Phytoremediation of indoor air – Current state of the art

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Kays, S.J. 2011. Phytoremediation of indoor air – Current state of the art. pp. 3-21, In: The Value Creation of Plants for
Future Urban Agriculture, K.J. Kim (ed.), Nat. Inst. Hort. Herbal Science, RDA, Suwon, Korea.
Phytoremediation of Indoor Air – Current State of the Art
Stanley J. Kays
University of Georgia
Athens, Georgia 30602-7273 USA
Phytoremediation
Phytoremediation of indoor air utilizes plants to remove or neutralize
environmental contaminants such as volatile organic compounds (VOCs) in the air of
homes, offices and other enclosed buildings. Certain plant species, working in tandem
with yet unidentified microorganisms in the root zone, have the ability to remove VOCs
and purify the air.
Hundreds of VOCs have been identified as indoor contaminants (ACGIH, 1995;
EPA, 1989; Won et al., 2005). For example, the U.S. Environmental Protection Agency
(EPA) reported detection of more than 900 VOCs in the air of public buildings (EPA,
1989). In a Finnish study, over 200 VOCs were identified in each of 26 homes
(Kostiainen, 1995). An example of the types of volatiles that might be encountered is
presented in Table 1 which lists the VOCs found in two houses surveyed in Athens,
Georgia that had serious air quality problems. The volatiles in the first house were
emanating from toxic drywall and in the second from insulation that had been blown into
the air space within the outside walls. Indoor air in cities has been reported to be as much
as 5 to 1000 times more polluted than exterior air (Brown et al., 1994; Godish, 1995;
Kostianen, 1995; Brown, 1997; Ingrosso, 2002; Yang et al., 2004; Zabiegała, 2006). The
chemicals are absorbed into human and animal bodies through inhalation and in some
instances, through direct penetration of the skin (McDougal et al., 1990).
While the initial work on phytoremediation of indoor air was done in the 1970s, it
has not been until recently that interest in the subject has spread. Currently, the leading
research programs are in South Korea. To date, a significant portion of the research has
been directed toward identifying superior phytoremediation species of indoor plants.
While the results have been very positive, the lack of adequate funding has impeded
exploring the basic mechanisms operative and making the transition from the laboratory
to real world homes and offices. As a consequence, what we currently know is vastly
exceeded by what we do not, a situation that is evident from, for example, the very
limited number of VOCs that have been tested.
The public has displayed tremendous interest in the potential of phytoremediation
and there is a growing awareness of the serious health issues arising from breathing
polluted indoor air. Four popular books on the subject are currently available (Son, 2004;
Son, 2009; Wolverton, 1996; Wolverton and Takenaka, 2010).
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History
In the early 1970s, NASA began monitoring the atmosphere inside spacecraft
during manned Skylab missions. Since the spacecraft is a tightly sealed space, volatiles
given off from materials and humans began to build up once the chamber was closed.
During Skylab III, over 300 compounds were found in the air. It was evident from
changes in the composition of the air that the quality deteriorated with time and that a
means of removing undesirable volatiles was needed. Studies on a cross-section of
methods of purifying the air within the spacecraft were undertaken and in 1984 the first,
detailing the ability of indoor plants to remove VOCs in sealed chambers, was published
(Wolverton et al., 1984). During this time, there was a growing body of research
published on the role of VOCs in the declining air quality of homes, offices, and other
buildings. The initial research on phytoremediation led to a relatively small number of
subsequent studies on the potential of indoor plants to remove volatile pollutants. In
nearly all instances, the research was done in closely controlled laboratory settings with
the aim of subsequently testing species with superior phytoremediation potential in real-
world settings to alleviate symptoms of “sick building syndrome” and in homes and
offices where it is not evident that air quality has been compromised (i.e., the absence of
distinct physical symptoms in the occupants).
Plants Tested
A cross-section of species have been assessed for their phytoremediation potential
with most being indoor plants that are adapted to low light conditions. A partial list of
plants is found in Table 2. While the assumption is that these plants will be used in homes
and offices where the light intensity and quality differ markedly from sunlight, many of
the phytoremediation tests have been conducted at light intensities that are appreciably
higher than might be expected in homes.
VOCs Tested
Of the 900 VOCs listed as indoor pollutants by the EPA, only a very small cross-
section have been tested for their ability to be removed by indoor plants. These include:
acetone (Oyabu et al., 2003; Tani and Hewitt, 2009); benzene (Cornejo et al., 1999; Liu
et al., 2007; Orwell et al., 2004; Tarran et al., 2007; Wolverton, 1986; Yang et al., 2009;
Yoo et al., 2006); benzaldehyde (Tani and Hewitt, 2009); n-butyraldehyde (Tani and
Hewitt, 2009); iso-butyraldehyde (Tani and Hewitt, 2009); crotonaldehyde (Tani and
Hewitt, 2009); formaldehyde (Aydogan and Montagu, 2011; Chen et al., 2010; Kim et
al., 2008; Kim et al., 2010; Oyabu et al., 2003; Sawada and Oyabu, 2008; Son et al.,
2000; Wolverton et al., 1984; Wolverton, 1986; Xu et al., 2011); methacrolein (Tani and
Hewitt, 2009); methyl ethyl ketone (Tani and Hewitt, 2009); diethyl ketone (Tani and
Hewitt, 2009); methyl n-propyl ketone (Tani and Hewitt, 2009); methyl iso-propyl ketone
(Tani and Hewitt, 2009); methyl isobutyl ketone (Tani and Hewitt, 2009); octane (Yang
et al., 2009); pentane (Cornejo et al., 1999); α-pinene (Yang et al., 2009);
propionaldehyde (Tani and Hewitt, 2009); toluene (Cornejo et al., 1999; Kim et al., 2011;
Orwell et al., 2006; Sawada and Oyabu, 2008; Yang et al., 2009; Yoo et al., 2006);
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trichloroethylene (Wolverton, 1986; Yang et al., 2009); xylene (Cornejo et al., 1999;
Orwell et al., 2004; Sawada and Oyabu, 2008; Wolverton, 1993). In some instances,
assessment has been only cursory. The methods utilized in a number of the studies differ
widely and the clarity and thoroughness of their description insufficient to make
meaningful comparisons among studies. Likewise, the list of VOCs represents only a
very small fraction of the number of volatiles encountered in the indoor air of homes and
offices.
