Impact of Air-Purifying Plants on the Reduction of Volatile Organic Compounds in the Indoor Hot Desert Climate

1 Introduction

Residents of Dubai, United Arab Emirates (UAE), spend 90% of their time indoors because of the sweltering summers and lack of clear demarcation between different seasons (Khaleej Times, 2011; UAE, 2018; Hachicha et al., 2019; Jung and Awad, 2021a). The interior environment of the apartments in the UAE is causing sick building syndrome (SBS), faster than in any other country (Amoatey et al., 2020; Jung et al., 2021a; Awad and Jung, 2021). Moreover, due to unprecedented urbanization, many new residential projects have been constructed in the UAE, with inappropriate material and ventilation (Nassar et al., 2014; Jung et al., 2021b). This social phenomenon is accelerating the spread of SBS (Awbi, 2015). In particular, in an apartment structure in which natural ventilation is difficult to achieve and due to more hazardous the building and furnishing materials, a large quantity of volatile organic compounds (VOCs) are generated indoors (Zheng et al., 2011; Frey et al., 2014; Jung et al., 2021c). In addition, various indoor air pollutants, such as carbon dioxide (CO₂), carbon monoxide (CO), formaldehyde (CH₂O), radon, and microorganisms, are present in buildings (Peng et al., 2017; Carrer and Wolkoff, 2018). Dubai municipality has stipulated an indoor air quality (IAQ) of less than 0.08 ppm of formaldehyde (less than 300 µ/m³ of the total volatile organic compounds (TVOCs)) and less than 150 micrograms/m³ of suspended particulates (less than 10 microns) during 8 h of continuous monitoring prior to occupancy (Gulf News, 2020; DEWA, 2021). As general methods of reducing indoor air pollution, ventilation using various air conditioners or the usage of ecofriendly interior finishing material (Pamonpol et al., 2020; Settimo et al., 2020; Jung et al., 2021d) has been adopted. In general, although mechanical ventilation using an air-conditioning system is effective, it is expensive (Turner et al., 2013; Zhao et al., 2018). As an alternative to these methods, interest in air-purifying plants has increased in late, which can reduce costs and create a green environment (Martini et al., 2014; Liu et al., 2017; Gong et al., 2019).

Recently, in the UAE, research on IAQ improvement using indoor plants in the university classrooms in Ajman in order to enhance the learning efficiency has been conducted (Jung and Awad, 2021b). In areas such as architecture, landscaping, and environment, natural purification of indoor air using plants is being actively investigated (Armijos Moya et al., 2021). However, existing studies measure only the effect in unit space, at a basic research level, for determining the effect of plants in improving the environment, in the area of landscaping (Balocco and Leoncini, 2020; Mata et al., 2021).

This study aims to investigate the effect of three different types of air-purifying plants, Pachira aquatica, Ficus benjamina, and Aglaonema commutatum, on reducing the volatile organic compounds (VOCs) and formaldehyde (CH₂O) in indoor spaces. For this purpose, an experiment was performed in which the effect of reducing pollutants in indoor space during each season was explored by varying the planting volume in an apartment. It is established that air purification using plants is possible by deriving practically usable results through this empirical experiment in a general apartment space (Gawrońska and Bakera, 2015). Moreover, it is possible to develop an environment-friendly system using air-purifying plants in the interior landscaping of apartments, which are multiunit dwellings.

2 Materials and Methods

NASA conducted an “air and water purification experiment” as part of a lunar base plan in the early 1980s and was the first to explore the improvement of the indoor air quality (IAQ) through air-purifying plants (Hort et al., 2012; Inbathamizh, 2020). NASA scientists were interested in the Earth’s self-cleaning ability, for which living plants were the solution (Berg et al., 2014; Wolverton and Nelson, 2020). Furthermore, they are exploring the possibility of creating ecological life support systems in enclosed spaces (Aydogan and Montoya, 2011).

Gas chromatography (GC) is the commonly used technique to separate smaller volatile and semi-volatile organic molecules such as hydrocarbons, alcohols, and aromatics, as well as pesticides, steroids, fatty acids, and hormones for environmental testing. Combined with the detection power of mass spectrometry (MS), gas chromatography–mass spectrometry (GC-MS) (Figure 1) can be used to separate complex mixtures, quantify analytes, identify unknown peaks, and determine trace levels of contamination. Based on GC-MS measurements, the air quality in the spacecraft raised the serious concern since more than 300 types of VOCs were detected from the indoor air of the spacecraft with the presence of the crew (Bandehali et al., 2021).

