Potted plants can remove the pollutant nitrogen dioxide indoors

Nitrogen dioxide (NO2) is a significant pollutant in both outdoor and indoor environments with exposure linked to serious respiratory illnesses, decreased lung function and airway inflammation. Here, we investigate whether potted plants can contribute as a simple and cost-effective indoor air pollution mitigation technique. Our study investigates the ability of the combination of the three plant species Spathiphyllum wallisii ‘Verdi’, Dracaena fragrans ‘Golden Coast’ and Zamioculcas zamiifolia with two different growing media to remove in situ concentrations (100 ppb) of NO2 in real-time at two typical indoor light levels (0 and 500 lx) and in ‘wet’ and ‘dry’ growing media conditions. All studied ‘growing medium–plant systems’ were able to reduce NO2 concentrations representative of a polluted urban environment, but to varying degrees. The greatest NO2 removal measured inside a 150 L chamber over 1-h period in ‘wet’ growing media at ~ 500 lx was achieved by D. fragrans. When accounting for dilution, this would correspond to a removal of up to 3 ppb NO2 per m2 of leaf area over the 1-h test period and 0.62 ppb per potted plant over the same period when modelled for a small office (15 m3) in a highly polluted environment. Depending on building ventilation rates and NO2 concentration gradients at the indoor-outdoor interface that will vary massively between polluted urban and rural locations, potted plants offer clear potential to improve indoor air quality—in particular in confined indoor spaces that are poorly ventilated and/or located in highly polluted areas

Interaction between plant species and substrate type in the removal of CO2 indoors

Elevated indoor concentrations of carbon dioxide [CO2] cause health issues, increase workplace absenteeism and reduce cognitive performance. Plants can be part of the solution, reducing indoor [CO2] and acting as a low-cost supplement to building ventilation systems. Our earlier work on a selection of structurally and functionally different indoor plants identified a range of leaf-level CO2 removal rates, when plants were grown in one type of substrate. The work presented here brings the research much closer to real indoor environments by investigating CO2 removal at a whole-plant level and in different substrates. Specifically, we measured how the change of growing substrate affects plants’ capacity to reduce CO2 concentrations. Spathiphyllum wallisii 'Verdi', Dracaena fragrans 'Golden Coast' and Hedera helix, representing a range of leaf types and sizes and potted in two different substrates, were tested. Potted plants were studied in a 0.15 m3 chamber under ‘very high’ (22000 lux), ‘low’ (~ 500 lux) and ‘no’ light (0 lux) in ‘wet’ (> 30 %) and ‘dry’ (< 20 %) substrate. At ‘no’ and ‘low’ indoor light, houseplants increased the CO2 concentration in both substrates; respiration rates, however, were deemed negligible in terms of the contribution to a room-level concentration, as they added ~ 0.6% of a human’s contribution. In ‘very high’ light D. fragrans, in substrate 2, showed potential to reduce [CO2] to a near-ambient (600 ppm) concentration in a shorter timeframe (12 hrs, e.g. overnight) and S. wallisii over a longer period (36 hrs, e.g. weekend)

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’

Thorn-dwelling ants provide antiherbivore defence for camelthorn trees, Vachellia erioloba, in Namibia

Ants are widely employed by plants as an antiherbivore defence. A single host plant can associate with multiple, symbiotic ant species, although usually only a single ant species at a time. Different plant-ant species may vary in the degree to which they defend their host plant. In Kenya, ant–acacia interactions are well studied, but less is known about systems elsewhere in Africa. A southern African species, Vachellia erioloba, is occupied by thorn-dwelling ants from three different genera. Unusually, multiple colonies of all these ants simultaneously and stably inhabit trees. We investigated if the ants on V. erioloba (i) deter insect herbivores; (ii) differ in their effectiveness depending on the identity of the herbivore; and (iii) protect the tree against an important herbivore, the larvae of the lepidopteran Gonometa postica. We show that experimental exclusion of ants leads to greater levels of herbivory on trees. The ants inhabiting V. erioloba are an effective deterrent against hemipteran and coleopteran, but not lepidopteran herbivores. Defensive services do not vary among ant species, but only Crematogaster ants exhibit aggression towards G. postica. This highlights the potential of the V. erioloba–ant mutualism for studying ant–plant interactions that involve multiple, simultaneously resident thorn-dwelling ant species