Removal of Indoor Carbon Dioxide and Formaldehyde Using Green Walls by Bird Nest Fern

Abstract

This paper presents an evaluation of the effectiveness of removing air pollutants by installing green vertical walls filled with potted plants. Most people in urban areas spend 80–90% of their lives indoors, indicating the significance of indoor air quality. Carbon dioxide (CO₂) and formaldehyde (HCHO) are the most common sources of indoor pollution; their levels can be reduced by using potted plants, which provide the additional benefit of beautification. In accordance with our previous study, for this purpose, we used bird nest fern (Asplenium nidus Linn.), which has a high rate of transpiration and is easy to grow indoors. Upon using 3 treatments involving the release of CO₂, HCHO, or CO₂ + HCHO, the experimental results showed that bird nest fern can reduce the concentration of CO₂ from 2000 ppm to a safe 800 ppm at an average of 1.984 ppm·h⁻¹ (per pot). By contrast, the concentration of HCHO was reduced from 2 ppm to the safe level of 0.1 ppm, at an average of 0.003 ppm·h⁻¹ (per pot). Regarding temperature and humidity, the results showed a decrease of 2°C indoors and an increase of 10% relative humidity. These results show that bird nest fern has high rates of CO₂ and HCHO removal, reduces temperature, and raises relative humidity.

Introduction

Currently, most city dwellers spend a great portion of their lives indoors (Lim et al., 2006; Llewellyn and Dixon, 2011; USEPA, 2002). Indoor air has been found to be up to 100 times more polluted than outdoor air (Fisk, 2000; Orwell et al., 2004). Wong et al. (2006) stated that 9 indoor air pollutants [i.e., airborne bacterial count (ABC), carbon monoxide (CO), carbon dioxide (CO₂), formaldehyde (HCHO), nitrogen dioxide (NO₂), ozone (O₃), radon (Rn), respirable suspended particulates (RSP), and volatile organic compounds (VOCs)] and 3 thermal comfort parameters [i.e., air velocity (V), relative humidity (RH), and temperature (T)] could affect people’s health to a certain extent. According to studies on the measurement of indoor air quality (IAQ) that have been conducted by Kim et al. (2013), Pilidis et al. (2009), and Wong et al. (2006), this variable is critical for health and quality of life.

Indoor plants beautify offices, allowing their occupants to experience positive physical and psychological effects and to increase their work efficiency (Bringslimark et al., 2007; Chang and Chen, 2005; Dravigne et al., 2008; Park et al., 2008; Raanaas et al., 2011). Fifty indoor plants were identified to purify the air by removing toxic chemical substances in terms of IAQ by the National Aeronautics and Space Administration (NASA) (Wolverton and Wolverton, 1993). Many other researchers confirmed that indoor plants purified the air by removing pollutants (Lohr and Pearson-Mims, 1996; Raanaas et al., 2011). Plants also reduce indoor temperatures and raise indoor humidity (Aydogan and Montoya, 2011); in addition, plant transpiration facilitates the convection of indoor air, which is an effective method of improving IAQ (Park et al., 2008). Uhde and Salthammer (2007) also studied the influence of construction materials and furniture on IAQ. Using indoor plants is usually cheaper than technological approaches to improve IAQ (Wolverton, 1986; Wood et al., 2002). Improvement of IAQ by using indoor plants has thus received much attention recently.

Although CO₂ is colorless and odorless, it reduces oxygen content and ion numbers; in a poorly ventilated environment, this promotes the accumulation of dust, bacteria, and body odor, causing discomfort to humans. Ping et al. (2011) stated that plants absorb large amounts of CO₂ and release O₂ during photosynthesis, making CO₂ and O₂ levels in indoor air relatively balanced and stable. The mechanisms that plants use to absorb harmful gases are leaf absorption, soil sorption, and decomposition of microorganisms around the plant-root system.

