## 2 Materials and Methods
### Procedure  
Two zero-measurement evaluations were performed to establish the conditions of the setup in the glass container in which the depletion of the formaldehyde took place: one at the beginning of the test series and one at the end. Similarly, two extra zero-measurement evaluations were performed with a plastic container that had the same characteristics of the containers that were used during every test. The measurements were executed for 1-1.5 h until the formaldehyde was depleted or stabilized in the chamber. Gas concentrations were measured in ppb in the case of formaldehyde and in ppm in the case of CO\({}_{2}\). For further analysis, the concentrations of these gases were expressed as micrograms per cubic meter (\(\upmu\)g/m\({}^{3}\)) and milligrams per cubic meter, respectively. For each test, \(\sim\)368.48 \(\upmu\)g/m\({}^{3}\) (\(\sim\)300 ppb) of formaldehyde was released in the chamber to generate exactly the same condition every time. Each set of experiments was conducted three times, in order to consistently evaluate each condition tested (Tables 1 and 2). For each test, the glass container was wiped with a wet paper towel after each measurement. The plastic container with the substrate or plant sample was placed in the center of the glass chamber. Depending on the height of the plant, a stainless-steel base was placed at the bottom (stainless steel is an inert material). A small plate connected to a heat source was placed in the lower hole, and 10 \(\upmu\)L of formaldehyde solution was placed on the plate with a pipette. After a drop of formaldehyde solution was placed on the plate, the hole was closed, and the heat source was activated in order to realize the solution in the air. This was the beginning of the test. During the tests with the Boston ferns, it was necessary to inject some CO\({}_{2}\) when the level was lower than \(\sim\)410 ppm (-738 mg/m\({}^{3}\)) which is the global atmospheric CO\({}_{2}\) concentration (average outdoor concentration) [35; 36] and is sufficient for the plants to grow, although some studies have shown that the optimal CO\({}_{2}\) concentration is around 900 ppm [37]. To calculate the amount of formaldehyde depleted inside of the chamber, the following formula was used [38]:  
\[-\lambda=\frac{ln\left(\frac{N(t)}{N(0)}\right)}{t} \tag{1}\]  
where \(\lambda\)= Decay rate (h\({}^{-1}\)). \(N(t)\) = Amount of pollutant after time \(t\) (\(\upmu\)g/m\({}^{3}\)) or (mg/m\({}^{3}\)). Figure 2: Selected plants: **(a)**_Spatihiphylum Wallisii Regel_ (common name: peace lily); **(b)**_Nephrolepis exulata_ L. (common name: Boston fern) in the glass container. \(N(0)\) = Initial amount of pollutant at \(t\) = 0 \(h\) (\(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\)) or (\(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\)). To calculate the rates of contaminant reduction in the test chamber, the clean air delivery rate (CADR) was calculated [39; 40]:  
\[CADR=(\lambda e-\lambda n-\lambda p)V \tag{2}\]  
where \(\lambda e\) = Total decay rate (\(\mathrm{h}^{-1}\)). \(\lambda n\) = Natural decay rate which is the reduction of the contaminant due to natural phenomena in the test chamber (\(\mathrm{h}^{-1}\)). \(\lambda p\) = Decay rate when the plastic pot was placed in the chamber (\(\mathrm{h}^{-1}\)). \(V\) = Volume of the chamber (\(\mathrm{m}^{3}\)), 0.033 (\(\mathrm{m}^{3}\)). To calculate the removal efficiency of the different test conditions, the following formula was used [38]:  
\[\eta=\left(\frac{N(0)-N(t)}{N(0)}\right)*100 \tag{3}\]  
where \(\eta\) = Efficiency (\(\%\)). \(N(t)\) = Amount of pollutant after time \(t\) (\(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\)) or (\(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\)). \(N(0)\) = Initial amount of pollutant at t = 0 \(h\) (\(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\)) or (\(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{m}^{3}\)).