FIGURE 1

FIGURE 1. GC-MS equipment (Agilent 6890N network series).

After extensive study, NASA established that plants were effective in removing formaldehyde (CH₂O) in a closed laboratory (Choi, 2021). Subsequently, they developed a small, completely enclosed structure called “bio-home” designed to block heat and energy as much as possible. The interior included plastics and other synthetic materials to enable the discharge of VOCs for causing the appearance of typical SBS symptoms (Nelson and Wolverton, 2011). The indoor air samples collected and measured before and after the placement of indoor plants in a planter (a large container for planting plants) in the bio-home indicated that the VOCs were reduced (Kaur et al., 2014). In addition, NASA tested the air purification effect of plants that can grow indoors (Schlacht et al., 2016). The results revealed that indoor plants could absorb and purify aromatic organic compounds, such as toluene (C₇H₈), benzene (C₆H₆), and formaldehyde (CH₂O), at 350–1,200 µg per 100 cm² of the leaf area per day (Pegas et al., 2012; Rodgers et al., 2013). Moreover, the usage of plants can be the most representative method for biological decontamination (Su, 2014; Brilli et al., 2018). Although the results of such a system are scientifically insufficient to establish the degree of indoor air pollution reduction, it is certain that it can be applied sufficiently as a method for reducing indoor air pollutants (Dela Cruz et al., 2014). As such, the ability of plants to purify air has been studied for a long time.

In this study, the changes in the VOC and formaldehyde (CH₂O) concentration in a room were identified by varying the volume of air-purifying plants, such as Pachira aquatica, Ficus benjamina, and Aglaonema commutatum (Park et al., 2010; Son et al., 2019) (Figure 2). The leaf area of the 3 target plants was 900 cm²/pot, calculated using an AutoCAD program after scanning the average leaf. 3 indoor air-purifying plants (pot size: 15 cm × 21 cm) are placed in line next to the windows in the laboratory. Plants were arranged in 10% (1.75 m²) and 5% (0.875 m²) of the laboratory space, and an experiment was conducted from March 4th, 2020, to February 24th, 2021 (Table 1).

FIGURE 2

FIGURE 2. Target plants (left: Pachira aquatica, middle: Ficus benjamina, and right: Aglaonema commutatum).

TABLE 1

TABLE 1. Annual outdoor temperature in the United Arab Emirates.

Planting of Pachira aquatica, Ficus benjamina, and Aglaonema commutatum (Table 2) was performed in 10% (1.75 m²) and 5% (0.875 m²) of the laboratory space volume (Pennisi and van Iersel, 2012). An actual apartment setting was divided as used in a laboratory, which included a living room and balcony (3.5 m in width, 3.5 m in length, and 2.4 m in height) (Kim et al., 2020a).

TABLE 2

TABLE 2. Comparison of Pachira aquatica, Ficus benjamina, and Aglaonema commutatum.

The experimental space included two rooms, Room 1 and Room 2, with and without the target plants, respectively, in which the changes in the formaldehyde (CH₂O) and VOC concentrations were measured under the same conditions for comparison (1.5 m high in the center of the room) (Kim et al., 2010a; Kim et al., 2010b; Kim et al., 2014). The target plants were 2 years old and had been propagated through bushes; there was a minor difference in the number of pots, but it was ensured that the volume of the plants was the same, in the experiment (Moya et al., 2019).

The plant volume was measured in terms of the width, length, and height (900 cm²/pot) (Lee et al., 2018). The concentrations of toluene (C₇H₈), ethylbenzene (C₈H₁₀), xylene (C₈H₁₀), styrene (C₈H₈), and formaldehyde (CH₂O) among the individual VOCs were measured under the same conditions in Room 1 and Room 2. The interior material of these two experimental spaces was the same. A certain quantity of VOCs was artificially generated, and the indoor air was collected with a minipump without manual intervention. In Room 1, the initial concentration was measured 24 h after the target plant was installed; Room 2, without plants, was measured simultaneously (Kim and Kim, 2016). The second measurement was performed after 48 h in both rooms, and the final measurement was performed after 72 h. In another experiment, measurement was performed five times a day (9:00, 12:00, 15:00, 18:00, and 21:00) (Nitmetawong et al., 2020). As a reduction in the VOC concentration was observed between 12:00 PM and 15:00 PM, measurements were performed only once a day at 15:00 PM (Cao et al., 2019). In addition, as a process test method for apartment houses, after 30 min of ventilation and 5 h of sealing, the experiment was conducted between 13:00 PM and 17:00 PM. Figure 3 shows the laboratory plan and the measurement locations.