Rate of VOC Removal
Assessing indoor plants for phytoremediation efficiency involves comparing the
purification capacity among species under standard conditions. As one might anticipate,
there are distinct differences in removal rate among plant species. For example,
comparing a cross-section of orchids, the formaldehyde removal efficiency of Sedirea
japonicum (L. Linden & Rchb. f.) Garay & H.R. Sweet was the highest while Cymbidium
sp. was the lowest of the species tested (Kim and Lee, 2008). Table 3 lists the highest and
lowest rates of removal from a selection of phytoremediation research papers after
conversion to a standard measure (e.g., µg m-3 m-2 h-1). The rate of removal varied
substantially among experiments, initial VOC concentration, plant species, and the VOC
in question (Table 3). Formaldehyde removal ranged from 95 to 18,582 µg mg-3 m-2 h-1,
benzene from 1,281 to 44,843 µg mg-3 m-2 h-1, toluene from 22 to 928 µg mg-3 m-2 h-1 and
hexane from 15,292 to 168,000 µg mg-3 m-2 h-1. It is doubtful, however, if valid
comparisons can be made among experiments due to differences in methods.
Concentrations, for example, ranged from 0.15 to 100 ppm and data from Wolverton et
al. (1984) indicates a marked increase in removal rate between 14 and 37 ppm
formaldehyde for the same species.
The half-life (time required for 50% removal) is occasionally used as an indicator
of the purification capacity of a plant and allows comparing the efficiency among species
under standardized conditions (Kim et al., 2008; Orwell et al., 2006; Oyabu et al., 2003).
In contrast, expression of VOC removal rate based on leaf area per unit time (µg m-3 m-2
h-1) is considered superior in that it allows comparing different experiments and plants of
varying size, the latter being essential for determining the number of plants needed for
specific indoor environments.
Microorganisms
It is now well established that microorganisms, in particular bacteria, in the media
are a critical part of the phytoremediation response (Chun et al., 2010; Wolverton and
Wolverton, 1993; Wood et al., 2002). Their role is important since they are more likely to
be able to effectively metabolize the diverse range of chemical pollutants found in indoor
air. The microbe populations are comprised of a number of organisms, the specific
composition and/or gene expression of which are thought to be capable of shifting in
response to the prevailing volatile composition. The ability to change adds tremendous
flexibility to the phytoremediation response. It is important to note that at this time we
know virtually nothing about the organisms involved: species/strains present, population
dynamics, changes in gene expression in response to specific VOCs, sensitivity to VOCs,
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relationship to the plant and a seemingly endless list of other questions.
The fact that certain microorganisms found in the growing media of potted-plants
are involved in the removal of VOCs is demonstrated by the fact that when the plant(s)
are removed from the media, the VOC concentration continues to decrease (Godish and
Guindon, 1989; Wolverton et al., 1989; Wood et al., 2002) and plants held in the dark
also remove VOCs (Orwell et al., 2004; Wolverton et al., 1984; Yoo et al., 2006). Media
with plants removed formaldehyde faster than the media alone and media that formerly
had plants, removed more formaldehyde than sterilized media. Media bacterial counts
varied with the plant species from which they were isolated (Sansevieria trifasciata
Prain. > Kalanchoe sp. > Ficus benjamina L. > Spathiphyllum sp.) (Wolverton and
Wolverton, 1993). The addition of selected microbe populations to the media also
increases the rate of VOC removal (Chun, et al., 2010). In addition, removal efficiency of
the media increased (~7-16%) with increased VOC exposure frequency (Kil et al., 2008)
suggesting an apparent stimulation of the organisms.
Bacterial populations within pots of Howea forsteriana (C. Moore & F. Muell.)
Becc., for example, could remove benzene and hexane even after the plant and roots were
removed and a cultured isolated bacterial population applied to a sterile media
(vermiculite) could likewise metabolize the two volatiles (Wood et al., 2002). Unused
media (i.e., never having had plants) could also remove benzene with removal increasing
after an initial lag period of around 5 days. Significant changes in the overall population
of microbes (expressed as bacterial colony-forming units per g potting soil) in response to
exposure to benzene were not found.
Since the microbes may vary with the plant species growing in the media, Chun et
al. (2010) isolated the bacterial populations from media in which 9 different plant species
were growing. The ability of isolated bacteria populations to remove benzene and toluene
varied with the VOC in question. When the populations growing on a culture medium
were exposed to VOCs, there were significant differences in the rate of removal of the
volatiles. Adding bacterial populations back to the media of Pachira aquatica Aubl.,
Ficus elastica Roxb. ex Home. and Spathiphyllum wallisii Regel, the isolate had a
significant impact on the removal of benzene, toluene and xylene (Chun et al., 2010). The
root-zone also eliminates a substantial amount of formaldehyde (Kim et al., 2008). VOC
removal efficiency is known to vary with the microbe population present, the type and
concentration of VOCs, plant species, potting media, and other factors.
Plants excrete into the root zone significant amounts of carbon that stimulates the
development of microorganisms in the rhizosphere (Kraffczyk et al., 1984; Schwab et al.,
1998) and appears to be important in maintaining a viable population. The phyllosphere
is also colonized by a diverse array of microorganisms (Mercier and Lindow, 2000).
