COMMENT ON THE USE OF PLANTS AS
                                                      A MEANS TO CONTROL IA POLLUTION
          Comment on the Use of Plants as a Means to Control Indoor Air Pollution

                                    John R Girman
                             Branch Chief, Analysis Brandt
                                 Indoor Air Division
                        U. S. Environmental Protection Agency
                                Washington, DC 20460

       Severalissues 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, 1
 from water and tnis 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 24 h 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
 equation C = C<,efc determines the concentration in ir.j chamber, where
                         ,       •        •     - >      •  •_ -       •  ' f         ^   -
              C  = concentration of the pollutant at time t,
              Q = the initial concentration of the pollutant,
              k = the first order pollutant removal rate constant,
              t = the time in hours since the beginning of the test. ,

 Rearranging the equation, we obtain

             -(l/t)ln(C/Co) = k.

 Since for our example, t = 24 h and C/C0 = 0.10, k or the pollutant removal rate is 0.096 h-i.
 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 process, ventilation; and (2) it allows us to extend realistically the pollutant
 removal rate of the plant to environments other than just a test chamber.

       The pollutant rempval rate of a plant in the test  chamber (with appropriate
1 considerations of scale) can be compared with ventilation rates (die most common pollutant
 removal process) of typical environments.  Office buildings have ventilation rates ranging
 from about 0.5 h-i (or half an air change per hour) to about 2 h-i. A typical residence "may
 have a ventilation rate of about 0.75 h-i and a tight  house may have a Ventilation rate of 0.25
 h-i.  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 and offices.                                               >         ,

      = -However, scale-up considerations are also important It appears that the average
      ter volume used in Wolverton's tests was 0.5 m?; 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
  JSv5£v?2.5°?£ ^l™1111*6 of a ^ical house in the US. is 340 m3 with a floor area of 139
  !^Q\'   " ' ""*?' the recommendation that one plant be used per 100 ft2, implies the use of
    >E e  *  a typical house. This would provide for 340 m3/i5 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
  toes 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
  Sm??? r«»°val rate which is only 1/46 of the rate it would have in the chamber, i.e., only
   •  r£ ; ""is, 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 poUutant removal rate as realized in the test chamber,  one
           y?   3VG ?6tSame IS™1* *****& U* 680 plants in a typical house (340 m3
            °-5 a*3 P^ 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 indoor air quality.  Similar concerns
 apply to the use of plants to control indoor air quality in office environments. It is hardly
 ^"?™S tnat the attempt to validate the test chamber results by Associated Landscape
 Contractors of America did not  provide measurable success.

       In addition, many of the reported tests relied upon a fan to  circulate air containing
 pollutants ; near the plant. This would serve to inflate the pollutant removal rate of a plant
 m a test chamber unless fans were also used to circulate air containing pollutants in a house
 or °™s flf e u56 Of fe*s 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 poUutant removal rate. This would not be
 practical in most indoor environments.
      '3fc  above jsnot intended as a criticism of small chamber testing. Small chamber
           **¥l in conjunction with modeling, is an important tool for improving indoor
            EPA has encouraged its use for source emission characterization, for product
comparisons and to evaluate various mitigation actions.

 - • _  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 poUutant
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
poUutant more of that same poUutant is being emitted at the same time by an indoor source
of that poUutant. These types of sources can be dealt with by chamber studies which
incorporate dynamic conditions, ie., continuous injection of a poUutant. In addition,
because mdoor environments, with few exceptions, always have some ventilation, realistic
                            1 	- vr.i-•!>.'•,:-2

 chamber tests usually incorporate some ventilation. The effect of this ventilation is easily
 accounted for by modeling.

       Using the same conditions as in the example above (0.5 nvf 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 
i                  III

                                  WASHINGTON, D.C.  20460
                                     FEB 241992
                                                                         OFFICE OF
                                                                      AIR AND RADIATION
 "Dear, Sir:     •"•     .           ;  '   y    '•   ., .-.  .-.-•• V-  •'.'''•.    ;     . \.

       1 am writing in response to your letter of January 12,1992, concerning EPA's position
 on the research conducted by Dr. Wolverton and others on the use of decorative foliage
 plants as an indoor air cleaning technology. Unfortunately, the position which EPA has
 taken with regard to this emerging technology has been misunderstood and I would like to
 make our position clear,

       EPA has no problem whatsoever with the use of plants in indoor environments for
 aesthetic reasons and most - if not all — of the Indoor Air Division staff have plants in our
 home and office environments because we like them. Your sources are mistaken if they are
 telling you that we are aggressively opposing the use of plants as indoor air cleaners. We
 are, however, skeptical of the claims being made by the decorative houseplant industry
 concerning the air cleaning capabilities of decorative foliage plants given the absence of any
 evidence that the technology is effective under real world conditions,  the absence of an
 EPA endorsement of this technology at this time, however, should not be construed as
 opposing further research in this area or as opposing in .any way the use of decorative
 foliage plants indoors.

       At this stage of research, however/ we consider the question of whether plants are an
 effective indoor air mitigation technology to be an open question since numerous
 significant uncertainties exist. For example:

       — While small chamber testing is extremely useful for certain purposes, the static
 conditions used in the Wolverton/NASA studies, (Le., no air exchange, enhanced air
 distribution, and one time injection of a pollutant vs. continuous emission typically
 encountered with many indoor sources) makes extrapolation to real world environments
 extremely uncertain. The lack of any successful validation studies is troublesome, given the
 degree to which this technology has been  embraced and promoted by the decorative
 houseplant industry.

      — To be a practical means of pollution control, the removal rate by plants must be
 comparable to that of other common pollutant removal processes, for example, ventilation.
 Our scale-up calculations, factoring in typical air exchange values in homes and offices,
 indicates that a much higher density of plants would be required (e.g. hundreds of plants in
 the typical house) to achieve the results you have postulated. No analyses of the relative
 effectiveness of plants vs. other removal mechanisms appears to have been undertaken in
 the studies we have reviewed. If the effect of plant removal mechanisms is overwhelmed by
common ventilation and air exchange rates, we question the practical utility of plants in
actually improving indoor air quality. It is also of concern if the industry'is implying that if

h°me °r
                                                             don't need adequate
                                       *™'*»8?**t conc^ over the potential for
                         *        become a source for unhealthy micro-organisms.
fBSSnofeS^SSlSftaSnS; w^S,    ^ acknowledgements are not visible in any

                                           l° COmment on the work Conducted by

precluding an :open-mmded .look at plant technology, has any basis in fact whatsoever. The
Agency has an excellent record of Interagency cooperation -on indoor air issues, as is
evidenced by the many documents and. projects which EPA has undertaken jointly with
other Federal agencies. In addition, we are conducting research in many areas/ of which
HVAC work is only a relatively small component

      Once agaln^ let me assure you that, we have rip desire to see research in this area
curtailed and hope that ss additional research is conducted, that the claims being made
today for houseplaBis as air cleaners will beWlidated, We do not particularly care to play
the role of spoiler for an otherwise desirable product and would welcome industry efforts
to conduct additional research in this area as well as to mfuse current promotional efforts
with some perspective on the aelative utiJitf ©f feotsseplanls versus other indoor air
   .   Attached for yoiar mformation a?e some additional comments from John Girman, the
head of our Analysis Branch, As always, we "would be happy to discuss any of these issues
with you farther as- well as to assist m jfoe dteslgn or review of future research protocols.

                        -•              Sncerely,
                                      Robert Axelrad, Director
                                      Indoor Air Division

                                                                    i:;u    i A" i,« .''n
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