Proceedings of Healthy Buildings 2009 Paper 667
Critical Review: How Well Do House Plants Perform as Indoor Air
, Tom Phillips
and Hal Levin
California Air Resources Board
Building Ecology Research Group
Corresponding email: firstname.lastname@example.org
In the late 1980’s, research indicated that plants had the capability to remove volatile organic
compounds (VOC) from indoor air. The findings were based upon chamber studies involving
injection of a pollutant into a small, sealed chamber and following the pollutant decay, with
and without plants present. The results were striking with removal rates up to 90% in 24 hr.
Other studies examining this effect followed. Today, even a casual search of the internet will
find many articles extolling the benefits of using plants as indoor air cleaners. However, there
has been little critical analysis of the application of plants to actual indoor environments and
only a few field studies have been conducted. A critical review of results of both laboratory
chamber studies and field studies leads to the conclusion that indoor plants have little, if any,
benefit for removing indoor air of VOC in residential and commercial buildings. Finally,
recommendations for improving future studies are presented.
Plants, pollution reduction, VOC, air cleaning
Using plants indoors to control indoor air pollution is an attractive, popular concept and many
articles in the popular press and internet extol and promote their use as indoor air cleaners.
Today, a search of the internet will find many articles promoting the use of plants as indoor
air cleaners. While several scientific papers have been published on studies of pollutant
removal by plants in small test chambers under controlled conditions, as yet, there has been
little critical analysis of the studies and their results. Far fewer field studies have been
published. This paper will briefly review results of both laboratory chamber studies and field
studies, followed by a critical analysis of these results and the implications for indoor air
cleaning. Finally, recommendations for improving future studies are presented.
STUDIES OF POLLUTANT REMOVAL BY PLANTS
In the late 1980’s, published research indicated that plants had the capability to remove
pollutants from indoor air (e.g., Wolverton et al., 1989). The findings were based upon
studies involving the introduction of a pollutant or pollutants into a small, sealed chamber.
The chamber volumes typically ranged from 0.31 to 0.88 m
. Many pollutants were studied,
including benzene, xylenes, tricholorethylene and formaldehyde at concentrations of ~15 to
20 ppm. The decay of the pollutant concentration over time, with and without plants present,
was then followed. The reported results were striking, with reductions that averaged 10 to
70% in a 24-hr period. Wolverton and colleagues later conducted tests on the removal of
benzene and trichloroethylene at concentrations ranging from 0.1 to 0.4 ppm. The reported
reductions ranged from 9.2 to 90%.
Proceedings of Healthy Buildings 2009 Paper 667
Studies examining this effect by other researchers followed. For example, Wood et al. (2003)
used small chambers (0.22 m
) and several plant species to study the removal of benzene and
hexane over 24 hours from initial concentrations of 25 ppm for benzene and 100 ppm for
hexane. Quantitative results were not given for the concentration reductions but estimated
concentration vs. time plots indicate reductions by potted plants exposed to daily
introductions of pollutants of ~80% for benzene and ~70% for hexane.
To test the validity of laboratory results, the Associated Landscape Contractors of America
(ALCA) worked with Healthy Buildings International to conduct a field experiment (HBI,
1992). HBI sampled for toluene, xylene, 1,1,1-trichlorethane and benzene for several months
in two very similar floors of an office building in Arlington, VA, USA. Identical ventilation
systems on both floors had their outdoor air damper set and unchanged for the duration of the
study. For the first month, no plants were on either floor; for the next four months, plants
were only on the 9
floor; and for the last four months, plants were on both the 9
and the 11
floors. The number of plants installed by ALCA was not reported but is probably consistent
with the ALCA recommendation of one plant per 9.29 m
). Pollutant concentration
maxima were all in the 10’s to 100’s of ppb range: toluene, ~210 ug/m
; xylene, ~300 ug/m
1,1,1-trichloroethane, ~700 ug/m
; and benzene, ~18 ug/m
. The presence of plants produced
no reduction of pollutant concentrations. The authors concluded that the “levels of VOCs on
the ninth floor remained essentially the same as those on the eleventh floor throughout the
duration of the study.”