Therefore, rhizospheric and phyllospheric microorganisms, as well as stomate-mediated
absorption, provide a means of biofiltration of VOCs from indoor air.
It is evident that our understanding of rhizospheric and phyllospheric biology
relative to the removal of VOCs is exceedingly limited and hinders the development of
biological processes for indoor air VOC removal (Guieysse et al., 2008). A better
understanding is essential for optimizing removal efficiency.
Contribution of the Plant versus Microbes
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The root-zone eliminates a substantial amount of formaldehyde during both the
day and night. The ratio of removal by aerial plant parts versus the root-zone was
approximately 1:1 during the day and 1:11 at night (Kim et al., 2008). Wolverton and
Wolverton (1993) found that the top to media ratio varied with the species of plant and
the VOC in question. Dieffenbachia maculata (Lodd. et al.) G. Don1 and Nephrolepis
exaltata (L.) Schott had similar ratios (~1:1 aerial plant parts : root zone) for xylene while
ratios for formaldehyde favored the root zone (37:63 Diffenbachia sp. to 40:60
Aglaonema sp.). While a number of soil microorganisms are capable of degrading toxic
chemicals (Darlington et al., 2000; Wolverton et al., 1989), few that are directly
associated with formaldehyde removal have been identified.
VOC Stimulation of Phytoremediation
Phytoremediation efficiency of many plant species increases with exposure to
VOCs (Kil et al., 2008; Kim et al., 2011; Orwell, et al., 2004), however, the
mechanism(s) controlling the response is not known. The increase in efficiency in
response to toluene appears to be relatively widespread in the plant kingdom (Kim et al.,
2011), though such changes in efficiency have only been demonstrated for a small
number of VOCs to date.
Changes in phytoremediation efficiency in response to toluene exposure (1.3
ppm) occurred quite rapidly (i.e., <3 d) and in 27 of 28 crops (Kim et al., 2011). The
greatest increase was between the 1st and 2nd exposure (i.e., after 3 d) and the increase in
efficiency between the 1st and 3rd exposure ranged from 378 µg·m-3 m-2 h-1 in Pinus
densiflora Siebold & Zucc. to -16.6 µg·m-3 m-2 h-1 in Salvia elegans Vahl, with a mean of
156 µg·m-3 m-2 h-1 for all crops. The percent change ranged from 614 % (Pittosporum
tobira (Thunb.) W. T. Aiton) to -8 % (Salvia elegans) but was not necessarily indicative
of phytoremediation value of a species.
It is also known that microbes within the growing media are involved in the
removal of VOCs, in addition to the plant (Chun et al., 2010; Kil et al., 2008; Orwell, et
al., 2004). As a consequence, increases in efficiency may be attributable to an effect on
certain microorganism species in the media. For example, inoculation using cultured
bacterial populations from nine indoor plant species significantly increased the removal
of a cross-section of benzenes (benzene, toluene, m,p-xylene, and o-xylene) (Chun et al.,
2010). Microbe populations extracted from the media of certain plant species were more
effective than others (Chamaedorea elegans (Aiton) R. Br. > Nephrolepis exaltata (L.)
Schott > Sanservieria trifasciata Prain) in the removing the VOCs and their removal
efficiency varied with the volatile in question.
The observed increases in efficiency could have been mediated through an effect
on the microbe population and/or the plant itself through altered gene expression. They
could also be due to a change in the population of microbes that effectively metabolize
toluene and presumably derive a benefit from its presence. The rapid rate of increase in
toluene removal efficiency is likely faster than could be accounted for by an increase in
beneficial microbe populations (i.e., in some instances, up to a 6-fold increase in
efficiency occurs). Regardless of the cause, increasing the removal efficiency of VOCs is
advantageous to the phytoremediation process in general and a better understanding of
1 D. maculata is a synonym of Dieffenbachia seguine (Jacq.) Schott var. seguine.
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the mechanism(s) responsible for this increase may enable end-users to take full
advantage of the response.
Enhancing Phytoremediation Efficiency
There are a number of possible avenues for enhancing phytoremediation
efficiency in plants. Pretreating the media with microorganisms selected for their ability
to remove certain targeted VOCs is one possibility. Alternatively, since the VOCs
encountered vary widely among buildings, it may be essential to enhance the rhizosphere
with a cross-section of specific microorganism isolates.
Another approach for increasing the phytoremediation potential of indoor plants is
through transgenic technology. Two examples have been reported thus far for facilitating
the removal of formaldehyde: 1) over expression of a glutathione-dependent HCHO
dehydrogenase (FALDH) pathway found in plants (Achkor et al., 2003) and 2) the
introduction of a bacterial ribulose monophosphate (RuMP) pathway (Chen et al., 2010).
In the former, an enzyme for formaldehyde oxidaion, glutathione-dependent
formaldehyde dehydrogenase, is over expressed. FALDH plants, however, remain
sensitive to formaldehyde and the system requires the addition of glutathione as a
cofactor and regeneration systems for the reduced gluthathione and NAD+. The latter
system, however, utilizes a gene from a methylotroph, an microorganism that utilizes one
carbon compounds (e.g., methane, formaldehyde) as its carbon source (Yurimoto et al.,
2005; Kato et al., 2006; Yurimoto et al., 2009.). The RuMP pathway, like the Calvin-
Benson pathway, fixes the single carbon unit to ribulose phosphate. In each case fructose-
6-phosphate cycles back to ribulose phosphate through a series of rearrangement
reactions. Therefore the CO2 fixation steps in the Calvin-Benson pathway are bypassed in
the RuMP pathway by 3-hexulose-6-phosphate synthase (HPS) and 6-phospho-3-
hexuloisomerase (PHI) catalyzed reactions (Chen et al., 2010). Two superior constructs
(rmpA and rmpB) were found and when regenerated Arabidopsis thaliana (L.) Heynh.
plants were crossed, the AB7 hybrid displayed far greater insensitivity to high levels of
formaldehyde that either parent or transgenic FALDH plants. Removal of formaldehyde
by the RuMP transgenic plants was estimated to be enhanced by around 20%.