FIGURE 3

FIGURE 3. 3D model of the laboratory and measurement locations.

VOCs samples such as toluene (C₇H₈), ethylbenzene (C₈H₁₀), xylene (C₈H₁₀), and styrene (C₈H₈) were collected using a charcoal tube, and formaldehyde (CH₂O) sample was collected using a DNPH cartridge (Brągoszewska et al., 2019), which were extracted with CS₂ (carbon disulfide) and acetonitrile solutions, respectively. After extraction, VOCs were analyzed via gas chromatography with flame ionization detection (GC-FID), whereas formaldehyde (CH₂O) was analyzed via high-performance liquid chromatography (HPLC) (Gunasinghe et al., 2021).

3 Results

3.1 Effect of 10% Planting on VOC Concentration Reduction

In order to investigate the effect of the volume of each plant in the reduction of the VOC and formaldehyde (CH₂O) concentrations, an experiment was conducted by planting 10% of the laboratory space volume (Gawronski et al., 2017). As the reduction in indoor air pollutants by plants is affected by solar radiation, all the plants were placed near a window (Kim et al., 2020b; Zheng et al., 2020). In addition, as the sizes of the three types of plants used in the experiment differed slightly, the number of pots was varied to ensure that the total volume was the same (Zhou et al., 2011; Han and Ruan, 2020). Among the three plants, Aglaonema commutatum had the widest and longest leaves, but the number was rather less. The Pachira aquatica and Ficus benjamina trees were tall with small and numerous leaves. These three types of target plants were selected among seven types of air-purifying plants based on their excellent results in previous experiments (Kim et al., 2016; Fasihi et al., 2019) (Table 3).

TABLE 3

TABLE 3. O₂ and CO₂ concentrations with different air-purifying plants.

3.1.1 Spring (March 2020–May 2020)

Pachira aquatica, Ficus benjamina, and Aglaonema commutatum trees were placed in 10% of the laboratory space during the spring season, from March to May 2020, and the change in the VOC concentration was measured. In the case of benzene (C₆H₆), there was a reduction in 22.7 µg/cm³ after the installation of Ficus benjamina, which was the maximum (Figure 4). For toluene (C₇H₈), the reduction effect of Ficus benjamina was the best. Toluene (C₇H₈) showed a reduction of approximately 19.2–35.4 µg/cm³ with each plant, which was relatively greater than that of benzene (C₆H₆) (Figure 5). The reduction of toluene (C₇H₈), whose concentration is generally high in apartments, is crucial; Ficus benjamina had the best effect in spring, among the three plants. The reduction amount of ethylbenzene (C₈H₁₀) when Pachira aquatica was installed was 11.9 µg/cm³, which was lesser than that of benzene (C₆H₆) or toluene (C₇H₈) (Figure 6). The effect of Ficus benjamina was excellent for benzene (C₆H₆), whereas, for toluene (C₇H₈), Pachira aquatica was the best. After Pachira aquatica, the ethylbenzene (C₈H₁₀) reduction effect of Ficus benjamina was excellent. In the case of xylene (C₈H₁₀) (Figure 7) and styrene (C₈H₈) (Figure 8), the effect was maximum when 10% Aglaonema commutatum was installed. The formaldehyde (CH₂O) reduction effect was maximum with 10% Ficus benjamina (Figure 9).

FIGURE 4

FIGURE 4. Concentration change in benzene (C₆H₆) according to the planting volume (10%).

FIGURE 5

FIGURE 5. Concentration change in toluene (C₇H₈) according to the planting volume (10%).

FIGURE 6

FIGURE 6. Concentration change in ethylbenzene (C₈H₁₀) according to the planting volume (10%).

