average chamber volume used in Wolverton's tests was 0.5 m3. This
means the results must be appropriately scaled-up for use in a larger
environment to allow for differences in volumetric loading (the number
of plants per volume of space). This does not appear to have been done.
The volume of a typical house in the U.S. is 340 m3 with a floor area
of 139 m2 (1500 ft2). Thus, the recommendation that one plant be used
per 100 ft2 implies the use of 15 plants in a typical house. [ALCA recommends
1 plant per 100 ft2. Wolverton recently told us he now recommends 2
or 3 plants/100 ft2, but he says "he is "just throwing a dart."] This
would provide for 340 m3/15 plants or 23 m3 per plant, not 0.5 m3 per
plant as in the chamber. This means that each plant would have to clean
46 times more volume than it did in the test chamber or, as would actually
happen, it will clean the larger volume less effectively. To be more
precise, each plant will have a pollutant removal rate which is only
1/46 of the rate it would have in the chamber, i.e., only 0.002 h-1.
Thus, plants at the volumetric loading recommended would be expected
to contribute relatively little to pollutant removal in any indoor environment
with typical ventilation."
"To achieve the same pollutant removal rate as realized in the test
chamber, one would need to have the same volumetric loading, i.e., 680
plants in a typical house (340 m3 divided by 0.5 m3 per plant). This
does not seem practical and this forms the basis for concern that adequate
and realistic scale-up considerations are necessary before the use of
plants can be recommended as a means to control IAQ. Similar concerns
apply to the use of plants to control IAQ in office environments. It
is hardly surprising that the attempt to validate the test chamber results
by Associated Landscape Contractors of America did not provide measurable
"In addition, many of the reported tests relied upon a fan to circulate
air containing pollutants near the plant. This would serve to inflate
pollutant removal rate of a plant in a test chamber unless fans were
also used to circulate air containing pollutants in a house or office.
(The use of fans in this manner would increase operating costs and requires
a separate analysis to determine if bringing in additional outside air
for ventilation would be more cost effective.) It also appears that
a large part of the test space was occluded by the plant itself, which
also tends to inflate the pollutant removal rate. This would not be
practical in most indoor environments."
"The above is not intended as a criticism of small chamber testing.
Small chamber testing, when used in conjunction with modeling, is an
important tool for improving IAQ. EPA has encouraged its use for source
emission characterization, for product comparisons and to evaluate various
"However, there are aspects of Wolverton's chamber test conditions
which must be addressed in translating his results to typical indoor
environments. The test method employed by Wolverton is a static test
method, in which a one-time injection of a pollutant occurs. This is
appropriate only for certain types of indoor air pollution, i.e., when
the pollutant source does not emit pollutants continuously. Many important
pollutant sources, such as building materials and furnishings, are continuous
emitters. In the case of continuous sources, plants would be even less
effective in real environments than the test results would indicate.
This occurs because, while the plant is removing a particular pollutant,
more of that same pollutant is being emitted at the same time by an
indoor source of that pollutant. These types of sources can be dealt
with by chamber studies which incorporate dynamic conditions, i.e.,
continuous injection of a pollutant. In addition, because indoor environments,
with few exceptions, always have some ventilation, realistic chamber
tests usually incorporate some ventilation. The effect of this ventilation
is easily accounted for by modeling."
"Using the same conditions as the example above (0.5 m3 chamber, one
plant per chamber; pollutant removal of 90% in 24 h under static conditions),
one can model that under dynamic conditions which include some ventilation
(a low rate of 0.5 h-1 and a continuous pollutant source, the pollutant
removal at steady state would be only 16% rather than 90%. This result,
when considered in concert with the need for very large amounts of plants
in indoor environments to achieve results comparable to those of small
test chambers, suggests that a great deal of validation remains before
the use of plants can be recommended for effective control of indoor
"Finally, few technologies produce only benefits; there is often some
drawback. Humidity and microbial contaminants are potential concerns
in some indoor environments and applications. The use of large numbers
of plants in an indoor environment could increase the humidity to problem
levels. The use of fans to draw air over the soil of large numbers of
plants may have the potential to cause microbial problems. In addition,
while our understanding of the degradation products produced by plants
metabolizing pollutants is limited, we must be certain that these products
are not problems themselves. For example, there are literature reports
that the degradation products of trichloroethylene metabolism by plants
are dichloroethylenes and vinyl chloride, which are also harmful pollutants.
Should the performance of plants in controlling air pollutants improve
greatly, this aspect would require a thorough examination."
