We set out to quantify how much of an impact simple potted houseplants that are widely available can have on indoor air quality. This work started when I was based in the Chemistry Department at the University of Reading discussing air pollution with Dr Tijana Blanusa, a horticultural scientist at the Royal Horticultural Society (RHS) and visiting scientist at Reading. With Tijana’s plant-focussed expertise and mine in atmospheric chemistry we designed a truly cross-disciplinary PhD project that was funded jointly by the Engineering and Physical Sciences Research Council (EPSRC) and the RHS. We already had a summer student working on a related project looking at green-roof plants, so it didn’t take long to identify the best candidate to carry out this work: Curtis Gubb was the perfect choice and happily accepted to stay on for a PhD. Thanks to Curtis’ enthusiasm and dedication, the work continued seamlessly when I decided to move to the University of Birmingham where all the experiments reported on our latest paper were carried out.
The key aims of the work were to establish how efficiently different indoor pollutants are removed by a carefully selected range of common houseplants. Thanks to the RHS involvement, we were able to monitor and control the plants’ health and activity during these studies which often is a shortcoming of these kind of experiments. We ensured plants are grown in identical conditions and found that we had to limit our measurement periods to one hour, to avoid plants getting “stressed” in our measurement chamber which would lead to completely different plant behaviours than encountered under natural conditions.
In the end, we decided to focus our experiments on removal of carbon dioxide (CO2), the impact on relative humidity (RH) and removal of nitrogen dioxide (NO2) by a total of seven different common houseplants. All these experiments were carried out in a 150 litre chamber where we could fully control the flow and composition of the air as well as the availability and energy of light. We picked this set-up to have as much control as possible of external factors to establish reproducible and quantifiable effects – there are of course limitation when applying the results from these chamber experiments to the real world (e.g. an office or a classroom) especially related to uncertainties about ventilation/air exchange in these rooms as well as intermittent pollutant sources and other sinks. Nevertheless, our well constrained results in a small chamber should be a good basis for estimates of the potential impact on indoor air quality when taking into account dilution and ventilation effects in the real world.
Our first results published in 2018 (Gubb et al., 2018) and 2019 (Gubb et al., 2019) showed that CO2 removal under normal indoor light levels will be very modest, so unless particular lighting arrangements are implemented (that would negatively affect the energy consumption of the indoor space), the impact of CO2 removal by indoor plants will be very limited especially in densely occupied indoor spaces given the amount of CO2 exhaled by people; the plants may have a positive impact by increasing RH, though. The ability of houseplants to provide a range of building services should thus be considered (Gubb et al., 2020).
Our latest work published in February 2022 (Gubb et al., 2022) demonstrated that unlike the CO2/RH effects, NO2 removal does not depend on light levels, so that a significant removal was observed without any further energy input (even a pot of soil without the plant removed NO2 significantly in our experiments, but this would be aesthetically much less pleasing than a peace lily!). NO2 was removed to similar extents by all of our three test plants for the NO2 experiments with the peace lily coming just top of the class (followed by the dragon tree and ZZ plant). So while the plants we chose were all very different from each other, they all showed strikingly similar abilities to remove NO2 from the atmosphere.
Based on the results in our chamber, we made estimates how much NO2 could be removed in offices of different sizes. For the size of my office (15 m3) with poor ventilation, five plants could potentially reduce a peak NO2 concentration of 100 ppb (regularly encountered near very busy roads in urban centres) by up to 20% with effects for larger and better ventilated offices being substantially smaller.
Part of the difficulty in getting a definitive verdict on these effects is that real-world homes and offices will have new air and new pollutants flowing in and out all the time and also that the long-term fate of the NO2 taken up by the plant-soil system is still a mystery.
Future research is likely to look at the exact mechanisms that these plants are using to remove NO2 from the air, and how they compare to 'active green walls' – entire walls of plant life that are likely to be much more effective at cleaning up air.
This research links directly to ongoing work on Air Pollution Solutions for Vulnerable Groups (CleanAir4V) and in a related project that has just started, we will be designing sophisticated tools for modelling air quality indoors encompassing a much wider range of variables. The new project, funded by the Met Office, will use mobile air quality measuring instruments to identify pollutants and test their effects in both residential and occupational spaces, producing a wealth of data to inform the tool’s development.
Gubb, C., Blanusa, T., Griffiths, A. and Pfrang, C., 2022: Houseplants can remove the pollutant nitrogen dioxide indoors. Air Quality, Atmosphere and Health, 15, 479–490, doi.org/10.1007/s11869-022-01171-6
Gubb, C., Blanusa, T., Griffiths, A. and Pfrang, C., 2020: Can plants be considered a building service? Building Services Engineering Research & Technology, 41(3), 374–384, doi.org/10.1177/0143624419899519
Gubb, C., Blanusa, T., Griffiths, A. and Pfrang, C., 2019: Interaction between plant species and substrate type in the removal of CO2 indoors, Air Quality, Atmosphere & Health, 12, 1197–1206, doi.org/10.1007/s11869-019-00736-2
Gubb, C., Blanusa, T., Griffiths, A. and Pfrang, C., 2018: Can houseplants improve indoor air quality by removing CO2 and increasing relative humidity? Air Quality, Atmosphere & Health, 11(10), 1191–1201, doi.org/10.1007/s11869-018-0618-9