FIGURE 7

FIGURE 7. Concentration change in xylene (C₈H₁₀) according to the planting volume (10%).

FIGURE 8

FIGURE 8. Concentration change in styrene (C₈H₈) according to the planting volume (10%).

FIGURE 9

FIGURE 9. Concentration change in formaldehyde (CH₂O) according to the planting volume (10%).

Formaldehyde (CH₂O) is generated by the building materials, preservatives, furniture, and adhesives used in interior finishing. In particular, its concentration is five times than those of the other VOCs; hence, reduction measures are urgently needed. Considering the concentration of the indoor air pollutants, the reduction of the other VOCs is also important; however, the placement of plants effective in reducing formaldehyde (CH₂O) in spring will have a good effect. In addition, it was found Ficus benjamina was highly effective in reducing most VOCs, including formaldehyde (CH₂O), benzene (C₆H₆), and toluene (C₇H₈).

3.1.2 Summer (June 2020–August 2020)

Another experiment was conducted from June–early September 2020, when the weather in Dubai was hot. Due to the characteristics of VOCs, their initial concentrations were mostly higher in summer when the temperature was high, compared to spring. When 10% Pachira aquatica was installed, the reduction in the concentration of benzene (C₆H₆) was maximum during each season (Figure 4). Among the VOCs, toluene (C₇H₈) was reduced by 45.9 µg/cm³ when 10% Ficus benjamina was installed; the reduction was maximum in summer, and the effect was excellent (Figure 5). In the case of ethylbenzene (C₈H₁₀) (Figure 6) and xylene (C₈H₁₀) (Figure 7), the concentration was mostly below 40 µg/cm³. Maximum reduction in the ethylbenzene (C₈H₁₀) and xylene (C₈H₁₀) concentrations was observed when Pachira aquatica and Aglaonema commutatum were installed, respectively. In particular, the reduction in xylene (C₈H₁₀) on planting Aglaonema commutatum was thrice, compared to Pachira aquatica or Ficus benjamina. In the case of formaldehyde (CH₂O) (Figure 9), the reduction was 534.4 µg/cm³, 330.7 µg/cm³, and 321.9 µg/cm³, when Ficus benjamina, Aglaonema commutatum, and Pachira aquatica were installed, respectively.

The initial concentration was approximately 800 µg/cm³, and it decreased in all the cases. The reduction in the concentration of formaldehyde (CH₂O) was almost the same during the 3 days of measurement, and the effect was most significant when Ficus benjamina was installed.

3.1.3 Autumn (September 2020–November 2020)

In autumn, the overall concentration was similar to that in spring because of the similarity in temperature, in the UAE. Benzene (C₆H₆) with similar initial concentrations of approximately 20 µg/cm³ in spring and autumn showed a reduction of 14.7–16.6 µg/cm³. Ficus benjamina achieved maximum benzene (C₆H₆) reduction (Figure 4). In the case of toluene (C₇H₈), the best effect was observed when Pachira aquatica was installed; Ficus benjamina was also effective (Figure 5). The reduction effect of ethylbenzene (C₈H₁₀) was the maximum with Pachira aquatica, whereas xylene (C₈H₁₀) and styrene (C₈H₈) showed the maximum reduction in concentration with Ficus benjamina (Figure 6). The initial concentration of formaldehyde (CH₂O) was 800 µg/cm³ in autumn, similar to that in spring; however, the decrease in concentration was slightly greater than that in spring (Figure 9). Among the three plants, Aglaonema commutatum was the most in reducing formaldehyde (CH₂O) to 361.6 µg/cm. The concentration was reduced from 812.2 µg/cm³ to 680.2 µg/cm³ after 48 h and 450.6 µg/cm³ after 72 h; a gradual reduction of 200 µg/cm³ was observed each day. Aglaonema commutatum exhibited the best effect for formaldehyde (CH₂O) reduction, followed by Pachira aquatica and Ficus benjamina with similar reductions of 289.9 µg/cm³. When the planting volume was 10% in autumn, Ficus benjamina had the best effect for each VOC substance; however, in the case of formaldehyde (CH₂O), which constituted a large portion of the pollutant concentration, Aglaonema commutatum had the best effect.