While these first transgenic plants with enhanced phytoremediation potential are
only capable of removing C1 VOCs, the potential for adapting this approach to other
VOC metabolizing pathways is attractive. It is thought that transgenic house plants might
be more acceptable to the general public than transgenic food plants (Chen et al., 2010)
VOC Considerations
A range of questions involving the interaction of VOCs with the plant, microbes,
and other volatiles remain to be addressed.
a. Fate of VOCs – Little is known about the fate of VOCs during
phytoremediation. Giese et al. (1994) demonstrated that the leaves of spider plant
Chlorophytum comosum (Thunb.) Jacques exposed to 7.1 [mu]L L-1 (8.5 mg m-3) gaseous
[14C]-formaldehyde could absorb the volatile and metabolize it to carbon dioxide.
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Radioactivity was found to be incorporated into organic acids, amino acids, free sugars,
and lipids as well as cell-wall components. Formaldehyde appears to be efficiently
detoxified by oxidation and subsequent C1 metabolism. The volatile is also known to be
removed by microorganisms in the media. The mechanism of uptake, degree of
adsorption versus absorption, and plant/microbe species differences in processing for a
number of critical VOCs await similar isotope experiments.
b. VOC concentration effects and toxicity to the plant and microbes – The
linearity of the phytoremediation response, upper and lower VOC concentration limits for
phytoremediation, changes in removal efficiency with concentration, and the
concentrations at which plant and microbe toxicity effects occur are not known. Toxicity
can be determined by assessing the development of visual symptoms. Likewise, a cross-
section of physiological responses (photosynthetic, respiratory and transpiration rates,
stomatal conductance and intercellular CO2) have been measured before and after
exposure to benzene (1.0 ppm), toluene (1.0 ppm) or both volatiles (0.5 ppm each) to
assess possible deleterious effects to the plant (Yoo et al., 2006). There were significant
differences in each of the parameters. For example, the photosynthetic rate decreased
when exposed to either benzene or toluene and even more so when exposed to both
simultaneously. While the treatments did not produce symptoms of physical damage, it is
probable that at certain concentration × exposure duration combinations, damage to the
plant and/or beneficial microbes will occur.
c. Effect of multiple VOCs – When assessing removal by the aerial portion of the
plant, the VOC removal efficiency of both benzene and toluene (0.5 ppm each) was
reduced both during the day and the night by the presence of the other volatile (Yoo et
al., 2006). The removal efficiency of Cissus rhambifolia Vahl. was the least effected of
the 4 plant species tested while Syngonium podophyllum Schott was the most. Day and
night rates of benzene removal by C. rhambifolia were reduced 27 and 18% respectively
when toluene was present, while toluene removal was reduced 38 and 56% when benzene
was present. While the rates will no doubt change markedly when the entire plant is
exposed, the effect of one VOC on another is indicative of an interaction or competition
among VOCs.
Cornejo et al. (1999) found that benzene but not toluene was removed when both
gasses were applied simultaneously (Kalanchoe blossfeldiana Poelln.). Yoo et al. (2006)
in contrast found that most species could remove both gases, with toluene being removed
more effectively than benzene (e.g., toluene was double that of benzene in Hedera helix
L.).
d. Synthesis of VOCs by the plant – Plants release a large number of VOCs into
their environment and in some cases a considerable volume [e.g., 6 Tg y-1 of isoprene are
produced by trees and shrubs (Guenther et al., 2006)]. Three primary pathways
(isoprenoid, shikimic acid, and the oxidative cleavage and decarboxylation of various
fatty acids) are responsible for the synthesis of many of the volatile compounds
(Dudareva et al., 2004; Kays and Paull, 2004). House plants also emit VOCs, however,
the rate of synthesis is relatively low such that they are not considered to represent a
health problem. The fragrance of flowers and the aroma of an apple pie represent benign
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and generally pleasurefull VOCs. The volatiles emitted from 4 species of house plants
(Chrysalidocarpus lutescens Wendl., Ficus benjamina L., Sansevieria trifasciata Prain,
Spathiphyllum wallisii Regel) were identified by Yang et al., (2009). Included were (-)-
alloaromadendrene, butyl butyrate, caryophyllene, copaene, (+)-β-costol, α-cubebene, β-
cubebene, (+)-cycloisosativene, 3,4-dimethyl-2-hexanone, (E)-4,8-Dimethyl-1,3,7-
nonatriene, farnesal, α-farnesene, (3Z,6E)-α-farnesene, (E)-β-farnesene, (Z)-β-farnesene,
E-farnesene epoxide, germacrene D, 1-hexanol, 2-heptanone, 3-hydroxy-2-butanone, 4-
hydroxy-4-methyl-2-pentanone, isopropyl myristate, D-limonene, linalool, (E)-linalool
oxide, (Z)-linalool oxide, methyl 4-tert-butylbenzoate, methyl salicylate, 2-nonanone,
(Z)-β-ocimene, 1-octanol, santalol, sesquirosefuran, 1-tetradecanol, and 3,3,5-
trimethylcyclohexanol. VOCs were also found to be emitted by the media + microbes, the
plastic pot and pesticides that had been applied to the plants. The volatile profile differed
markedly among the species tested and between day and night with the total
concentration ranging from 61,465 pg 100 g-1 dwt h-1 (day)/42,958 pg 100 g-1 dwt h-1
(night) for S. wallisii to 427 pg 100 g-1 dwt h-1 (day)/130 pg 100 g-1 dwt h-1 (night) for S.
trifasciata. Night time rates were lower, in part reflecting the greater diffusion resistance
with the stomata closed. S. wallisii was in flower and emitted substantial amounts of α-
farnesene which represented 90.3 % of the total pg of volatiles from the plant. In general,
however, the levels of VOC synthesis were very low and those emitted did not appear to
represent a health concern.