The International Agency for Research on Cancer (IARC) demonstrated that HCHO is a Group 1 carcinogen (IARC, 2014) that it is the most commonly emitted pollutant among indoor volatile organic compounds. HCHO has been extensively used in interior decorations and renovations. Because Taiwan is located in a subtropical climate zone, the high temperature and humidity in this country accelerate the emission of HCHO. Ping et al. (2011) suggested that traditional construction materials release numerous harmful pollutants, and these materials have been a major source of indoor air pollution.

Higher concentrations of HCHO may also induce nausea, vomiting, cough, chest tightness, and asthma, or even pneumonia, pulmonary edema, or death when exposure to over 6.5 mg·m⁻³ HCHO occurs (Pilidis et al., 2009). High temperature, high humidity, negative pressure, and high loading significantly accelerate HCHO diffusion. Therefore, indoor HCHO levels could exceed the standard allowable concentration under certain conditions. Moreover, the release of HCHO requires a long period, generally between 3 and 15 years (Pilidis et al., 2009). In addition, the air-purifying effect of plants is correlated with the number of them planted indoors (Aydogan and Montoya, 2011; Su and Lin, 2013).

Previous studies demonstrated that indoor plants can improve the IAQ. However, few papers have mentioned consideration of the direct effect of indoor plants. CO₂ and HCHO are among the most widely detected indoor gas pollutants. Su and Lin (2013) indicated that bird nest fern (Asplenium nidus Linn.) is one of the most popular and easy plants to grow indoors and was shown to have a high transpiration rate among 50 plants recommended by the Environmental Protection Administration (EPA) of Taiwan Executive Yuan, suggesting that its use is suitable in daily life. Therefore, in this study, bird nest fern was used to alter the levels of the most commonly detected indoor gas pollutants, CO₂ and HCHO. The results are expected to provide insight into the application of this technique in daily life.

Materials and Methods

Measurement site setting

The experiment was conducted in a laboratory with a volume of about 38.88 m³ (L360 × W360 × H300 cm). One of the laboratory walls (W360 × H300 cm) was covered with 189 Ø10-cm pots of bird nest fern; altogether, the pots occupied an area of 5.72 m² (W220 × H260 cm) (Fig. 1A). Opposite the greened wall, a fan was placed inside the laboratory to ensure even dispersal of the CO₂ and HCHO gas, instead of natural ventilation, to produce complete mixing of the gas in the space, which was introduced through a tube into the confined space. All plants were irrigated sufficiently on the day before measurements in each experiment and were not irrigated during the test period. Additional fertilization was not used during the test period.

Fig. 1.

Setting of the vertical green walls. (A) A sketch of the vertical green walls, (B) one of the walls (W360 × H300 cm) in a laboratory was covered with 189 Ø10-cm pots of bird nest fern; altogether, the pots occupied an area of 5.72 m² (W220 × H260 cm); (C) plastic flower pots and spaces between the soil and plants were coated with aluminum foil to reduce the effect of interference.

To ensure statistical accuracy, the laboratory was sealed when the data measurements were conducted. A 1-cm-thick polystyrene sheet covered the windows and the surrounding walls and ceiling were covered with tinfoil. A galvanized steel sheet covered the ground, and special caution was taken to close the gaps around the door to prevent the natural decrease of HCHO. Monitoring equipment was placed in the center of the room (Fig. 1B) and another monitor was placed outside the laboratory to collect data.

We isolated all the materials in the laboratory before the experiments, including plastic flower pots, the soil in the pots, lamps, and the wall all being coated with aluminum foil, except the leaves of bird nest fern, to verify the effect of the removal of harmful gases and to ensure the accuracy of the measurements.

After the laboratory lighting was installed, the illumination of the lamps was measured at heights from the ground of 80 cm and 280 cm, and at intervals of 75 cm, so that a total of 39 measurement points were established. The light intensity of the laboratory was 512.5 lux (approximately 6.99 μmol·m⁻²·s⁻¹). The lamps were used for daytime lighting and for nighttime lighting during the test period.