3.1.4 Winter (December 2020–February 2021)

Furthermore, the experiment was conducted in winter between December 2020 and February 2021, and the measurements were made at a slightly lower temperature. VOC release in the winter experiment was at a slightly lower concentration compared to the other seasons because of the low temperature. The reduction effect was not very good for most VOC substances. In the case of benzene (C₆H₆), the reduction was 5.5–9.3 µg/cm³, and the effect of Aglaonema commutatum was the best at 9.3 µg/cm³ (Figure 4). In the case of ethylbenzene (C₈H₁₀), xylene (C₈H₁₀), and styrene (C₈H₈), the reduction amount was approximately 10.0 µg/cm³ (Figures 6–8). Except for ethylbenzene (C₈H₁₀), the effect was excellent when Aglaonema commutatum was installed; for ethylbenzene (C₈H₁₀), Pachira aquatica was effective. The initial concentration of formaldehyde (CH₂O) in winter was found to be approximately 200 µg/cm³ lower than those of the other seasons (Figure 9). Pachira aquatica was the most effective in reducing the concentration of formaldehyde (CH₂O). With the other air-purifying plants, concentration reduction was observed, but it was less on the second day.

3.1.5 Reduction Rate According to the Season

With 10% planting, the reduction of VOCs and formaldehyde (CH₂O) was expressed as a reduction ratio. In most cases, the best effect was observed in summer (Stevenson et al., 2020). In the case of benzene (C₆H₆) in Figure 4, the reduction effect was maximum in summer and autumn, similar to toluene (C₇H₈) in Figure 5. Ethylbenzene (C₈H₁₀) showed a reduction of 86.4% in Figure 6 when Pachira aquatica was planted in summer, and most of the benzene (C₆H₆) was removed. According to the season, the reduction ratio of benzene (C₆H₆) was 29.4–36.8% in spring, 47.4–86.4% in summer, 15.8–70.1% in autumn, and 34.4–53.4% in winter. In some cases, the reduction amount and reduction ratio were slightly different depending on the difference in the initial VOC and formaldehyde (CH₂O) concentrations (Suárez-Cáceres and Pérez-Urrestarazu, 2021). However, most of the experimental results indicated that the concentration reduction effect was excellent in spring, summer, and autumn. In winter, when the temperature was low, the reduction amount and reduction ratio were poor compared to the other seasons (Weyens et al., 2015) (Table 2).

Figures 4–9 depict the changes in the concentration of benzene (C₆H₆), toluene (C₇H₈), ethylbenzene (C₈H₁₀), xylene (C₈H₁₀), styrene (C₈H₈), and formaldehyde (CH₂O), respectively, when the three types of air-purifying plants occupy 10% of the laboratory space volume. The VOC and formaldehyde (CH₂O) concentrations were measured thrice, 24 h, 48 h, and 72 h after planting.

3.2 Effect of 5% Planting on VOC Concentration Reduction

To investigate the effect of the planting volume of each plant on the reduction of the VOC and formaldehyde (CH₂O) concentrations, plants were placed in 5% of the laboratory space, and the following experiment was conducted. The plants were placed near a window, considering the effect of solar radiation. The experimental method was the same as in that in the case of 10% planting, and the only change was in the number of plants.

3.2.1 Spring (March 2020–May 2020)

The changes in the concentration of formaldehyde (CH₂O) and the VOCs were identified after setting the planting volume to 5%, in each season. In a previous experiment, the planting volume was set to 10, 5, and 3%, respectively, and the results were compared. It was found that the concentration reduction effect was effective when plants were installed in more than 5% of the laboratory space.

As shown in Figure 10, the reduction in benzene (C₆H₆) concentration was ranged from 14.9 to 34.9 µg/cm³. In spring, the reduction in benzene (C₆H₆) concentration was maximum when Ficus benjamina was installed; Pachira aquatica was effective as well. However, with Aglaonema commutatum, the reduction was lesser than twice that of Pachira aquatica and Ficus benjamina. The toluene (C₇H₈) concentration, as shown in Figure 11, was reduced in all the seasons, and it was found that the reduction effects of Aglaonema commutatum and Ficus benjamina were considerable. Among these two plants, the effect of Aglaonema commutatum was slightly better. The initial concentration value of toluene (C₇H₈) in spring was approximately 60.0 µg/cm³, similar to that in autumn; however, the decrease in concentration was slightly more than that in autumn. In addition, the initial concentration was lower than that in summer, and the amount of reduction was also less. However, all the results were better compared to winter.