Plant Considerations
The precision by which one can determine the mechanisms operative during
phytoremediation and the potential application of the results to homes and offices is
dependent on the care and precision at which the experiments are conducted.
Experimental protocol can be separated into three general topics: plant factors, treatment
environment and considerations associated with the volatile(s) being tested.
Plant factors – the age, size, health, condition, leaf area, fertilization program,
growing medium, and water status of the plant(s) and the acclimatization
method and duration.
Treatment environment – temperature, O2 and CO2 concentrations, relative
humidity, air exchange rate or frequency, type of system (closed versus flow
through), length of time in the chamber and light intensity, quality, duration,
and photoperiod.
Volatile considerations – initial exposure concentration, method of introduction
and quantification, equilibration time, and duration of exposure.
Additional desirable information needed when reporting phytoremediation experiments
include the analytical method used, least detectable quantity, source and purity of
analytical standards, number of replications and statistical method utilized. The
presentation of the data should be in a manner that allows comparison among
experiments whenever applicable, (e.g., µg m-3 m-2 h-1).
Essential Requisites for Widespread Adoption of Indoor Air Phytoremediation
a. Development of an accurate analytical method for measuring the volatile status of the
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air in homes and offices that is economically acceptable to the public and is provided
by a credible organization.
From existing scientific studies, it is readily evident that a large number of homes
and offices have VOC levels that potentially compromise the health of the occupants. The
absence of a means for the public to determine the presence of a VOC problem is a major
obstacle. For example, even though complaints of serious health problems were
repeatedly voiced due to the volatile emissions from U.S. government trailers used as
temporary housing after hurricane Katrina, it was 2.5 years before the CDC assessed the
air quality and found excessively high levels of formaldehyde (Brunker, 2008), a highly
toxic volatile thought to be a carcinogen. Likewise, contamination due to
methamphetamine synthesis in clandestine laboratories typically results in the structures
being sufficiently toxic to be uninhabitable even though the presence of pollutants is not
necessarily readily apparent (Lim Abdullah and Miskelly, 2010). One striking
technological deficiency is the absence of an accurate, affordable means of determining
the VOC status of homes and offices. Currently the majority of commercial services are
either prohibitively expensive for the average homeowner or lack sufficient analytical
accuracy. Existing University Extension Service analytical laboratories represent an
excellent possible source for such a service.
Indoor air VOC composition is known to vary widely among structures. Plant
species, likewise, vary in their ability to remove volatiles. As a consequence, knowledge
of the building (e.g., volume, air exchange rate) and its volatile composition is essential
for determining the appropriate species and number of plants of each to reduce the VOC
concentrations to a safe level.
b. Additional information on the toxicology of volatiles found in air of homes and offices.
A major problem with interpreting the volatile composition of indoor air is that
there is toxicology data for only a relatively small portion the volatiles found. Currently
the CAS Registry identifies more than 56 million organic and inorganic substances with
an additional 12,000 new substances added daily (CAS, 2011). In 1976, when congress
passed the Toxic Substances Control Act (TSCA, 1976), there were 60,000 known
chemicals in the US marketplace. Today, that number has grown to 80,000 chemicals
(http://www.potomac riverkeeper.org/chemical-pollution) plus impurities and compounds
formed via reactions occurring during fabrication of products.
While there are exposure guidelines for some chemicals, the data is absent or
incomplete for an exceedingly large number. The need for increased testing was recently
made in a Chemical and Engineering News article by Erickson (2011) indicating “...EPA
still has to deal with the tens of thousands of chemicals already in commerce. Only a few
hundred of them have been assessed for their toxicity, and the EPA needs to prioritize
which of the remaining ones should be evaluated.”
The lack of adequate information is also indicated by order of magnitude
differences among countries in the acceptable concentrations allowed in the air for some
volatile compounds [e.g., the threshold limit value (TLV) for toluene in homes is 0.27
ppm in Korea (Korean Ministry of Environment, 2006), 0.07 ppm in Japan (Japanese
9

Ministry of Health, Labour and Welfare, 2000), 20 ppm (time-weighted average) in the
U.S. (ACGIH, 2008), and 100 ppm in Australia (NOHSC, 2001)].
c. Support for phytoremediation research
Research on phytoremediation of indoor air has proceeded in spite of virtually no
government sponsored funding outside of South Korea. The rate of progress has been
exceedingly slow and critical deficiencies (e.g., fate of VOCs, assessing the effectiveness
of plants in different types of buildings, maximizing the effectiveness of microorganisms
in the media) represent research that is expensive and requires a team of scientists from a
cross-section of disciplines. Better prioritization of grant funding and focusing more
research dollars on projects that have the potential for a tremendous impact on the health
and well-being of humans simply makes good sense. A better understanding of the
biological processes operative in phytoremediation will allow maximizing the potential of
plants and their associated microbes to remove volatile pollutants. While plants will not
be the solution for indoor VOC problems in all structures (e.g., large structures), they
represent an affordable solution to the majority of citizens especially with the current
economic conditions.
Conclusions
While our understanding of the basic biology and chemistry of phytoremediation
of indoor air remains extremely limited, the possible impact of this inexpensive means of
air purification on the health and wellbeing of humans is potentially tremendous. Indoor
plants not only create an aesthetically pleasing but potentially healthier environment for
people worldwide that live and/or work in enclosed buildings. Facilitating research
through additional funding is the key to maximizing the many positive advantages of this
technology, assessing its real world potential, and if appropriate expediting its
widespread use.
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14

15
Table 1. Volatile organic compounds found in two houses in Athens, Georgia.
