## 5 Conclusions and Recommendations  
A series of tests was performed to evaluate the effect of potted plants on reducing formaldehyde and CO\({}_{2}\) levels in a controlled glass chamber. The outcome of the tests showed some clear advantages and disadvantages of the different test conditions to consider for the design of an indoor plant-based system.  
In terms of air \"cleaning\" of formaldehyde, the measurements and analysis showed that soil, in general, was most effective in reducing formaldehyde concentrations in the chamber (\(\sim\)0.07-0.16 m\({}^{3}\)/h). Plants (\(\sim\)0.03 m\({}^{3}\)/h) were as effective as dry expanded clay (0.02-0.04 m\({}^{3}\)/h). Wet and dry soil, wet expanded clay, and dry activated carbon performed better than the selected plants in formaldehyde depletion. In this study, it became clear that the substrate is an important ally in reducing gaseous pollutants such as formaldehyde.  
Regarding CO\({}_{2}\) reduction in the chamber, potted plants (peace liles: 0.01 m\({}^{3}\)/h) (Boston ferns: 0.02-0.03 m\({}^{3}\)/h) were more effective than the other tests, especially Boston fern, which has a higher stomatal conductance than the peace lily, indicating the possibility of allowing more gaseous pollutants to be absorbed in the long term.  
Studies with potted plants in closed chambers were shown to be useful for isolating factors that may enhance removal efficiency and contribute towards the improvement of plant-based systems (e.g., plant species and growth medium). However, the impact of one potted plant on the cleaning of the indoor air was insignificant. Therefore, several potted plants will be required to improve the IAQ, taking into account the specific characteristics of the place, such as size and the ventilation system.  
It must be noted, however, that in this study, the formaldehyde was introduced in a glass chamber in which the plant and its substrate were located, hereby surrounding the plant and its substrate with formaldehyde. In a \"normal\" indoor environment, usually the source of formaldehyde may not be close to the plant system. For the plant system to take up the formaldehyde, the polluted air needs to be transported to the vicinity of the plant. This could be realized, for example, by an active plant-substrate system, in which the contaminated air is forced to go through the plant leaves and through the substrate roots. Further research with active plant-based systems on the depletion of formaldehyde and other pollutants is required.  
Conceptualization, T.A.-M., P.M.B. and P.J.V.; methodology, T.A.-M., P.M.B. and P.J.V.; formal analysis and investigation, T.A.-M. and P.M.B.; data curation, T.A.-M.; writing--original draft preparation, T.A.-M., writing--review and editing, T.A.-M., M.O., A.v.d.D. and P.M.B.; visualization, T.A.-M.; supervision, P.d.V., M.O., A.v.d.D. and P.M.B. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** The study did not involve humans.  
**Informed Consent Statement:** The study did not involve humans.  
**Data Availability Statement:** More information on the data presented in this study is available on request from the corresponding author.  
**Acknowledgments:** The work described in this paper is part of a PhD project, supported by the National Secretariat of Higher Education, Science, Technology and Innovation of Ecuador (Senescyt) and Delft University of Technology. The authors would like to express their gratitude to the Wageningen Plant Research Group at Wageningen University for their skillful help and support during the performance of this set of experiments in their laboratory. [PERSON] and [PERSON] are thanked for their help during the execution of the experiments.  
**Conflicts of Interest:** The authors declare no conflict of interest.