FIGURE 10

FIGURE 10. Concentration change in benzene (C₆H₆) according to the planting volume (5%).

FIGURE 11

FIGURE 11. Concentration change in toluene (C₇H₈) according to the planting volume (5%).

In the case of ethylbenzene (C₈H₁₀), as shown in Figure 12, all the plants were effective in all the seasons. For reducing ethylbenzene (C₈H₁₀) in spring, 5% Aglaonema commutatum was the most effective, followed Pachira aquatica and Ficus benjamina; however, the reduction was greater on the third day than the second. The reduction of xylene (C₈H₁₀) (Figure 13) and styrene (C₈H₈) (Figure 14) was high in summer but low in spring. Both xylene (C₈H₁₀) and styrene (C₈H₈) were effectively reduced when Pachira aquatica was installed; Aglaonema commutatum was also effective in reducing xylene (C₈H₁₀). The concentration value of styrene (C₈H₈) ranged from 15.0 to 40.0 µg/cm³; in spring, the concentration gradually decreased from the initial concentration of 30.0 µg/cm³ and dropped significantly on the third day.

FIGURE 12

FIGURE 12. Concentration change in ethylbenzene (C₈H₁₀) according to the planting volume (5%).

FIGURE 13

FIGURE 13. Concentration change in xylene (C₈H₁₀) according to the planting volume (5%).

FIGURE 14

FIGURE 14. Concentration change in styrene (C₈H₈) according to the planting volume (5%).

As depicted in Figure 15, the concentration of formaldehyde (CH₂O) gradually decreased from approximately 800.0 µg/cm³ at the beginning of the measurement. Among the three plants, when Aglaonema commutatum was installed, the reduction amount was 200.0 µg/cm³, which was the maximum. In spring, the initial concentration was similar or slightly lower than that in summer, but the reduction amount was lesser.

FIGURE 15

FIGURE 15. Concentration change in formaldehyde (CH₂O) according to the planting volume (5%).

3.2.2 Summer (June 2020–August 2020)

All the measurements were made from June to late August during summer, and the concentrations were higher than those in the other seasons. The effect of 5% planting was lesser than that of 10% planting. As shown in Figure 10, the reduction in benzene (C₆H₆) concentration in summer was maximum at 20.0 µg/cm³ when Pachira aquatica was planted. The initial concentration of benzene (C₆H₆) was approximately 20.0 µg/cm³ in the other seasons, but in summer, the initial concentration was 40.0–50.0 µg/cm³. In the case of toluene (C₇H₈), Aglaonema commutatum was most effective when planted in the summer, compared to other seasons (Figure 11). When 5% Aglaonema commutatum was installed, 60.0 µg/cm³ of toluene (C₇H₈) was reduced.

For ethylbenzene (C₈H₁₀), the concentration reduction effect of Pachira aquatica was the highest in summer, as shown in Figure 12. The reduction in ethylbenzene (C₈H₁₀) and styrene (C₈H₈) was maximum with Pachira aquatica, whereas Aglaonema commutatum was most effective in reducing xylene (C₈H₁₀). The effect of 10% as well as 5% Ficus benjamina planting was excellent in the case of formaldehyde (CH₂O) (Figure 15). With 10% Ficus benjamina planting, the reduction amount was 534.4 µg/cm³, whereas 5% planting showed a reduction of 414.1 µg/cm³.

3.2.3 Autumn (September 2020–November 2020)

In autumn, the VOC and formaldehyde (CH₂O) concentrations were measured in October and November. The effect of Aglaonema commutatum was excellent for almost all the VOCs, whereas the reduction effect of Pachira aquatica for benzene (C₆H₆) and formaldehyde (CH₂O) was excellent. The initial concentration of benzene (C₆H₆) in autumn was lower than that in spring and summer (Figure 10). The reduction effect of Aglaonema commutatum was good in autumn, whereas Pachira aquatica and Ficus benjamina had excellent effect in spring. With Pachira aquatica, the concentration reduction of toluene (C₇H₈) was maximum at 25.0 µg/cm³ in autumn (Figure 11). In the case of toluene (C₇H₈), the concentration was reduced to 10.8 µg/cm³ and 22.0 µg/cm³ with Aglaonema commutatum and Ficus benjamina, respectively. Ethylbenzene (C₈H₁₀) (Figure 12), xylene (C₈H₁₀) (Figure 13), and styrene (C₈H₈) (Figure 14) showed the best reduction when Pachira aquatica was installed at 5%. In the case of formaldehyde (CH₂O), the reduction was lower than that in summer, but when Aglaonema commutatum was installed, a large reduction of 268.8 µg/cm³ was observed (Figure 15). In autumn, 10% planting realized a greater reduction of approximately 92.7 µg/cm³. The greater the planting volume, the greater was the reduction effect.

