Research suggests succulents such as Zamioculcas zamiifolia could be effectively used in indoor green walls
Plants feature in many ways within indoor environments – from simple houseplants to complex, species-rich green walls – and they offer multifaceted services, including pollutant removal and reduction of building energy consumption.
This review identifies pollutants that have been measured at harmful concentrations indoors and gives health assessments of each. It explains which plants remove ‘2019’s priority pollutants’ effectively and directs research to those that have not been investigated. Finally, it consolidates the current research, presenting why plants should be considered a building service.
Plants deliver an array of benefits indoors, offering improvements in human health (pollutant removal) and in building energy consumption by the removal of carbon dioxide (CO2) and relative humidity (RH) regulation – which, in turn, reduces ventilation requirements (1, 2).
Numerous airborne pollutants are present in indoor environments: these include volatile organic compounds (VOCs), inorganic gases/vapours (CO2, nitrogen dioxide – NO2) and particulate matter (PM) (3). The main sources of such pollutants are indoor human activities, construction materials, and the infiltration of outdoor-produced particles and pollutants. (4-6)
Activities such as cooking, cleaning and painting produce numerous indoor pollutants (6,7). In addition, the closure of windows – and a push for more tightly sealed buildings in an attempt to reduce energy consumption – leads to an accumulation of indoor pollutants (5, 8).
In western Europe people are commonly exposed to more than 20 hours per day of indoor air (9).Thus, quantifying the concentration of indoor pollutants with relevant safe exposure guidelines/standards is imperative – a guideline is based upon scientific evidence or expert opinion and a standard contains enforceable regulations adopted by regulatory authorities (10). Such health guidelines produced by organisations such as the World Health Organisation (WHO) and US Environmental Protection Agency (EPA) contain only a limited number of indoor pollutants (6, 11).
Indoor pollutants cause an array of acute and long-term (chronic) health problems, contribute to poor indoor air quality (IAQ) and are the probable cause of sick building syndrome (SBS), a phenomenon describing health issues experienced by the occupants of a building, caused by spending time within the building but, where no specific cause can be found (6, 12). Moreover, indoor pollutants also react with ozone and produce radicals and secondary organic aerosol (SOA) – both, considered harmful to health (13, 14).
Indoor pollutants vary in toxicity and prevalence. Prolonged exposure to an indoor pollutant, at a concentration greater than the recommended guideline, can cause symptoms such as mild sensory irritation (in the presence of alpha-pinene, for example) to significant respiratory problems (NO2) and cancer (benzene) (6, 15).
Indoor plants have been shown to remove a wide variety of organic and inorganic pollutants (1, 16), PM (17-19) and ozone (20, 21). Houseplants can also help alleviate the symptoms of sick building syndrome (SBS) (22-24).
High indoor concentrations of CO2 are harmful to human health, increase absenteeism and reduce cognitive performance (25-30), so HVAC systems are designed to keep concentrations low, with ventilation increasing energy consumption (31). Indoor plants can act as a simple, low-cost ventilation surrogate, contributing to CO2 removal indoors and reducing the requirement for traditional HVAC systems by about 6%.
Indoor plants can also reduce energy consumption by increasing RH. HVAC systems typically attempt to keep RH in the range of 40-60% – where the majority of adverse health effects can be avoided (32). A RH that is either too high (> 60%) or too low (< 40%) can cause health and building issues (32). High RH encourages fungal and mould growth, and contributes to the deterioration of building materials (33-36). Low RH can cause dryness of the eyes, skin and mucus membrane, enhance indoor ozone, increase the likelihood of influenza transmission, and exacerbate problems of static electricity (32, 33, 35, 37-39).
Our review aims to improve the current understanding of which indoor pollutants – and at what concentrations – are harmful to health.
A systematic review of the literature was conducted to determine the indoor pollutants measured in home environments, up to and including 2018.
Logue et al compared indoor pollutant concentrations with relevant health guidelines produced by the Environmental Protection Agency (EPA) and California Office of Environmental Health Hazard Assessment (OEHHA) for 67 home environments between 1998 and 2010. They identified nine ‘priority’ indoor pollutants (see Figure 1, which does not include butadiene) considered to be harmful. (40) All were chosen on the basis of the measured concentration data exceeding health guidelines and the number of homes affected.
Since 2010, an assessment of ‘Logue’s priority pollutants’ and their mean concentrations in indoor environments has not been carried out. So we have used data from home environments after 2011 to determine if concentrations of these nine pollutants have changed since.
Furthermore, we compare the post-2011 results with up-to-date chronic health guidelines produced by the World Health Organization (WHO) and US EPA (Figure 1). Any pollutants with an average long-term concentration greater than the appropriate guideline will be designated a ‘2019 priority pollutant’.
The data collected in Figure 1 suggests that the mean concentrations of four indoor pollutants have increased in studies after 2010 – namely, benzene, naphthalene, NO2 and PM2.5. Reductions in concentrations of acetaldehyde, acrolein, dichlorobenzene – 1,4 and formaldehyde were measured, perhaps because of a large body of research focusing on lowering pollutant emissions from building materials (52-54).