[end of Girman comment]
We think Girman has raised some excellent points while being rather
generous with the NASA research. The 90% removal rate was one of the
highest reported. The average NASA study measurement was 45.1%, about
half the value used by Girman. We believe Wolverton's claim that research
will allow selecting the most effective plants, but he told us that
a variety of plants were likely to be needed to deal with the wide range
of indoor air contaminants. Thus, the removal rate for all chemicals
per plant may be near the average.
How much of the reported removal occurred by adsorption of the chemicals
on the chamber walls? We asked some of the best indoor air scientists
we know to speculate on this question. Given the results reported by
NASA, some theoretical considerations, and each one's experience, the
estimates we feel comfortable reporting are between 10 and 20% of the
total mass introduced into the chamber.
The question arises as to whether Wolverton made "initial" measurements
before or after the occurrence of any possible sink effect. As we read
his reports, in some cases his measurements were made very quickly,
while in others they waited for 30 or even 60 minutes. The removal rates
were calculated by subtracting the final concentration from the initial
concentration to determine the percent removed. Theoretically, the control
test with the pot full of soil without a plant should be a good indicator
of the total removed by adsorption on the chamber walls, pot, and soil
and by leakage from the chamber. However, it does not allow us to separate
these various potential loss mechanisms. Thus, the removal by plants
may be even less.
We do not think the research reported to date suggests a significant
role for plants in cleaning indoor air. Phil Morey told IAB: "I've been
in buildings where there are hundreds of plants, and I've never considered
them a significant factor [in terms of controlling VOC concentrations].
Morey said it is perfectly reasonable that a bacterium at the root-hair
interface could consume VOCs.
Indeed, Morey cautioned that there is a large literature on plants
themselves being a source of VOCs. Leaves have chemicals for insect
defense, and some of these chemicals are semi-volatile compounds that
sit on the leaf surface. Some are volatiles like terpenes. We need more
work to check the possible negative consequences of introducing large
numbers of plants into building environments.
Both Snyder and Wolverton were critical of Girman's memo and of EPA's
attitude as they see it. However, Wolverton told IAB he has seen progress
and is optimistic from his conversations with EPA officials. IAB contacted
NASA officials connected with the research; they think the idea is interesting
but that more research is needed. They also said NASA has not advocated
using plants to clean indoor air.
We think EPA should guide Wolverton, ALCA, NASA, and others interested
in testing or promoting the use of plants to clean indoor air. Both
chamber and full-scale testing should be encouraged, but careful experimental
design is required. The research done to date does not demonstrate familiarity
with many of the techniques now widely used by indoor air researchers.
We feel that the reporting and the limited methodologies reported in
the NASA study and a more recent study conducted by Dr. Wolverton are
inadequate. We hope that their future work will address some of these
B. C. Wolverton, Anne Johnson, and Keith Bounds, "Interior Landscape
Plants for Indoor Air Pollution Abatement, Final Report September
15, 1989." Stennis Space Center, Mississippi: National Aeronautics and
Space Administration. Contact: NASA, John C. Stennis Space Center, Science
and Technology Laboratory, Stennis Space Center, MS 39529-6000.
Stuart Snyder, Letter to Robert Axelrad, January 12, 1992.
Robert Axelrad, Director, Indoor Air Division, EPA, Letter to Stuart
Snyder, President, Aqua/Trends, Boca Raton, FL. February 24, 1992.
John R. Girman, Branch Chief, Analysis Branch, Indoor Air Division,
U. S. Environmental Protection Agency. "Comment on the Use of Plants
as a Means to Control Indoor Air Pollution." Undated.
B. C. Wolverton, Scientific Spokesperson, Plants for Clean Air Council,
Falls Church, Virginia. "Response to the Comments of John R. Girman
on the Use of Plants as a Means to Control Indoor Air Pollution." Undated.
B. C. Wolverton and John D. Wolverton, "Bioregenerative Life Support
Systems for Energy Efficient Buildings." Proceedings of the International
Conference on Life Support and Biospherics, University of Alabama, Huntsville,
AL. February 18-20, 1992.
B. C. Wolverton, Wolverton Environmental Services, Inc., Letter to
Erich Bretthauer, Assistant Administrator, Office of Research and Development,
U. S. EPA. March 10, 1992.
Stuart Snyder, Letter to Robert Axelrad, March 31, 1992.
For more information:
B. C. Wolverton, Wolverton Environmental Services, Inc., 726 Pine Grove
Road, Picayune, Mississippi 39466. (601) 798-6875.
Stuart Snyder, President, Aqua/Trends, Box 810444, Woodlandia Station,
Boca Raton, FL. 33461. (407) 272-9838.
Hal Levin email@example.com
Building Ecology Research Group
2548 Empire Grade, Santa Cruz, CA 95060
tel 831 425 3946 fax 831 426 6522