3.2.4 Winter (December 2000–February 2021)

In winter, the concentration reduction effect was insignificant compared to the other seasons, and the reduction amount was less than 10.0 µg/cm³ in most cases. In the case of benzene (C₆H₆), the effect was greater when Pachira aquatica was planted (Figure 10). Aglaonema commutatum was most effective in reducing toluene (C₇H₈) (Figure 11), ethylbenzene (C₈H₁₀) (Figure 12), and styrene (C₈H₈) (Figure 14), whereas Ficus benjamina was the most effective for xylene (C₈H₁₀) (Figure 13). Formaldehyde (CH₂O) exhibited a greater reduction in concentration than other VOCs, and the effect was excellent when Pachira aquatica was installed (Figure 15). The formaldehyde (CH₂O) concentration changes in winter were 60.0 µg/cm³ or more, and the maximum reduction was 126.0 µg/cm³ when Pachira aquatica 5% was planted.

3.2.5 Reduction Rate as per Season

In the experiment with 5% planting volume, the reduction in the concentration of each VOC and formaldehyde (CH₂O), depicted as a reduction ratio, was maximum in summer (Pichlhöfer et al., 2021). In the case of benzene (C₆H₆), when Ficus benjamina was planted in summer, the reduction effect was maximum at 61.1%, whereas it was 40–50% in spring and autumn (Figure 10). In the case of toluene (C₇H₈), when Aglaonema commutatum was planted in summer, the effect was excellent at 62.7%, and the reduction effect was close to 50%, except in winter (Figure 11). However, as the initial concentrations were slightly different, the same reduction amount and reduction ratio were not realized in all the seasons (Li et al., 2016). For example, the reduction amount as well as reduction ratio was maximum with the planting of Aglaonema commutatum in summer. However, in spring, when Pachira aquatica was planted, the reduction amount was 15.2 µg/cm³; Ficus benjamina was more effective, with a reduction of 28.6 µg/cm³. Due to the difference in the initial concentration, the reduction ratio with Pachira aquatica was 38.1%, which was better than the reduction ratio of 36.7% with Ficus benjamina.

The effect on the reduction ratio and reduction amount of formaldehyde (CH₂O) was the same (Figure 15). When Ficus benjamina was planted in summer, the reduction amount was 414.1 µg/cm³, and the reduction ratio was 42.0%, which was the most effective. With Aglaonema commutatum, Pachira aquatica, and Ficus benjamina, the reduction ratios were 9.7–35.3%, 19.1–37.7%, and 14.6–42.1%, respectively. The experimental results with the three plants demonstrated that the reduction amount of formaldehyde (CH₂O) was greater than those of the other individual VOCs. However, the reduction ratio was approximately 20–40%, which was lesser than those of the other VOCs. There was a difference in the initial concentration of each VOC. For the reduction ratio, it was found that a slight decrease in concentration at a low initial concentration was more effective than a decrease in concentration at a high concentration (Teiri et al., 2018).

Figures 10–15 depict the change in the concentration of benzene (C₆H₆), toluene (C₇H₈), ethylbenzene (C₈H₁₀), xylene (C₈H₁₀), styrene (C₈H₈), and formaldehyde (CH₂O), respectively, when plants occupy 5% of the laboratory space volume. Table 4 lists the reduction amount of the VOCs and formaldehyde with respect to the planting volume.

TABLE 4

TABLE 4. VOC and formaldehyde (CH₂O) concentration reduction with respect to the planting volume (10 and 5%).

4 Discussion

This study addresses the improvement of the indoor air quality using three types of indoor air-purifying plants alone. However, in future research, more diverse air-purifying plants will be integrated with building design and ventilation facilities as green walls or green ceilings to achieve better effects.

