## 3 Results  
Figures 4-7 show the measured formaldehyde concentrations for the different test configurations. Figure 8 presents the measured CO\({}_{2}\) concentrations when the selected potted plants were included. Figure 9 presents the measured formaldehyde and CO\({}_{2}\) concentrations when the Boston ferns were included. In general, three measurements were executed for every test condition, and the figures present the mean values including standard errors (\(\pm\)SE). In Tables 1 and 2, the CADRs of, respectively, formaldehyde and CO\({}_{2}\) depletion inside of the chamber for the different tests are presented. The CADRs were calculated using Equations (1) and (2). The depletion time used in Tables 1 and 2 to calculate the CADR is based on when the lowest levels of formaldehyde and CO\({}_{2}\) were detected inside of the chamber. It is important to mention that the time of the depletion of the pollutant inside of the chamber determines the efficacy of the depletion of every element tested. Figure 4: Measured formaldehyde concentration ((\(\mu\)g/m\({}^{3}\))/h) when expanded clay samples were tested: zero measurement (ZM), zero measurement with the plastic pot (ZM_P), dry expanded clay (EC_D), wet expanded clay (EC_W). Data means \(\pm\) SE, \(n=3\). Figure 5: Measured formaldehyde concentration ((ug/m\({}^{3}\))/h) when soil samples were tested: zero measurement (ZM1), zero measurement with the plastic pot (ZM_P), dry soil (SD), wet soil (SW). Data means \(\pm\) SE, \(n\) = 3. Figure 6: Measured formaldehyde concentration ((ug/m\({}^{3}\))/h) when activated carbon samples were tested: zero measurement (ZM), zero measurement with the plastic pot (ZM_P), dry activated carbon (AC_D), wet activated carbon (AC_W). Data means \(\pm\) SE, \(n\) = 3 (AC_D), and \(n\) = 2 (AC_W; the third test was excluded). Figure 8: Depletion of CO\({}_{2}\) (mg/m\({}^{3}\)): for the three Boston ferns (NEPH_1, NEPH_2, and NEPH_3), and for the three peace lilies (SPA_1, SPA_2, and SPA_3). Data means \(\pm\) SE, \(n\) = 3. Figure 7: Measured formaldehyde concentration ((mg/m\({}^{3}\))/h) when plant samples were tested: zero measurement (ZM), zero measurement with the plastic pot (ZM_P), peace lily (SPA), Boston fern (NEPH). Data means \(\pm\) SE, \(n\) = 3. During the zero measurements of the setup, the sensor indicated the presence of around 30.7 \(\mathrm{\SIUnitSymbolMicro m^{3}}\) (25 ppb) of formaldehyde in the system. It is believed that this value was due to the calibration process. The zero-measurement tests indicated that the formaldehyde decreased slowly in the chamber (Figures 3-6), which could be the natural decay of the gas or because it was partially adsorbed by the setup. When the plastic container was placed inside of the chamber, the reduction slightly increased, which shows that the formaldehyde was adsorbed by the container. These two values have to be taken in account when analyzing the real effect of the substrates and plants regarding formaldehyde depletion (Table 1). Therefore, to calculate the CADR and establish the real air-cleansing impact of every test condition, the natural decay of the chamber (\(\lambda n\) = 0.11 \(\mathrm{h^{-1}}\)) and the decay rate of the plastic container (\(\lambda p\) = 0.15 \(\mathrm{h^{-1}}\)) were subtracted from the total decay rate (Tables 1 and 2). Figure 4 presents the depletion of formaldehyde when expanded clay was tested, under dry and wet conditions, indicating that wet expanded clay was more effective on depleting formaldehyde than under dry conditions. Among all the conditions tested, soil was the most effective element to reduce formaldehyde in the chamber, especially under wet conditions (Figure 5). Figure 6 shows that activated carbon under dry conditions was more efficient than under wet conditions in reducing formaldehyde in the chamber.