2020 Volume 89 Issue 3 Pages 319-327
This research evaluated the attention capacity of 70 pupils in the sixth grade with the intervention of indoor air quality regulated by indoor plant placement in the classrooms of two elementary schools in Seoul, South Korea. Two sets of three-week measurements were conducted with an interval of 12 weeks from 27th June to 7th October, 2016. We divided subjects into two groups (Group I and II): subjects in Group I occupied classrooms without indoor plants and those in Group II occupied classrooms with indoor plants. The classrooms with indoor plants had indoor levels with constant air temperature (approximately 26°C), relative humidity (around 50%), and carbon dioxide (CO2) (around 1100 mg·m−3). Additionally, 12-week placement of indoor plants reduced the indoor concentrations of airborne contaminants. After 12 weeks of the experiments, the subjects’ attention capacity improved as demonstrated by a standard questionnaire (Frankfurt Aufmerksamkeits-Invertar, FAIR). Indoor plant placement showed little difference in terms of efficiency (FAIR-E) and continuity (FAIR-C) scores, but exhibited a significant improvement for performance (FAIR-P) (increasing from 0.964 to 0.989) and quality (FAIR-Q) scores (increasing from 0.945 to 0.973). Based on multiple regression, the current study suggested that indoor plant placement was one of the most important factors to improve the attention capacity of pupils in classrooms.
Spending more of our lives indoors is one of the most widespread current trends. Earlier studies reported that the general public nowadays spend approximately 80% of their daily lives indoors (Orwell et al., 2004; United States Department of Labor, 2006; WHO, 2010). Consequently, it could be postulated that the health of indoor occupants is more greatly influenced by the indoor environment quality than outdoor factors.
Indoor occupants have often tried to maintain their indoor environments at a comfortable level using frequent mechanical ventilation. However, the ventilation tends to be done without considering the contamination level in indoor spaces (Moya et al., 2018). Researchers have suggested that indoor plant placement could be an alternative method for regulating indoor environments efficiently with the expectation of physical, as well as psychological benefits (Adachi et al., 2000; Bringslimark et al., 2009; Dijkstra et al., 2008; Lohr et al., 1996; Orwell et al., 2006; Wood et al., 2002). In addition to the visual attractiveness, indoor plant placement provides additional benefits to indoor environments via plants’ metabolic actions (Bringslimark et al., 2009; Gary and Birrell, 2014; Ottele, 2011; Raanaas et al., 2011; Wolverton, 1997).
That is, the placement of indoor plants regulates air temperature and humidity through evapotranspiration (Davis and Hirmer, 2015; Mangone and van der Linden, 2014; Perez-Urrestarazu et al., 2016). After the National Aeronautics and Space Administration (NASA) Clean Air Study presented a study about the metabolic actions of plants to remove toxic agents from the air in the 1980s (Wolverton et al., 1984, 1989), researchers have reported similar results that indoor plant placement helped reduce the indoor level of airborne contaminants (Kim et al., 2011; Lim et al., 2009; Orwell et al., 2006; Wood et al., 2002).
The health of indoor occupants is greatly influenced by the indoor environment quality and the accumulation of carbon dioxide (CO2) (Shendell et al., 2004) and volatile organic compounds (VOCs) (Daisey et al., 2003; De Kluizenaar et al., 2016) are caused by insufficient ventilation. Based on previous studies, it is postulated that the indoor environment quality plays an important role in the health and work performance of indoor occupants (Al Horr et al., 2016; Blueyssen et al., 2016; Frontczak et al., 2012; Kosonea and Tan, 2004; Raanaas et al., 2011; Wyon, 2004). Other reports also pointed out that the interaction between indoor occupants and plants in indoor environments can alter the attitudes, behavior, and physical responses of the indoor occupants, improving their productivity and overall satisfaction (Gary and Birrell, 2014; Lohr et al., 1996; Relf, 1990; Shoemaker et al., 1992).
Young individuals in their growth phase are highly sensitive to environmental factors (Faustman et al., 2000). They usually spend more than half of their daily lives in classrooms. Classrooms can be distinguished from other indoor environments by their overpopulation and abundant pollution sources (Becker et al., 2007; Fromme et al., 2008; Theodosiou and Ordoumpozanis, 2008). Some researchers suggested that exposure to a cleaner natural environment could help people improve attention capacity (Felsten, 2009; Kaplan, 1995; Tennessen and Cimprich, 1995; Wells, 2000). Others also proposed that proper management of the indoor environment quality using plants could enhance the attention capacity of indoor occupants (Momovic et al., 2009; Raanaas et al., 2011).