As the awareness of sustainability increases in hot desert climate, the use of indoor air-purifying plants will become more active due to over 41°C scorching summer. Especially in Dubai, UAE, without much green landscape in the urban context, residents will be eager to have indoor plants in their houses. Not only as the form of a potted plant but there are also many methods to implement indoor air-purifying plants seamlessly with building design such as green wall and green ceiling.

These indoor plant implementations could help to achieve better LEED (leadership in energy and environmental design) since LEED has indoor environmental quality (IEQ) category. As a biophilia design, a biofilter wall could be installed with three indoor air plants from this research such as Pachira aquatica, Ficus benjamina, and Aglaonema commutatum as hydroponic plants. Since these hydroponic green walls help conserve water in conditions where active humidification is required, they reduce HVAC loads by 15% and boost the relative humidity by up to 5–30% based on the 2008 study carried out by the Indian government.

5 Conclusion

In this study, the effect of the planting volume on the reduction of the formaldehyde (CH₂O) and VOC concentrations was investigated for three types of air-purifying plants, namely, Pachira aquatica, Ficus benjamina, and Aglaonema commutatum.

The experimental results indicated that the greater the planting volume (10% of the laboratory space, 1.75 m²), the greater was the reduction effect for each VOC. Investigation of the change in the formaldehyde (CH₂O) and VOC concentrations according to the season established that the initial concentration was high in summer, and the reduction effect was also excellent. In the case of benzene (C₆H₆), the reduction effect was maximum in summer and autumn, similar to toluene (C₇H₈). Ethylbenzene (C₈H₁₀) showed a reduction of 86.4% when Pachira aquatica was planted in summer. According to the season, the reduction ratio of benzene (C₆H₆) was 29.4–36.8% in spring, 47.4–86.4% in summer, 15.8–70.1% in autumn, and 34.4–53.4% in winter.

The reduction of the concentration of formaldehyde (CH₂O) and toluene (C₇H₈) was particularly high, and in the case of toluene (C₇H₈), the concentration reduction effect was excellent with Aglaonema commutatum and Ficus benjamina in summer. It was determined that the concentration reduction effect was the best when the plants were installed in summer. Toluene (C₇H₈) was reduced by 45.9 µg/cm³ when 10% Ficus benjamina was installed; the reduction was maximum in summer, and the effect was excellent. In the case of ethylbenzene (C₈H₁₀) and xylene (C₈H₁₀), the concentration was mostly below 40 µg/cm³. Maximum reduction in the ethylbenzene (C₈H₁₀) and xylene (C₈H₁₀) concentrations was observed when Pachira aquatica and Aglaonema commutatum were installed, respectively. In particular, the reduction in xylene (C₈H₁₀) on planting Aglaonema commutatum was thrice, compared to Pachira aquatica or Ficus benjamina. In the case of formaldehyde (CH₂O), the reduction was 534.4 µg/cm³, 330.7 µg/cm³, and 321.9 µg/cm³, when Ficus benjamina, Aglaonema commutatum, and Pachira aquatica were installed, respectively.

Although the reduction of all the VOC substances in an apartment is crucial, as formaldehyde (CH₂O) and toluene (C₇H₈) are more hazardous, planting Ficus benjamina in indoor spaces could be more effective. The initial concentration was approximately 800 µg/cm³, and it decreased in all the cases. The reduction in the concentration of formaldehyde (CH₂O) was almost the same during the 3 days of measurement, and the effect was most significant when Ficus benjamina was installed.

This study is meaningful in revealing the trend of the purification effect using the three plants for a long period of time, and the limitation of the study, however, is that it was not possible to do the experiment in apartment conditions where various conditions can occur because the actual place of the experiment is inhabited by people.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, and further inquiries can be directed to the corresponding author.

Author Contributions

All the authors contributed significantly to this study. CJ and NA identified and secured the example buildings used in the study. The data acquisition system was designed, and the sensors were installed by CJ and NA. NA was responsible for data collection. Data analysis was performed by CJ. The article was compiled by CJ and reviewed by NA. All the authors have read and agreed to the published version of the article.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Acknowledgments

The authors would like to express their gratitude to Ajman University for the generous support in publishing this paper.

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