Dingle et al. (2000) reported on a field study of three portable office buildings in Perth,
Australia to test removal of formaldehyde by plants. Five plants (five species) were added to
each room every two days to a maximum of 20 plants (at 2.44 plants per m
) after nine days.
Two adjacent portable office buildings were used as controls with no plants. The mean
formaldehyde concentrations were about 850 ppb, except with 20 plants. The authors state
that the results show “no change in formaldehyde concentrations with the addition of 5 or 10
plants in the rooms and only an 11% reduction in formaldehyde concentrations with 20 plants
in the room.” They did not indicate that this reduction was statistically significant.
Wood et al. (2006) reported on field studies of pollutant reductions using plants in three office
buildings in Sydney, Australia. In one building, the nine offices studied were served by three
separate air-conditioning systems; in the second building, the eight offices studied were
served by a single air-conditioning system, supplying about 0.6 to 1.2 outside air changes per
hour; and the third building was naturally ventilated with windows almost always closed
during the study. In the third building, nine offices were studied in the first phase, and eight
offices were studied in the second phase. All offices were designed for single occupancy and
had 10-12 m
in floor area. Five-minute samples of total volatile organic compounds (TVOC)
were measured weekly with a portable photoionization detector, and individual VOC were
measured using passive samplers and gas chromatography/mass spectroscopy.
In the first and third buildings, after one month of pretesting, subsets of three office buildings
were randomly supplied with 0, 3 or 6 potted “Janet Craig” plants. Weekly measurements
were made over nine weeks and then the potted plants were randomly reassigned among the
offices for a second nine-week period. For the second part of the investigation, two types of
potted plants were used in the second and third buildings. After one month of pretesting, four
offices were randomly supplied with 0 or 6 plants. Air was sampled for nine- and five-week
periods in the second building and for nine weeks in the third building.
Proceedings of Healthy Buildings 2009 Paper 667
With no plants in the first and third buildings, the mean indoor TVOC concentration was 110
+ 15 ppb. Periods with 3 or 6 plants had a pooled TVOC concentration of 80 + 7 ppb, a 27%
reduction but only at p < 0.09. TVOC concentrations were identical with either 3 or 6 plants.
When only periods with TVOC concentrations greater than 100 ppb (9 of 18 weeks) were
used to calculate means, the reductions were statistically significant (p < 0.05): 0 plants,
mean concentrations 190 + 40 ppb; 3 plants, 105 + 15 ppb; and 6 plants, 100 + 10 ppb.
Results of the nine-week study in the second investigation were similar. Concentrations for
14 individual VOC are also reported. No trend is evident from these data: individual VOC
concentrations with 6 plants appear to be randomly higher or lower than those with no plants.
At first glance, the pollutant reduction by plants in chamber studies seems remarkable.
However, closer examination suggests otherwise. Little has changed in terms of quantitative
VOC reductions by plants in chamber studies since the early Wolverton studies, i.e., the best
result for the removal of a single injection of a VOC remains about 90% in a 24-hour period.
Thus, the conclusions of a previous analysis using a mass-balance model by Girman (reported
by Levin, 1992) are still valid. This analysis concluded that pollutant removal in a chamber of
90% in 24 hours was only 0.096 hr
, less than the removal achieved by the natural ventilation
rate of a very tight house (e.g., 0.2 h
). Moreover, this removal was achieved with a plant
loading in chambers (approximately one plant per 0.5 m
) far in excess of what would be
reasonable for indoor environments. To achieve results equivalent to those of chamber
studies would require 680 plants for a 340 m
) house. Yet ACLA recommends one
plant per 9.29 m
and a reduction in plant loading to 1 plant per 0.5 m
means that the
plants in such an environment would have a removal rate equivalent to only 0.002 hr
Significant methodological issues also plague these chamber studies. The chamber test was a
static test method, i.e., pollutants are injected and then the pollutant decay is measured. This
does not mimic the behaviour of pollutants such as formaldehyde that are continuously
emitted. Reductions for pollutants continuously emitted would be much lower. In addition,
pollutant removal rates in these studies are too often reported as only the percent removed,
rather than mass of pollutant removed per hour per plant. This makes it difficult to translate
the results to other scenarios, e.g., to proposed use in an actual room or building, or to
compare this method to more traditional pollutant removal methods such as ventilation or air
cleaning with filters or sorbents. It should also be noted that the chamber studies used
pollutant concentrations an order of magnitude or more higher than those generally found in
indoor environments. Also, many chamber studies employed air circulation fans, which
would tend to increase pollutant losses to interior surfaces.