Considering the points above, the current study tried to clarify the relationship between the attention capacity of indoor occupants and their indoor environment quality when regulated by indoor plant placement. The aim of this study is to propose an appropriate indoor environment quality for elementary school pupils during their time indoors to improve attention capacity.
This research evaluated the attention capacity of 70 pupils in the sixth grade with the intervention of indoor air quality regulated by indoor plant placement in the classrooms of two elementary schools in Seoul, South Korea. Two sets of three-week measurements were conducted with an interval of 12 weeks from June 27 to October 7, 2016. The authors followed the same patterns as previous reports by Kim et al. (2013, 2016) in arranging facilities and subjects, placing indoor plants, and measuring the indoor environment. Detailed descriptions of the current study were as follows.
Arrangement of facilities and subjectsThe current study used two groups of classrooms (Group I and II) at two elementary schools (Schools A and B) which were established in 2011. Each school provided two classrooms: one classroom without indoor plants (Group I) and the other classroom with indoor plant placement (Group II). The classrooms of both schools had two opposing walls with windows. The classrooms of both schools had the same rectangular dimensions of 9.0 m long and 7.7 m wide. To regulate the indoor environment at an air temperature of 26°C, individual systems of mechanical ventilation and air conditioning were used for two hours twice a day (from 10:00 a.m. to 12:00 p.m. and from 2:00 p.m. to 4:00 p.m.).
We recruited 70 pupils in the sixth grade of elementary school who could attend school for the experimental duration without absence. All the pupils participated in this experiment with the permission of their parents after fully understanding the experimental procedure. We enrolled 40 participants for each of two classrooms at School A and 30 participants for each of two classrooms at School B, respectively (Table 1).
General description of the subjects.
The current study followed the descriptions of certain previous reports regarding the types, quantity and methods for indoor plant placement (Kil et al., 2008; Soreanu et al., 2013; WHO, 1986). We placed indoor plants in the classrooms of Group II at both schools for 12 weeks (from 6th March to 28th May) to acclimatize the pot plants to the indoor environment and removed them for four weeks to reduce any environmental damage to them prior to the actual examination. Then, indoor plants were again placed in the classrooms for the entire experimental duration (12 + 3 weeks, 27th June to 7th October).
We classified pot plants into two categories (large and small) based on a previous report by Lee and Kim (2005): a large pot plant was one with a leaf area of 3000 to 5000 cm2 and a small pot plant was one with a leaf area of less than 1000 cm2. Considering capacity for formaldehyde decomposition, earlier researchers recommended the suitable pot plants: lady palm, rubber plant, areca palm, and heavenly bamboo as large plants and bird’s-nest fern, peace lily, golden pothos, and tiny ardisia as small plants (Kil et al., 2008; Kim et al., 2009; Lee and Kim, 2005). A large pot plant was grown in a pot with a size of ø30 cm × 41 cm (h) (28.976 L) or ø24 cm × 23.5 cm (h) (10.626 L), and a small plant was grown in a pot with a size of ø18 cm × 15 cm (h) (3.815 L). Then, the researchers placed more than one large pot plant and one small pot plant in an area of 6 m2 by the window wall of each classroom, following the description of a previous report by Lee and Kim (2005) (Table 2; Fig. 1).
List of indoor plants placed in classrooms.
Photographs of classrooms for attention capacity measurements according to indoor plant placement.
The current study assessed indoor air conditions by recording the indoor levels of air temperature (°C), relative humidity (%), and CO2 (mg·m−3), and evaluated indoor air quality by measuring the indoor concentrations (μg·m−3) of formaldehyde and additional VOCs including benzene, toluene, ethylene, and xylene (BTEX).
The measurements were conducted over two measurement periods: the first and second measurement periods. The first measurement was conducted just prior to indoor plant placement (from 27th June to 15th July) and the second measurement was carried out just after 12 weeks of observation (from 19th September to 7th October). Each measurement period consisted of nine measurement days over three weeks (every Monday, Wednesday, and Friday). On every measurement day, we conducted the measurement at 9:00 a.m. and 1:00 p.m. with the pupils in an active state after closing all the windows and doors and ceasing mechanical air ventilation to prevent possible contamination from the outside environment.