Plant material

Bird nest fern (Asplenium nidus Linn.) was transplanted in pots. A pot of Ø10 × H7.6 cm (0.6 L) was filled with mixed medium of peat, composting bark, cane-bagasse, paddy, and others. Each bird nest fern pot included approximately 8 large leaves (18 × 4 cm), 6 medium-sized leaves (15 × 3 cm), and 8 small leaves (8 × 2 cm). In general, plants need a period of acclimatization to avoid problems associated with adapting to a new environment. Factors affecting the acclimatization of plants are temperature, water, light, air pressure, soil, fertilizer, and medium. In accordance with experiment by Aydogan and Montoya (2011), the bird nest ferns employed in this study were acclimatized indoors for 2 weeks before starting the experiment in the laboratory, where the plants demonstrated a gas pollutant removal effect and removed the indoor CO₂ and HCHO effectively.

The plastic flower pots, the soil in the pots, and the walls were all coated with aluminum foil to reduce the effect of interference (Fig. 1C). All items that could potentially emit HCHO (e.g., plastic flower pots and the composite walls used for greening) were covered with aluminum foil before the experiment to ensure the accuracy of the measurements.

Experimental set-up

According to the standard value of IAQ of the Indoor Air Quality Management Act (IAQMA) introduced by the EPA of Taiwan Executive Yuan on 31 December, 2005, this study adopted a CO₂ concentration standard of 800 ppm, which is the standard for Type II space as shown in Table 1. The standard for HCHO is 0.1 ppm, which is the same as that in Korea and Germany. The World Health Organization (WHO-ROE, 2006) recommended a limit of indoor HCHO of 0.08 ppm. Gases were released up to the initial concentration, and then monitoring of the data was started until the mean concentration dropped to the IAQMA Standard.

Table 1.

The standard value of IAQ of Indoor Air Quality Management Act.

In accordance with previous research, such as that by Chen (1990), Huang (2008), and Jiang (1997), the initial dose of CO₂ was 2000 ppm in this study. The CO₂ measurement started when it reached a concentration of 2000 ppm. A maximum CO₂ concentration of 1000 ppm for acceptable comfort is recommended by The American Society of Heating, Refrigeration and Air-Conditioning Engineers (Swift et al., 2010) and is also generally recognized in Australia (Environment Australia, 2001).

In accordance with previous research, such as that by Chen (1990), Jiang (1997), and Shao (2006), the initial dose of HCHO was 2 ppm in this study. The HCHO measurement started when its concentration exceeded 2 ppm. Aydogan and Montoya (2011) adopted an initial concentration of 1.63 ppm HCHO in his experiment. Temperature and humidity changes were recorded under conditions with and without plants during daytime and nighttime.

The experiments were conducted in three groups. We examined the effect of reducing the level of pollutant gas by bird nest fern in Group A, Group B, and Group C. Each group was subjected to three different experimental conditions (1. without plants in daytime lighting, 2. with plants in daytime lighting, and 3. with plants in nighttime lighting), as shown in Table 2.

• (1) Group A: CO₂ was released with its concentration continually monitored.
• (2) Group B: HCHO was released with its concentration continually monitored.
• (3) Group C: CO₂ and HCHO were released simultaneously with their concentrations continually monitored.

Table 2.

Time to reach IAQMA standard by bird nest fern in three experimental groups.

Besides Group A, Group B, and Group C, a comparison group was established to examine the original indoor conditions (without gas released); this represented the removal rates of CO₂ and HCHO in 24 h under conditions with and without plants, as shown in Table 3.

Table 3.

The average removal rate of the comparison group in 24 h.

Sampling and analysis techniques

In this study, the following equipment was used: CO₂ monitor (KD AirBoxx; KD Engineering, Blaine, WA, USA), HCHO monitor (Formaldemeter htV; PPM Technology Ltd., Caernarfon, UK), temperature and humidity monitor (iLog; Escort, Cryopak, TCP Company, Edison, NJ, USA), and SC-1 Leaf Porometer (Decagon Devices, Inc., Pullman, WA, USA). We used the IBM SPSS statistical software to analyze the experimental data on an hourly basis. The data were calculated once per minute.