Aliphatic hydrocarbons
2-n-Butylacrolein 1*
Decane 1, 2
2,3-Dimethylheptane 1
2,6-Dimethyloctane 1
2,6-Dimethylundecane 1
Dodecane 1
(E)-6-Dodecene 1
(Z)-5-Dodecene 1
3-Ethyl-2-methylheptane 1
1-Ethyl-3-methylcyclohexane
(c,t) 1
(E)-1-Ethyl-4-methyl-
cyclohexane 1
Heptadecane 1, 2
Hexadecane 1, 2
4-Methyldecane 1
4-Methyldodecane 1
3-Methylnonane 1
7-Methylpentadecane 1
3-Methyltetradecane 1
15-Methyltricyclo
[6.5.2(13,1)0(7,15)] pentadeca-
1,3,5,7,9,11,13-heptene 2
2-Methyltridecane 1
4-Methyltridecane 1
5-Methyltridecane 1
Nonadecane 1
Nonane 1, 2
Aliphatic alcohols
1-Butanol 1
2-Butoxyethanol 1, 2
1-(2-Butoxyethoxy)ethanol 1
1-Butoxy-2-propanol 1
1,3-Dichloro-2-propanol 1
Dimethylsilanediol 1
2-(2-Ethoxyethoxy)ethanol 1
2-Ethyl-1-hexanol 1
1-Hexanol 1
Hexylene glycol 1
2-(Hexyloxy)ethanol 1
2-(2-Methoxyethoxy)ethanol 1
1-(2-Methoxy-1-methylethoxy)-
2-propanol 1
1-(2-Methoxypropoxy)-2-
propanol 1
2-Methylene cyclopentanol 1
3-Methyl-1-butanol 1
5-Methyl-1-hexanol 2
1-Phenyl-1,4-butanediol 2
2-Phenylisopropanol 1
1-Octanol 1
1-Pentanol 1, 2
1-Propoxy-2-propanol 1
Propylene glycol 1
Aliphatic aldehydes
Decanal 1
(Z)-2-Decenal 1
Heptanal 1, 2
(E)-2-Heptenal 1
Hexanal 1, 2
(E)-2-Hexenal 1
2-Methyl-3-phenylpropanal 1
Nonanal 1
Octanal 1
(E)-2-Octenal 1
Pentanal 1
Aliphatic ketones
Cyclohexanone 2
2-Decanone 1
Geranylacetone 1
2-Heptanone 1
2-Hexanone 1
6-Methyl-2-heptanone 1
6-Methyl-5-hepten-2-one 1
4-Methyl-2-pentanone 1
1-(4-Methylphenyl)ethanone 1
2-Nonanone 1
(+)-Nopinone 1
Aromatic compounds
Acetophenone 1
p-Allylanisole 1, 2
Benzaldehyde 1
Benzyl acetate 1
Benzyl alcohol 1
Butylated hydroxytoluene 1
Decahydro-1,6-
dimethylnaphthalene 2
Decahydro-4,4,8,9,10-
pentamethylnaphthalene 1, 2
Decahydro-8a-ethyl-1,1,4a,6-
tetramethyl-naphthalene 1
1,2-Dichlorobenzene 2
1,4-Dichlorobenzene 2
1,3-Diethylbenzene 1
1,2-Diethylbenzene 2
3,4-Diethylbiphenyl 1
2,3-Dihydro-1,4,7-trimethyl-1H-
indene 1
2,3-Dihydro-4-methyl-1H-indene
1
2,6-Di-tert-butyl-1,4-
benzoquinone 1
Esters
Butyl acetate 1, 2
Butyl butyrate 1
2-Ethoxyethyl acetate 1
2-Ethyl-1-hexanol benzoate 1
Fenchyl acetate 1
1-Methoxy-2-propyl acetate 1
3-Methyl-1-butanol acetate 1
Methyl salicylate 1
Nonyl chloroformate 2
2-Pentyl acetate 1
Acids
Acetic acid 1
Butanoic acid 1
2-Ethylhexanoic acid 1
Hexanoic acid 1
Terpenoids
Borneol 1
Camphene 1
d-Camphor 1, 2
3-Carene 1
2-Carene 2
d-Carvone 1
N-containing compounds
2-Anthracenamine 1
Benzonitrile 1
Methylpyrazine 1
Pyrazine 1
N,S-containing compound
Benzothiazole 1
______________________________________________________________________________________
* Numbers indicate in which house the chemical was present.

Table 2. Plant species tested for their ability to remove selected VOCs.
Plant Species Reference Chemical Plant Species Reference
Chemical
Adiantum capillusveneris L.
Aglaonema modestum Schott ex Engl.
Aglaonema Schott
Aloe vera (L.) Burm. f.
Aloysia triphylla (L’Hér.) Britton
Anthurium andreanum Linden
Arachniodes aristata (G. Forst.) Tindale
Araucaria heterophylla Franco
Ardisia crenata Sims.
Ardisia japonica (Thunb.) Blume
Ardisia pusilla A. DC.
Asparagus densiflorus (Kunth) Jessop ‘Sprengeri’
Aspidistra elatior Blume ‘Milky Way’
Asplenium nidus L. ‘Avis’
Begonia maculata Raddi
Botrychium ternatum (Thunb.) Swartz.
Brassaia arboricola Endl.
Calathea makoyana E. Morr.
Calathea roseopicta (Linden) Regal
Camellia japonica L.
Camellia sinensis Kuntz.
Chamaecyparis obtusa Endl.
Chlorophytum bichetii Baker
Chlorophytum comosum (Thunb.) Jacq. ‘Fire Flash’
Chlorophytum elatum (Aiton) R. Br. var. vittatum
Chlorophytum elatum (Aiton) R. Br.
Chrysalidocarpus lutescens H. Wendl
Chrysanthemum morifolium (Ramat.) Hemsl.