The indoor levels of air temperature and relative humidity were recorded at one site at a height 1.5 m above floor level in the mid-part of the classroom (less than 4.0 m from plants) using a digital recorder (Testo 950; Testo SE & Co. KGaA, Lenzkirch, Germany). The indoor concentration of CO2 was measured at two sites (front and back sites, 3.0 to 6.0 m from plants) using a digital gas analyzer (Gas Analyzer OXYBABY M+; Witt-Gasetetechnik, Witten, Germany).
The current study measured the indoor concentrations of formaldehyde and BTEX at two sites in the classrooms (front and back sites, 3.0 to 6.0 m from plants). We installed a personal air sampler (Minipump ΣMP-100H; SHIBATA SCIENTIFIC TECHNOLOGY LTD., Saitama, Japan) in serial connection with an ozone scrubber cartridge (Waters Corp., Milford, MA, USA), which was filled with 350 mg of DNPH-silica (100 mg of DNPH). Then, indoor air was absorbed at a height of 1.5 m above floor level at a flow rate of 0.5 L·min−1 for 30 min and the concentration of formaldehyde was measured using high-performance liquid chromatography system equipped with ultra-violet absorption (Alliance 2690 and 2487; Waters Corp.). The indoor concentrations of BTEX using a solid thermal desorption method were also measured (Kim et al., 2010, 2011; Lim et al., 2009). We collected indoor air using another personal air sampler (Minipump ΣMP-100H; SHIBATA SCIENTIFIC TECHNOLOGY) equipped with a Tenax-TA tube (1/4" × 10 cm stainless steel; Supelco, Bellefont, PA, USA) at a flow rate of 10 L·min−1 for 30 min, and analyzed the indoor concentrations of BTEX.
Evaluation of attention capacityThe attention capacity of all subjects was assessed twice a day at 9:00 a.m. and 1:00 p.m. for the nine measurement days (every Monday, Wednesday, and Friday over three weeks) for each measurement period. We used the Frankfurt Aufmerksamkeits-Invertar (Frankfurt Attention Inventory, FAIR) questionnaire for the assessment through the scores for performance (FAIR-P), efficiency (FAIR-E), quality (FAIR-Q), and continuity (FAIR-C), carefully following the guideline to prevent the accidental errors from learning effects (Fink et al., 2002; Moosbrugger and Oehlsclagel, 1995, 1996; Schweizer et al., 2000). To analyze the factors affecting attention capacity, the authors used a model for multiple regression analysis based on the questionnaire. Independent variables included the levels of indoor air compounds and the concentrations of indoor airborne contaminants.
Statistical analysisA paired t-test was used to analyze the variations in indoor air conditions and indoor air quality with the intervention of indoor plant placement. Factors affecting the attention capacity of subjects were analyzed using multiple regression.
After 12 weeks of observation, the classrooms without indoor plants had increased air temperature (from 25.1 to 29.4°C) and CO2 (from 1087 to 1277 mg·m−3), and decreased relative humidity (from 57 to 51%). The placement of indoor plants did not alter the increasing tendency of air temperature or CO2, or the decreasing trend in relative humidity. However, the increases of air temperature and CO2 were reduced and the decreasing trend of relative humidity was facilitated in the classrooms with indoor plants (Fig. 2).
Thermal conditions and CO2 concentrations in the classrooms. Vertical bar on top bar indicated standard error. 1st Period; measurement before indoor plant placement (June 27 to July 15): 2nd Period; measurement after indoor plant placement (September 19 to October 7). Without; classrooms without indoor plants: With; classrooms with indoor plant placement. NS: non-significance, * significant at P < 0.05.
The indoor concentration of formaldehyde significantly decreased from 31.02 to 12.01 μg·m−3 over time in the classrooms without indoor plants (P < 0.001). The placement of indoor plants slightly facilitated a decreasing tendency of formaldehyde. In the classrooms without indoor plants, benzene showed a lower concentration at the second measurement (5.50 μg·m−3) than at the first measurement (5.92 μg·m−3) (P < 0.01). Indoor plant placement significantly facilitated a decreasing trend in benzene from 5.25 to 3.84 μg·m−3 (P < 0.001). After an interval of 12 weeks, we found the indoor concentration of toluene slightly increased from 65.77 to 68.51 μg·m−3 in the classrooms without indoor plants, but decreased from 75.42 to 39.10 μg·m−3 in the classrooms with indoor plants. The current study found little difference in the indoor concentrations of ethylbenzene and xylene, with no significant differences over time. However, indoor plant placement led to clear decreasing tendencies in the indoor concentrations of the two contaminants with clear significance (decreasing from 9.82 to 5.06 μg·m−3 and from 7.75 to 6.36 μg·m−3, respectively) (Table 3).