Results

Group A: Reaction rates for CO₂ release

The results showed that, when CO₂ at a concentration of 2000 ppm was released, 5 h 53 min was required to reduce the concentration to 800 ppm without any plants (A-1). In the laboratory with plants (A-2), 3 h 49 min in daytime lighting and 3 h 40 min in nighttime lighting (A-3) were required to reduce the CO₂ to the same concentration (Table 2 Group A). The time reduction was the greatest in the A-3 experiment, which was 2 h 13 min less than the time in the experiment without plants. The plant photosynthesis continues as long as light is available, even during the nighttime when illumination is provided. We used the IBM SPSS statistical software to analyze the experimental data on an hourly basis. As shown in Figure 2, the mean CO₂ concentration in Group A was higher than the IAQMA standard (800 ppm) until the 5th hour.

Fig. 2.

Time-course change of CO₂ concentration in Group A (CO₂ released) in a 38.88 m³ laboratory by pot installation with or without bird nest fern plants. In A-1, CO₂ was reduced to 800 ppm in 5 h 53 min. In A-2, CO₂ was reduced to 800 ppm in 3 h 49 min. In A-3, CO₂ was reduced to 800 ppm in 3 h 40 min.

The results showed that the temperature of the laboratory without plants (A-1) was approximately 2°C higher than that with plants, which was 22.9–23.9°C. With plants, the temperature of the laboratory was 21.8–22.1°C during daytime (A-2) and 21.9–22.2°C during nighttime (A-3). The humidity in the laboratory without plants was relatively low, but RH rose by about 10% when plants were present (data not shown).

Group B: Reaction rates for the release of HCHO

The results showed that, when an initial dose of HCHO (2 ppm) was pulse-injected into the laboratory, 9 h was required in the experiment without plants (B-1) to reduce the concentration to 0.1 ppm (IAQMA standard). When plants were used, 3 h 18 min in daytime lighting (B-2) and 3 h 8 min in nighttime lighting (B-3) were required for the HCHO to be reduced to the same concentration (Table 2 Group B). The greatest time reduction occurred in the B-3 experiment, which was approximately 6 h less than the time in the experiment without plants (B-1).

We used the IBM SPSS statistical software to analyze the experimental data on an hourly basis. As shown in Figure 3, the mean HCHO concentration in Group B was higher than the IAQMA standard (0.1 ppm) until the 5th hour.

Fig. 3.

Time-course change of HCHO concentration in Group B (HCHO released) in a 38.88 m³ laboratory by pot installation with or without bird nest fern plants. In B-1, HCHO was reduced to 0.1 ppm in 9 h. In B-2, HCHO was reduced to 0.1 ppm in 3 h 18 min. In B-3, HCHO was reduced to 0.1 ppm in 3 h 8 min.

The results showed that the temperature of the laboratory without plants (B-1) was approximately 1.5°C higher than that with plants, which was 22.6–23.4°C. With plants, the temperature of the laboratory averaged 21.5–22.2°C (B-2) during daytime and 21.6–22°C (B-3) during nighttime. The average humidity in the laboratory without plants was 61.5% RH. With plants, the average humidity was 70.4% RH (B-2) and 72.6% RH (B-3). During the experiment, the temperature dropped 1.5°C in the laboratory with plants; consequently, the humidity rose by 10% RH (data not shown).

Group C: Reaction rates with the release of CO₂ and HCHO

Monitoring reaction rates with CO₂

The results showed that, when CO₂ at a concentration of 2000 ppm was released, 5 h 30 min was required to reduce the concentration to 800 ppm without plants (C-1). When plants were used, 3 h 14 min in daytime lighting (C-2) and 3 h 12 min in nighttime lighting (C-3) were required to reduce the CO₂ to the same concentration (Table 2 Group C). The greatest time reduction occurred in Group C-3, which required approximately 5 h 22 min less than in the experiment without the plants (C-1).