Cinnamomum camphora (L.) J. Presl
Cissus rhombifolia Vahl.
Citrus medica var. sarcodactylis
(Hoola van Nooten) Swingle
Clivia miniata Regal
Codiaeum variegatum (L.) Blume
Coniogramme japonica (Thunb.) Diels
c3
c3j3
h2
j3
d7
c3l1,2,5, 6
c3
c3
c3d7
d7
c3d7
l1,2,5, 6
l1,2,5, 6
c3
d7
c3
j3
c3
l1,2,5, 6
c3
c3
c3
c3
l1,2,5, 6
i3
j3
c3
a3
d7
m2,7
e2
c3
l1,2,5, 6
c3
Crassula portulacea Lam. e2
Cupressus macrocarpa Hartweg
‘
Gold Crest’ c3
Cycas revoluta Thunb. c3
Cymbidium Sw. ‘Golden Elf’ e2
Cyrtomium caryotideum Nakai ‘Coreanum’ c3
Cyrtomium falcatum (L.f.) Presl. c3
Davallia mariesii Moore ex Baker c
3d7
Dendranthema morifolium (Ramat.) Tzvelev e2
Dendropanax morbifera Nakai Lév. c3
Dieffenbachia sp. a3
Dieffenbachia amoena Hort. ex Gentil ‘Marianne’ c3
Dieffenbachia amoena Hort. ex Gentil ‘Tropic Snow’ e2
Dieffenbachia seguine (Jacq.) Schottz l1,2,5, 6
Dizygotheca elegantissima R. Vig. & G. c3
Dracaena sp. f2
Dracaena concinna Kunth c3
Dracaena deremensis Engl. k2,4
Dracaena deremensis Engl. ‘Variegata’ e2
Dracaena deremensis Engl. ‘Warneckii’ c3
Dracaena fragrans (L.) Ker-Gawl. l1,2,5, 6
Dracaena fragrans (L.) Ker-Gawl. ‘Massangeana’ c3j3
Dracaena marginata Lam. f2
Dryopteris nipponensis Koidz c
3
Elaeocarpus sylvestris Hara ‘Ellipticus’ c3
Epipremnum aureum Bunt. a3c3f2l1,2,5, 6
Eugenia myrtifolia ‘Compacta’ c3
Eurya emarginata (Thunb.) Makino b3c3d7
Farfugium japonicum (L.) Kitam. b3d7
Fatsia japonica Decne. et Planch. b3c3b3
Ficus benjamina L. b3c3g3l1,2,5, 6
Ficus elastica Roxb. ex Horne. c3
Ficus elastica Roxb. ex Home. ‘Rubra’ l1,2,5, 6
Ficus microcarpa L. f. var. fuyuensis e
2
Fittonia argyroneura Coem. l1,2,5, 6
Fittonia verschaffeltii (Lem.) Van Houtte d7
References: a Aydogan et al., 2011; b Kim et al, 2008; c Kim et al., 2010; d Kim et al., in press; e Lui et al., 2007; f Orwell et al., 2004; g Son et al., 2000; h Tarran
et al 2007; i Wolverton et al., 1984; j Wolverton, 1986; k Wood et al., 2002; l Yang et al, 2009; mYoo et al., 2006.
VOCs: α-pinene 1; benzene 2; formaldehyde 3; hexane 4; octane 5; TCE 6; toluene 7.
16

Table 2. Plant species tested for their ability to remove selected VOCs.
Plant Species Reference Chemical Plant Species Reference
Chemical
Gardenia jasminoides Ellis c3
Guzmania sp. l1,2,5, 6
Haemaria discolor Lindl. c3
Hedera helix L. a3c3d7l1,2,5, 6m2,7
Hemigraphis alternata (Burm.f.) T. Anders ‘Exotica’ l1,2,5, 6
Howea belmoreana (C. Moore & F. Muell.) Becc. c3l1,2,5, 6
Howea forsteriana (C. Moore & F. Muell.) Becc. f2k2,4
Hoya carnosa (L.f.) R.Br. c3
Hoya carnosa (L.f.) R.Br. ‘Variegata’ e2,3,6, 7
Hydrangea macrophylla (Thunb.) Ser. e2
Ilex cornuta Lindl. & Paxton b3d7
Ilex crenata Thunb. c3
Jasminum polyanthum Franchet c
3
Jasminum sambac (L.) Aiton c
3
Laurus nobilis L. c3
Lavandula sp. c3
Ligustrum japonicum Thunb. c3d7
Maranta leuconeura E. Morren l1,2,5, 6
Melissa officinalis L. d7
Mentha piperita L. d7
Mentha piperita L. ‘Citrata’ d7
Mentha suaveolens Ehrh. d7
Mentha suaveolens Ehrh. ‘Applemint’ c3
Mentha suaveolens Ehrh. ‘Variegata’ d7
Microlepia strigosa (Thunb.) Presl. c3
Nandina domestica Thunb. c3
Nephrolepis exaltata (L.) Schott g3
Nephrolepis exaltata (L.) Schott ‘Bostoniensis’ e2
Osmunda japonica Thunb. c3
Pachira aquatica Aubl. c3g3
Pelargonium sp. c3
Pelargonium graveolens L’Her. ex Ait i4l1,2,5, 6
Peperomia clusiifolia (Jacq.) Hook c3
Peperomia clusiifolia (Jacq.) Hook ‘Variegata’ l1,2,5, 6
Peperomia obtusifolia (L.) A. Dietr. j3
Philodendron domesticum G. S. Bunting j3
Philodendron oxycardium Schott j3
Philodendron scandens K. Koch & Sello
ssp. oxycardium (Schott) G. S. Bunting l1,2,5, 6
Philodendron selloum C. Koch. c3j3
Philodendron sp. ‘Sunlight’ d7
Phoenix roebelenii O’Brien. c3
Pinus densiflora Siebold & Zucc. d7
Plectranthus tomentosus Benth. ex E. Mey. d7
Polypodium formosanum Baker c
3
Polyscias balfouriana Bailey c3
Polyscias fruticosa (L.) Harms l1,2,5, 6
