Potted-plants Can Significantly Reduce
Urban/Indoor Air Pollution
July 2012
Margaret D Burchett PhD,
Director, Plants and Indoor Environmental Quality (PIEQ) Group, and
Adjunct Professor, Faculty of Science  University of Technology, Sydney (UTS)
 
Summary statement
Urban buildings account for about one third of world energy use. International  research shows that plant installations could routinely be used to lower the energy load of city building air-conditioning, saving money and reducing the carbon footprint of the city, while improving health and wellbeing of building occupants. For example:
  • A University of Technology, Sydney (UTS) study47 with 60 offices (avr. area ~12 m2) in three buildings, found that threeDracaena deremensis ‘Janet Craig’ (300 mm pots), or Spathiphyllum wallisii ‘Sweet Chico’ (200 mm pots):
    –reduced total volatile organic compound (TVOC) levels by up to 75% (steadily maintaining values below 100 ppb; ie near external ambient, and regarded as of negligible respiratory health risk)40, and
    –lowered CO2 levels by from 10 to 25% and
    -carbon monoxide (CO) levels by 90%37.
  • An English survey of family homes (avr. area 100-110 m2) found that those with six or more potted-plants (various species and sizes) had nitrogen oxides levels more than one third lower than those without indoor plants9.
  • A Canadian project10,11 found that a building space fitted with plants maintained total VOC (TVOC) levels at or below those of the rest of the building, but with a ~50% lower air ‘refresh’ rates.
  • A Norwegian study17 found that introducing indoor plants into the workspace reduced sick-leave absences by more than 60%, and the improvement was maintained over the subsequent year of testing.
  • Studies across four primary schools and some 18 classrooms in the Brisbane area (Aust.), recorded that placement of three potted-plants (mixed species, in 200 mm pots) over the school term, resulted in 11-15% higher performance in mathematics and English than for those without plants51,52.
 
These results are in line with numerous other studies showing the capacity of indoor plants to remove urban/indoor air pollutants and improve wellbeing of building occupants.
 
A background and summary of examples of the growing world body of evidence, are presented below.
 
Indoor plants can remove all types of urban/indoor air pollution
International research shows that, like other plants, indoor potted-plants can remove all major types of urban/indoor air pollutants (90% of which come from fossil fuel emissions), including nitrogen oxides9,43,49 sulfur oxides24, ozone30,33, carbon monoxide12,20,26,43, carbon dioxide4,7,8, ‘air toxics’ (i.e. VOCs)6,44-48,50, polycyclic aromatic hydrocarbons (PAHs)19, and particulates28.
 
And indoor air pollution is almost always from 2 to 10 times (and sometimes 100 times) higher indoors than outside15,42, because of two main classes of air contaminant which are always in higher concentrations indoors, since they also arise from indoor sources. They are: more VOCs, from low, continuous emissions from plastic/synthetic furnishings, fittings and equipment5,19; and higher CO2, because of occupant respiration1,16. The cocktail of air contaminants, even at imperceptible levels, can produce symptoms of sick building syndrome/ building-related illness (eg sore nose, eyes, throat, wheezing, loss of concentration, slight nausea and/or dizziness)16,40.
 
Potted plants remove VOCs and CO2 from indoor air
All biotechnology research needs a combination of complementary laboratory and real-world field studies, as presented below. Field studies report correlations (eg plant presence and lowered air pollution levels), while laboratory studies are needed to establish cause-effect and dose relationships (eg with different species, specific test VOCs, and measured concentrations and exposures). 
 
(a) VOC removal
(a.1)Field studiesThe US EPA has identified over 900 VOCs that have been found in indoor air39, and in any one building it is common to find a mixture of between 10 and 300 different substances5,47. As mentioned earlier, an initial UTS field study showed that plant treatments reduced TVOC loads by up to 75%, always to below 100 ppb; meanwhile, ambient (non-planted) indoor levels were sometimes as high as 350-500 ppb47. However, a second UTS office study7, in two newer buildings, with similar plantings, recorded much lower removal rates (maxima of 5-10%) for either type of contaminant, because of the more up-to-date, hence efficient air conditioning (HVAC) systems. The results of the two studies, taken together, indicate that plant installations could be used to lower building ventilation needs, and hence reduce ‘building hyperventilation’.
 