Indoor concentrations of airborne contaminants in classrooms (unit: μg·m−3).
After an interval of 12 weeks in the classrooms without indoor plants, subjects’ FAIR scores increased. The placement of indoor plants facilitated the increase in some, but not all, FAIR scores. The variations in FAIR-P scores between the two measurement periods were 0.017 in the classrooms without indoor plants (0.970 for the first period and 0.986 for the second period) and 0.025 in the classrooms with indoor plants (0.964 for the first period and 0.989 for the second period). Based on the independent t-test with the intervention of indoor plant placement, we found that the variation in FAIR-P scores between the two measurement periods was significantly higher in the classrooms with indoor plants than in those without them.
Regardless of indoor plant placement, subjects’ FAIR-E scores increased over the experimental period. The placement of indoor plants had a negligible impact on FAIR-E scores between the two measurement periods. The variations in FAIR-E scores between the two measurement periods were 135 in the classrooms without indoor plants (433 for the first period and 568 for the second period) and 135 in the classrooms with indoor plants (439 for the first period and 574 for the second period).
The mean value of FAIR-Q scores increased for the subjects of both groups over the experimental period, and was significantly facilitated by indoor plant placement. The variations in FAIR-Q scores were 0.013 in the classrooms without indoor plants (0.938 for the first period and 0.945 for the second period) and 0.028 in the classrooms with indoor plants (0.951 for the first period and 0.973 for the second period). FAIR-C scores increased over the experimental period. The placement of indoor plants did not cause a significant difference in FAIR-C scores, with scores of 136 in the classrooms without indoor plants (412 for the first period and 548 for the second period) and 139 in the classrooms with indoor plants (421 for the first period and 560 for the second period) (Table 4).
Changes in attention capacity scores according to indoor plant placement.
The authors found the scores for subjects’ attention capacity had a highly positive correlation with indoor plant placement through multiple regression analysis. The t-values of indoor plant placement showed highly significant values with all the factors of attention capacity as follows: 3.144**, 9.883**, 6.044**, and 10.094** for FAIR-P, FAIR-E, FAIR-Q, and FAIR-C, respectively.
We also found that the scores for subjects’ attention capacity had a weak relationship with indoor air conditions and air airborne contaminant concentrations. The scores of FAIR-P, FAIR-E, and FAIR-C exhibited a positive correlation with the indoor concentration of CO2 with t-values of 2.745**, 4.547**, and 4.169**, respectively, but showed a negative correlation with ventilation frequency with t-values of −2.088*, −5.692**, and −5.277**, respectively.
The scores for subjects’ attention capacity were negatively correlated with the indoor concentrations of formaldehyde and ethylbenzene: a t-value of −3.481** between formaldehyde concentration and FAIR-P score, and t-values of −3.009**, −2.074**, and −2.205* between ethylbenzene concentration and FAIR-P, FAIR-E, and FAIR-C scores, respectively. In contrast, the indoor concentration of benzene exhibited a positive correlation with the scores of FAIR-P, FAIR-E, and FAIR-C, having t-values of 2.696**, 2.427**, and 2.466*, respectively (Table 5).
Multiple regression analysis of attention capacity.
In addition to being visually pleasant, indoor plant placement provides psychological benefits as well as physical benefits to indoor occupants (Bringslimark et al., 2009; Gary and Birrell, 2014; Mangone and van der Linden, 2014; Raanaas et al., 2011; Wolverton, 1997). We studied 70 individuals in two elementary schools to examine their attention capacity with the intervention of indoor environment quality regulated by indoor plant placement.
In the present study, the indoor environment quality of each classroom was properly managed through individual systems of mechanical ventilation and air conditioning. Therefore, we only found slight differences in the indoor air temperature, levels of relative humidity, and CO2 between the two groups of classrooms. An observation period of 12 weeks revealed variations to some degree in the indoor air temperature, levels of relative humidity and CO2 in the classrooms without indoor plants. The indoor level of relative humidity decreased (48%) and the air temperature (26.3°C) and CO2 concentration (1125 mg·m−3) were constant in the classrooms with indoor plants. Previous researchers have reported that indoor plant placement can regulate air temperature and relative humidity in indoor spaces through evapotranspiration (Davis and Hirmer, 2015; Mangone and van der Linden, 2014; Moya et al., 2018; Qin et al., 2014). Other previous studies pointed out that a classroom is a unique space because of its overpopulation of students (Becker et al., 2007) and the increased indoor level of CO2 mainly by the metabolic processes of indoor occupants (Cetin and Sevik, 2016). Therefore, it is thought that the CO2 level in the classrooms was increased by the metabolic processes of pupils and regulated by the metabolic processes of indoor plants (Fig. 2).