Data were analyzed per hour using IBM SPSS software. As shown in Figure 4, the mean CO₂ concentrations in Groups C-2 and C-3 were higher than the IAQMA standard (800 ppm) until the 4th experimental hour.

Fig. 4.

Time-course change of CO₂ concentration in Group C (CO₂ + HCHO released) in a 38.88 m³ laboratory by pot installation with or without bird nest fern plants. In C-1, CO₂ was reduced to 800 ppm in 5 h 30 min. In C-2, CO₂ was reduced to 800 ppm in 3 h 14 min. In C-3, CO₂ was reduced to 800 ppm in 3 h 12 min.

Monitoring reaction rate with HCHO

When HCHO at a concentration of 2 ppm was released, 11 h 58 min was required to reduce the concentration to 0.1 ppm without plants (C-1). When plants were used, 5 h 22 min in daytime lighting (C-2) and 6 h 4 min in nighttime lighting (C-3) were required to reduce the HCHO to the same concentration (Table 2 Group C).

Data were analyzed per hour using IBM SPSS software. As shown in Figure 5, the mean HCHO concentrations in Groups C-2 and C-3 were higher than the IAQMA standard (0.1 ppm) until the 7th experimental hour.

Fig. 5.

Time-course change of HCHO concentration in Group C (CO₂ + HCHO released) in a 38.88 m³ laboratory by pot installation with or without bird nest fern plants. In C-1, HCHO was reduced to 0.1 ppm in 11 h 58 min. In C-2, HCHO was reduced to 0.1 ppm in 5 h 22 min. In C-3, HCHO was reduced to 0.1 ppm in 6 h 4 min.

The results showed that, with plants in Group C, the average humidity was 71.5% RH during daytime. During the experiment, the temperature dropped 1.8°C in the laboratory with plants. Consequently, humidity rose by 14–17% RH (data not shown).

Comparison group: Reaction rates without release of CO₂ and HCHO

The results indicated that, in the absence of plants, the indoor CO₂ level without the release of additional CO₂ gas was reduced from 430 ppm to 400 ppm in 24 h, whereas in the presence of plants, the CO₂ level was reduced from 430 ppm to 330 ppm in 24 h, as shown in Figure 6. This result indicates that the CO₂ removal efficiency was higher in the presence of plants than in their absence. Similarly, in the absence of plants, the indoor HCHO level without the release of additional HCHO gas was reduced from 0.047 ppm to 0.040 ppm in 24 h into the monitoring, whereas in the presence of plants, the HCHO level was reduced from 0.047 ppm to 0.031 ppm in 24 h, as shown in Figure 7. This result indicates that the HCHO removal efficiency was higher in the presence of plants than in their absence.

Fig. 6.

Time-course change of CO₂ concentration in the comparison group in a 38.88 m³ laboratory with or without plants and no gas released. Without plants, CO₂ was reduced from 430 ppm to 400 ppm in 24 h. With plants, CO₂ was reduced from 430 ppm to 330 ppm in 24 h.

Fig. 7.

Time-course change of HCHO concentration in the comparison group in a 38.88 m³ laboratory with or without plants and no gas released. Without plants, HCHO was reduced from 0.047 ppm to 0.040 ppm in 24 h. With plants, HCHO was reduced from 0.047 ppm to 0.031 ppm in 24 h.

In the comparison group, the results showed that the laboratory temperature with plants was approximately 1.5°C lower than that without plants. The humidity in the laboratory increased to 10% RH with plants (data not shown).

Discussion

Indoor air pollutants are mainly composed of HCHO and VOCs (Kim et al., 2013; Wolkoff, 2003). Plants can absorb various harmful gases such as HCHO, benzene, VOCs, trichloroethylene, CO, and sulfur dioxide (Ping et al., 2011). Researchers have confirmed that various indoor plants can purify the air by removing pollutants (Lohr and Pearson-Mims, 1996; Raanaas et al., 2011; Wolverton and Wolverton, 1993). To expand the database for characterizing different indoor plants, this study was established to measure the absorption rate of CO₂ and HCHO concentrations by bird nest fern, which is easy to grow and exhibits high transpiration in Taiwan. Since the concentration of air pollutants was shown to be higher in an enclosed space than that outdoors (Adgate et al., 2004), this study focused on the effect of removing CO₂ and HCHO by bird nest fern in an indoor green wall.

