``` 402-F-92-003 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 ------- UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 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 ------- fa 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 pollution . 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 , '4. I I 1(11 I ill I I t iCitltlll l!«iillJllliliiii»aiK IIIBIti:! Ill 111 II 1111 111 II 1111 II II II 111 II III ,11 I > I I I I II II I II 111 111 11II III 1111111 1111111 ------- ```