Impact of plants on indoor air quality

An indoor environment that does not detrimentally affect our health and is comfortable to spend time in should be considered a fundamental human right. Previous studies have shown that various species of indoor plants are able to remove pollutants, including those shown to be both prevalent and harmful in indoor environments and contributors to poor Indoor Air Quality (IAQ). However, a lack of consensus exists on whether plants are able to improve IAQ in real dynamic environments with complex and everchanging ventilation, indoor sources and occupant numbers. Additionally, the processes underlying pollutant removal in plants are evidently equally complex, with numerous environmental parameters and likely physiological traits influencing species’ removal ability. In terms of Indoor Environmental Quality (IEQ), relative humidity regulation and control are vital for both occupant comfort and reductions in disease transmission – the latter ever more relevant with the current pandemic. However, humidity control indoors through mechanical systems is energy intensive, thus, exploration of plants as a passive technique is worthwhile. The focus of this study was to investigate a representative range of houseplants – with differing metabolisms, leaf types, and sizes – for their potential to improve indoor environments through pollutant removal (namely, CO\(_2\) and NO\(_2\)) and relative humidity regulation under differing environmental conditions and experimental scales. Alongside this, the study looked to address some of the inherent issues with plant-pollutant removal experiments in literature, namely, pollutants tested at much higher concentrations than what is typically measured in indoor environments. For CO\(_2\) removal, both studied experimental scales (leaf and chamber scale) drew the conclusions that for measurable removal supplementary lighting is required (at ~ 22 200 lux), and to elicit room scale concentration changes the number and density of plants offered by a green wall is necessary. At typical indoor light levels (0 – 500 lux), little potential is offered for CO\(_2\) removal, however, respiration rates were equally found to be negligible in terms of increasing CO\(_2\) concentrations at the room scale. The type of growing media (GM) was found to have a significant influence, with peat GM contributing to a greater reduction of CO\(_2\). Additionally, substrate moisture content (SMC) was deemed to have a negligible effect, especially when removal rates were extrapolated to the room scale. All studied plant types were able to reduce NO\(_2\) concentrations representative of a polluted urban environment to varying degrees at typical indoor light levels (0 – 500 lux). Few statistical differences were measured between differing environmental factors at the single plant scale namely, GM type, light level, and substrate moisture content. This research suggests that approximately five plants in a small, unventilated office could provide broadly similar health benefits in terms of life years saved, as are estimated to result from clean air policies in urban areas. As a method for measuring low VOC concentrations to improve the current plant-pollutant experimental methodology, little potential was offered by solid phase micro extraction (SPME) as a technique over what has been previously utilised, the gas-tight syringe. Moreover, we found that plant species which assimilated the most CO\(_2\) also contributed most to increasing relative humidity (RH) namely, Hedera helix and Spathiphyllum wallisii ‘Verdi’