Polystichum tripteron (Kunze.) Presl. c3
Psidium guajava ‘Safeda’ c3
Pteris dispar Kunze c3
Pteris ensiformis Burm. ‘Victoriae’ c3
Pteris multifida Poir. c3
Quercus acuta Thunb. c3
Quercus glauca Thunb. c3
Raphiolepis umbellata Makino c
3
Rhapis excelsa Wendl. c3
Rhapis humilis Blume g3
Rhododendron fauriei Franch. d7
Rosmarinus officinalis L. c3d7
Saintpaulia ionantha H. Wendl c3
Salvia elegans Vahl d7
Sansevieria trifasciata Prain c3j3l1,2,5, 6
Schefflera arboricola (Hayata) Merr. g3
Schefflera arboricola (Hayata) Merr. ‘Variegata’ l1,2,5, 6
Schefflera arboricola (Hayata) Merr. ‘Hong Kong’ c3
Schefflera elegantissima (Veitch ex Masters)
Lowry & Frodin d7l1,2,5, 6
Scindapsus aureus (Linden & André) G. S. Bunting g3i3j3
Selaginella tamariscina Spring c
3
Serissa foetida (L.F) Lam. c3
References: a Aydogan et al., 2011; b Kim et al, 2008; c Kim et al., 2010; d Kim et al., in press; e Lui et al., 2007; f Orwell et al., 2004; g Son et al., 2000; h
Tarran et al 2007; i Wolverton et al., 1984; j Wolverton, 1986; k Wood et al., 2002; l Yang et al, 2009; mYoo et al., 2006.
VOCs: α-pinene 1; benzene 2; formaldehyde 3; hexane 4; octane 5; TCE 6; toluene 7.
17

18
Table 2. Plant species tested for their ability to remove selected VOCs.
Plant Species Reference Chemical Plant Species Reference
Chemical
Soleirolia soleirolii (Req.) Dandy d7
Spathiphyllum sp. ‘Petite’ f2
Spathiphyllum sp. ‘Sensation” f2
Spathiphyllum sp. ‘Supreme’ e2
Spathiphyllum patinii (R. Hogg) N. E. Br. g3
Spathiphyllum wallisii Regel c3k2,4l1,2,5, 6m2,7
Spathiphyllum wallisii Regel ‘Clevelandii’ j3
Stauntonia hexaphylla (Thunb.) Dence. c3
Syngonium podophyllum Schott c3i3l1,2,5, 6m2,7
Thelypteris acuminate (Houtt.) Morton c
3
Thelypteris decursivepinnata Ching c
3
Thelypteris torresiana (Gaudich.) Alston ‘Calvata’ c3
Tillandsia cyanea Linden ex C. Koch c3
Trachelospermum asiaticum Nakai c
3
Tradescantia pallida (Rose) D.R. Hunt ‘Purpurea’ l1,2,5, 6
Tradescantia sillamontana Matuda j3
Viburnum awabuki K. Koch c
3
Zamia pumila L. c3
Zamioculcas sp. h2
Zamioculcas zamiifolia (Lodd. et al.) Engl. c3
References: a Aydogan et al., 2011; b Kim et al, 2008; c Kim et al., 2010; d Kim et al., in press; e Lui et al., 2007; f Orwell et al., 2004; g Son et al., 2000; h
Tarran et al 2007; i Wolverton et al., 1984; j Wolverton, 1986; k Wood et al., 2002; l Yang et al, 2009; mYoo et al., 2006.
VOCs: α-pinene 1; benzene 2; formaldehyde 3; hexane 4; octane 5; TCE 6; toluene 7.

Table 3. Variation in phytoremediation rates among species and experiments.
Chemical
Initial Concentration Plant Species Removal Rate
µg m-3 m-2 h-1 Citation
Formaldehyde
14-37 ppm Syngonium podophyllum Schott 574 Wolverton et al., 1984
Chlorophytum elatum
(Aiton) R. Br.var. vittatum 2,920
22 ppm Philodendron oxycardium Schott 832 Wolverton, 1986
Philodendron domesticum G. S. Bunting 95
1.5 ppm Ficus benjamina L. 18,582 Son et al., 2000
Schefflera arboricola (Hayata) Merr. 8,313
1.63 ppm Chrysanthemum morifolium
(Ramat.) Hemsl. 270 Aydogan et al., 2011
Dieffenbachia sp. 309
2.0 ppm Fatsia japonica Decne. et Planch. 1,000 Kim et al., 2008
Ficus benjamina L. 1,100
2.0 ppm Osmunda japonica Thunb. 13,300 Kim et al., 2010
Dracaena deremensis Engl. 1,300
Toluene
1.3 ppm Rhododendrom fauriei Franch. 928 Kim et al., 2011
Pittosporum tobira (Thunb.) W. T. Aiton 22
Benzene
25 ppm Howea forsteriana
(C. Moore & F. Muell.) Becc. 23,833 Wood et al., 2002
Spathiphyllum wallisii Regel 28,583
25 ppm Dracaena marginata Lam. 44,843 Orwell et al., 2004
Spathiphyllum sp. ‘Sensation’ 7,857
150 ppb Crassula portulacea Lam. 15,725 Lui et al., 2007
(continuous flow) Dracaena deremensis Engl.‘Variegata’ 1,281
25 ppm Aglaonema Schott 7,024 Tarran et al 2007
Zamioculcas sp. 4,152
Hexane
100 ppm Howea forsteriana
(C. Moore & F. Muell.) Becc. 168,000 Wood et al., 2002
Dracaena deremensis Engl. 15,292
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