(a.2)  Laboratory studies A range of test-chamber studies, including altogether over 200 species and 20 test VOCs, demonstrate conclusively that indoor plants have a universal, very high capacity to remove any type of VOCs3,6,23,27,44-48,50. Several detailed studies at UTS (ie lasting weeks, rather than the more usual testing time of several hours), using a total of 16 species and four test VOCs, have been able to demonstrate conclusively that the main VOC removal agents are normal root-zone bacteria (functioning steadily in light and dark – 24/7); the plant is acting symbiotically to nourish and maintain its root-associated microorganisms, and influences small species-specific differences in VOC removal rates6,7,31,32,47,48.  The fact that removal is primarily brought about by the substrate microorganisms explains why a robust capacity for VOC removal is to be found in every species tested, and with every VOC tested, which strongly suggests that most other species would share this ability.
 
This conclusion is also consistent with the results of a UTS comparative study of three species, using benzene as test VOC, in bench-top test chambers (216 L).  The rates of benzene removal in plants in 200 mm pots were identical with those of larger plants in 300 mm pots, and a batch of two to three smaller plants in 125 mm pots also worked just as fast6.  In all of the treatments, once the response had been induced by initial exposure to the VOC, repeated 5 ppm doses were completely removed in an average of 22 hours. (The dosage was chosen because 5 ppm benzene is equal to the maximum allowable occupational 8-hr averaged exposure concentration in Australia).  The findings are a further indication of the robust capacity of the substrate microorganisms to absorb and degrade VOCs. And the most recent UTS study shows that the indoor plant VOC removal system works in hydroculture as well as in standard potting mixtures21.
 
(b) CO2 removal – findings
(b.1)Field studiesIt is well known that, with adequate light, any green plant will photosynthesise, in the process absorbing CO2 and emitting equimolecular amounts of O2 – a two-way air-freshener system for the planet. Indoor plants, being very shade-tolerant, can absorb CO2 at very low light intensities, but even so, getting adequate light inside buildings can be problematic. Nevertheless, as mentioned above, in the first UTS office study the plantings used resulted in CO2 reductions of from 10 to 25%37. However, in the second study7, as with the VOCs, only very low reductions (~5%) were recorded. The two UTS office studies taken together show that plant installations could be used routinely to reduce the HVAC energy load of city buildings, saving money and lowering the carbon footprint of the city.
 
(b. 2) Laboratory studies  Precise light/shade tolerances and photosynthesis rates are species-specific38; and even within any one species, CO2 uptake rates depend not only on leaf area, but also on factors such as substrate moisture, surrounding humidity, etc. The interior planstscape industry has categorised species into grades of shade tolerance.  A recent UTS study of nine indoor species8, across three different industry-based grades of tolerance, found that the three potted-palm species trialled were the most tolerant of ‘normal’ (i.e. low) indoor light intensities (in the range 10 – 30 µmol PAR* m-2 sec-1, as found among spaces with or without windows) namely: Howea forsteriana, (Kentia palm); Chamaedorea elegans (Parlour palm); Dypsis lutescens (Golden Cane Palm). It was also found that Aspidistra elator (Cast Iron Plant), Aglaonema commutatum (Chinese evergreen), and Dracaena deremensis ‘Compacta’ performed satisfactorily, as did Spathiphyllum ‘Petite’ in an earlier study4.
 (*PAR – quanta of photosynthetically active radiation – a slightly narrower spectrum range than that of visible light; intensity of full  sunlight is about 2000 µmol PAR m-2 sec-1,)
 
Conclusions
  • Indoor plantings already in use are helping cleanse indoor air, though their contribution may be masked by modern building HVAC systems; they could readily be designed and fitted specifically to reduce building energy costs and the carbon footprint of the city. 
  • It has been demonstrated also, from other studies, that indoor plants improve occupant health and wellbeing, reduce illness symptoms and sick leave percentages, and more than pay for themselves in increased productivity7,13,14,17,22,25,29, 34,36.   
  • Indoor plants can therefore be used to contribute in achieving the triple-bottom-line of city sustainability: social-environmental-economic.
 Acknowledgements
Thanks to my PIEQ group colleagues, Dr Fraser Torpy, Jason Brennan and Peter Irga, who carried out most of the recent UTS work cited here.  Thanks also to numerous other UTS colleagues, past and present, many of whom participated in our office studies.  Thanks also to the funding bodies that have supported this research: the National Interior Plantscape Association (Australia), Ambius, Horticulture Australia Ltd, and UTS.
 
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