Acetaldehyde, benzene, formaldehyde, and NO2 are the indoor pollutants commonly measured at concentrations greater than the appropriate guidelines (Figure 1) – causing long-term health issues and, thus, being classified as 2019’s priority pollutants.
Forty studies have investigated numerous indoor plants for their ability to remove the ‘2019 priority pollutants’ benzene (> 45 species/cultivars) and formaldehyde (> 150 species/cultivars). The results from the most robust, well-cited and informative studies from these have been selected and are presented in Table 2.
To the author’s knowledge, no studies have investigated the potential of indoor plants to sequester either acetaldehyde or NO2 – although the removal of NO2 by outdoor plants has been thoroughly studied, with promising results (71, 72).
Plants as a building service
The main sources of CO2 indoors are human respiratory emissions and the outdoor air-supply rate. Several health guidelines exist for maximum safe CO2 concentrations, with the lowest eight-hour guideline being recommended by ASHRAE, at 1,000ppm. (1,77)
A number of studies have focused on indoor plants and their ability to reduce CO2 concentrations, with several focusing on houseplants specifically. Studies vary in scale and focus, but most use experimental chambers enclosing a single or small number of houseplant species.
Studies generally find that significant reductions can occur with the correct environmental conditions: namely, the light level. We found that raising the light level to 22,000 lux – made achievable with supplementary LED lighting – increased the CO2 removal 50-fold in some species.
Moreover, we estimated that 15 spathiphyllum wallisii verdi – a number that could, realistically, be installed in a small green wall – could offset 10% of a human’s respiratory contribution. A similar study by Torpy et al found that a 5m2 green wall containing chlorophytum comosum could balance the respiratory emissions of a full-time occupant using a similar lighting level.
Along with high CO2 concentrations in indoor environments, an additional challenge is extreme RH (low < 40% and high > 60%). Both can cause previously described issues, mainly concerning human and building health. Several studies have investigated the effect of indoor plants on RH, with mixed results. Indoor plants have been shown to increase, decrease and have no statistically significant effect on RH indoors. Houseplants release water vapour into an environment through transpiration and would be expected to increase RH indoors.
Plant-species choice and ventilation rate would both significantly influence results, and most likely explain the mixed results in literature. However, correct employment of indoor plants, with species consideration, could help reduce the energy consumption of HVAC systems.
Our research suggests that less physiologically active plants – such as Guzmania sp, dracaena fragrans and succulents such as zamioculcas zamiifolia – could be used in larger numbers (10+), as part of indoor living walls within even smaller offices, without a risk of raising office RH above 60%.
Conversely, hedera helix (ivy) and spathiphyllum (peace lily) would be suitable in smaller numbers (five or fewer), or in larger rooms with greater overall volume, where their RH-influencing effect would be diluted.
A significant body of research has looked at the ability of plants to remove indoor pollutants such as VOCs. Most, however, focus on pollutants that are detected infrequently indoors or at concentrations too low to damage human health. Experiments also commonly test pollutant concentrations that are not measured in real life (in situ).
This review highlights the range of concentrations present in situ and which indoor pollutants can be considered unsafe, to help direct future research.
Experiments suggest that the growing substrate, and the microorganisms within, are predominately involved in the removal of pollutants; plants themselves are only used indirectly to maintain and support substrate microorganisms.
Results generally suggest that the plant-substrate system can remove a wide variety of pollutants, but – with a lack of testing at in situ concentrations – extrapolation of the results to room level lacks accuracy. Further experiments should focus on the untested 2019 priority pollutants identified in this review, acetaldehyde and NO2 – preferably at in situ concentrations.
Certain houseplants can remove CO2 at significant quantities that would affect room-level concentrations, but only with the correct environmental conditions – for example, light level. Studies often suggest that a greater number of potted plants than would be feasible indoors (1, 2, 83) are required to measure concentration reductions, so the density provided by green walls would be more suitable.
Studies are now beginning to investigate green walls (77) and, additionally, how the substrate may influence removal – as measured with VOCs. RH literature has produced conflicting results.
Anecdotally, plants would be expected to increase RH indoors (2, 85, 86) , but this is not always the case (24, 84). We suggest a ‘standard’ method be devised – controlling chamber/room size and ventilation rate – to facilitate effective comparison between different plant species.
We believe plants should not be considered as a single entity, expected to provide all the above described benefits. There is large performance variability between species, so we recommend consulting literature to ascertain their suitability for a given benefit.
Although some benefits of indoor plants are less clear, when considered as a whole – with all the benefits combined – we believe plants should be considered as a building service, alongside traditional ventilation systems.
Read the full paper at www.cibse.org.uk/symposium
About the authors
Curtis Gubb and Christian Pfrang, Department of Earth, Geography and Environmental Science, University of Birmingham CXG857@student.bham.ac.uk
Tijana Blanusa and Alistair Griffiths, Science Department, Royal Horticultural Society
Curtis Gubb’s PhD project, which forms part of this work, is funded by the Royal Horticultural Society (RHS) and the Engineering and Physics Research Council (EPSRC).
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