In a previous study, households with plants exhibited significant decreases in the contents of CO and CO₂ (Lim et al., 2006). Tarran et al. (2007) reported that rooms with plants were associated with a 25% reduction in CO₂ concentration in a non-air-conditioned building. Torpy et al. (2014) reported that CO₂ removal activities of any species cannot be wholly predicted; they must be empirically tested.

The obtained results revealed that the absorption in Group A was not more effective than that in Group C (Table 2). The removal rate of CO₂ with plants in Group A-3 was 160% higher than that without plants in Group A-1 and the removal rate with plants in Group C-3 was about 268% higher than that without plants in Group C-1.

Group C-3 exhibited the highest CO₂ removal rate, in which CO₂ and HCHO were released together in nighttime lighting, suggesting the most effective results for CO₂ reduction. The best CO₂ removal rate of bird nest fern (per pot) was 1.984 ppm·h⁻¹ [average removal rate in C-3 (375 ppm·h⁻¹) divided by the total number of plants (189 pots) = 1.984 ppm·h⁻¹ (per pot)]. The test results demonstrated that bird nest fern could remove CO₂ and HCHO concurrently in an indoor environment.

As shown by similar results from Aydogan and Montoya (2011) and Lim et al. (2006), bird nest fern led to a marked decrease in HCHO for the first hour in this study, but induced a slight decrease for the next 4 hours. Group B-3 demonstrated the most effective results for HCHO reduction, in which HCHO was released with plants in nighttime lighting. Bird nest fern showed superior efficiency in removing two gases (CO₂ and HCHO) rather than one. Although plants typically do not undergo photosynthesis during nighttime, the provision of artificial lighting enables the plants to function normally with higher efficiency. As a result, a potted bird nest fern had an average reducing rate of 0.003 ppm·h⁻¹ [average removal rate in B-3 (0.607 ppm·h⁻¹) divided by the total number of plants (189 pots) = 0.003 ppm·h⁻¹ (per pot)]. The removal rate of HCHO with plants in Group B-3 was about 288% higher than that without plants in Group B-1, whereas the removal rate with plants in Group C-2 was about 223% higher than that without plants in Group C-1. The obtained results indicate that the CO₂ concentration would restrain the removal rate of HCHO.

Similarly to the results from Asumi et al. (1995), Lim et al. (2006), Kim et al. (2013), and Park et al. (2008), the experiments indicated that bird nest fern could cool the environment to maintain a moderately stable room temperature and increase relative humidity in Group A, Group B, Group C, and the comparison group.

Bird nest fern can continue photosynthesis under artificial lighting during nighttime, and the photosynthesis was more effective at removing gas pollutants after having done so during the daytime, suggesting that bird nest fern may perform long-term and continuous removal of gas pollutants. The results demonstrate that bird nest fern could effectively remove indoor CO₂ and HCHO, thus purifying the indoor air.

The plants can beautify rooms and adjust indoor air temperature and humidity. Installation of indoor vertical green walls is an easy approach for improving IAQ and has little effect on horizontal space. The installation of green walls is increasing in the developed world (Almusaed, 2010). The major emphasis for green walls to date has been on their esthetic and psychological functions; however, there is growing understanding that they can also have significant effects on IAQ as well. Indoor plants represent a potential low-cost, easily maintained, air-cleansing component in the built environment (Torpy et al., 2014). As a subtropical-climate plant, bird nest fern is popular, economical, well-shaped, and easy to maintain in Taiwan. Therefore, the use of this plant in vertical green walls could be a useful technique in daily applications.

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