Can plant walls improve indoor air quality?

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Urban air quality has deteriorated over the last 60 years, and air pollution has become a serious threat to the health and wellbeing of city dwellers. It is known that indoor air can be at least 3-5 times more polluted than outdoor air, and as most office workers spend 90% of their time indoors, this presents significant health and productivity concerns. Plants have been shown to provide psychological and physiological benefits to building occupants. It is only in recent years that their phytoremediation benefits have been established, as potentially sustainable indoor biofilters. The aim of this study was to investigate the proficiency of a novel, optionally fan-aerated, plant wall system, in removing four major indoor air pollutants, CO₂, PM₁₀, PM₂ ̣₅ and VOCs. The planted modules were comprised with either Chlorophytum comosum or Epipremnum aureum. An airborne fungal spore assay was also conducted, to determine the biosafety of the system in an office environment. It was found that the plant walls did not proliferate hazardous fungi such as Aspergillus fumigatus, nor increase the fungal spore density in the office tested. Their density was less than one twentieth of the recommended WHO maximum of 500 cfu/m³. With sub-substrate fans operating, the modules were also highly efficient in reducing concentrations of PM₁₀ and PM₂ ̣₅, by upwards of 70% in 10 min, well below the Australian maxima. Benzene, a model VOC, was reduced by more than 60% over a 12 hr period, however the walls were more effective in passive rather than active mode. Plant uptake of PM₁₀, PM₂ ̣₅ and VOCs continues night and day. The walls were also shown capable of significantly reducing CO₂ levels. However, as expected, the uptake was highly dependent on the light intensity. Finally, an in situ survey of seven city buildings, all using air-conditioning systems (HVACs), found that the same type of plant wall modules did have some effect in reducing indoor CO₂ levels, depending on the areas of the walls involved. These findings show potential for plant walls to be used as a complementary means of reducing building air pollution, and hence the lowering energy usage of current HVAC systems. This innovation could reduce overall building energy use, thus improving future urban sustainability. To achieve this goal, further research is needed on the ability of a range of indoor plant species to reduce CO₂ concentrations, and optimum formulations for substrates, to ensure maximum improvements of building IAQ.
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