Results from the field studies are more difficult to assess. The methods used to measure
formaldehyde (passive monitor) and VOCs (passive sampler, photoionization detector) are not
very accurate. In addition, although ventilation dominates the VOC removal processes in
virtually all real world buildings, ventilation was not measured in any of these studies. It is
not possible to obtain meaningful quantitative results of pollutant removal in a field study
without also measuring ventilation rates. The ventilation rate variability in most buildings is
simply too large a confounder.
With this caveat firmly in mind, it is hardly surprising that the HBI study failed to find any
effect on pollutant removal by plants, despite a reasonably strong study design in terms of
using controls. The study by Dingle et al. found only an 11% reduction in formaldehyde with
Proceedings of Healthy Buildings 2009 Paper 667
the highest loading of plants (20 plants in a room or a loading of 2.44 plants per m
), which is
not feasible in the real world and is probably not statistically significant.
Only in the field study by Wood et al. are pollutant reductions statistically significant, and
then only when indoor TVOC concentrations are above 100 ppb. However, these results are
not consistent with the fact that doubling the number of plants did not cause a statistically
significant reduction (i.e., a reduction of only 105 + 15 ppb to. 100 + 10 ppb) and with the fact
that individual VOC concentrations did not appear to be reduced. It is possible that variations
in ventilation may have been responsible for any apparent pollutant reductions. In this regard,
the indoor carbon dioxide (CO
) concentrations in the buildings ranged from 285 to 420 ppm
was not sampled), suggesting that building ventilation rates were high,
occupancy was low, or both conditions existed during the study. It is also likely that sampling
for TVOC for 5 minutes per week is insufficient to characterize indoor concentrations.
Several laboratory studies have shown that plants can remove airborne VOC. However, a
careful examination of studies does not find convincing evidence that the use of plants indoors
can result in meaningful reductions in indoor VOC concentrations. Several improvements
should to be made to studies intended to demonstrate that plants can be used to improve
indoor quality. Concentrations used in chamber studies should be representative of
concentrations found in actual indoor environments. Also, such studies should use analytical
methods of high accuracy and sensitivity to measure VOC concentrations and should focus on
individual VOC. They should also use mass-balance models to design and assess study
results. Chamber study results should be reported as mass of pollutant removed per hour per
plant to facilitate comparisons with other removal methods to assist building designers,
managers and owners in determining whether using plants is an appropriate pollution control
technique. For the same reasons, plant loadings should be reported. Finally, to be
convincing, any field study must also measure ventilation rates since ventilation rates
typically dominate pollutant removal processes. At present, it is premature to recommend that
using plants indoor is viable means of controlling indoor air pollution.
The opinions expressed in this paper are those of the authors and do not necessarily reflect the
positions of the California Air Resources Board.
Dingle P, Tapsell P, and Hu S. 2000. Reducing formaldehyde exposure in office
environments using plants. Bull. Environ. Contam. Toxicol., 64, 302-308.
HBI. 1992. Can plants help clean up the indoor air?, Healthy Buildings International
Magazine, 2(1), 10-11.
Levin H. 1992. Can house plants solve IAQ problems? Indoor Air Bulletin, 2(2), 1-5.
Wolverton B.C, Johnson A, and Bounds K. 1989. Interior landscape plants for indoor air
pollutant abatement, Final Report – Sept 1989. Stennis Space Center, National
Aeronautics and Space Administration, Mississippi, USA, 25 pages.
Wood R.A, Orwell R.L, Tarran J., Torpy F, and Burchett M. 2003. Potted-plant/growth
media interactions and capacities for removal of volatiles from indoor air. In:
Proceedings of Health Buildings 2003 – HB2003, Singapore, 1, pp. 441-445.
Wood R.A, Burchett M.D, Alquezar R, Orwell R.L, Tarran J, and Torpy F. 2006. The
potted-plant microsm substantially reduces indoor air voc pollution: I. office field-study.
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