An overall observation of the indoor concentrations of airborne contaminants during the experimental period revealed that some of the contaminants decreased, while others increased. The placement of indoor plants facilitated the tendency to decrease contaminants and reversed the increasing trend to a decreasing one. Previous researchers defined this phenomenon as phytoremediation, by which plants remove contaminants from the air, water and soil (Moya et al., 2018). This phenomenon has been consistently reported by many researchers in various studies (Kim et al., 2009, 2010; Lim et al., 2009; Orwell et al., 2006; Wood et al., 2002). However, a closer observation of the indoor concentration of formaldehyde revealed that indoor plant placement had little effect on the decreasing tendency. According to earlier reports, the decomposition degree of airborne contaminants varied depending on the kinds of plants placed, the growing media used (Salt et al., 1998) and the chemical properties of the contaminants (Bacci et al., 1990; Trapp, 2007). Based on all the observations above, we postulate that indoor plant placement facilitated the decomposition of airborne contaminants to various degrees via the interaction of indoor plants and airborne contaminants (Table 3).
We also conducted three weeks of indoor plant placement for plant acclimation, followed by four weeks of indoor plant removal for a balanced comparison in the actual study. As a result, the numeric values of indoor conditions and indoor air quality exhibited little difference in the first observation period as shown in Figure 2 and Table 3. Based on the hypothesis above, it is thought that the current study produced a fair comparison of attention capacity for the pupils of in the two groups.
After 12 weeks, all the subjects perceived that their attention capacity had increased. The placement of indoor plants made little difference to the scores of FAIR-E and FAIR-C, but brought about significant improvement in the FAIR-P and FAIR-Q scores. This investigation using the FAIR-questionnaire had a high possibility of accidental errors such as a bias caused from repeated testing and the natural improvement in attention capacity over time.
Therefore, the appropriate guidelines (Moosbrugger and Oehlsclagel, 1995, 1996) were followed to reduce the experimental errors. We believe the results of this investigation are significant based upon previous reports on the reliability of the questionnaire (Fink et al., 2002; Schweizer et al., 2000).
In previous reports, occupying a natural environment enabled people to increase their attention capacity (Adachi et al., 2000; Dijkstra et al., 2008; Felsten, 2009; Kaplan, 1995; Tennessen and Cimprich, 1995; Wells, 2000). Furthermore, green areas with plants in indoor spaces could alter the attitude and behavior of indoor occupants (Moya et al., 2018) and improve their perception (Mangone and van der Linden, 2014; Qin et al., 2014). A similar result was also found in an earlier report of Lohr et al. (1996) in which the placement of indoor plants in a computer lab at a university improved the attention capacity of students. Considering all the results above, we conclude that the interaction of indoor plants could partly enhance the attention capacity of indoor occupants (Table 4).
Through statistical analysis using multiple regression, we revealed a relationship between the attention capacity of subjects and the indoor contaminant levels in their environments and identify the most significant environmental factors affecting subjects’ attention capacity. The multiple regression showed that indoor plant placement was the most significant factor affecting subjects’ attention capacity.
It is clear that the indoor environment quality can play an important role in the work performance and productivity of indoor occupants (Al Horr et al., 2016; Blueyssen, 2016; Frontczak et al., 2012; Perez-Urrestarazu et al., 2016). Other researchers also reported that appropriate management of the indoor environment could improve the learning ability of students indoors (Mendell and Heath, 2005; Momovic et al., 2009). We also found that indoor plant placement resulted in airborne contaminant level reductions and enhanced psychological stability, which improved the attention capacity of indoor occupants (Table 5).
While a great number of studies have been conducted to investigate the effects of indoor plant placement on indoor environment quality, most of them did not achieve consistent results (Kim and Mattson, 2002; Park and Mattson, 2008, 2009; Shibata and Suzuki, 2001, 2002). The inconsistency could result from the differences in measurement methods, indoor environment, and plant conditions (Bringslimark et al., 2009; Kim and Mattson, 2002; Shibata and Suzuki, 2004).
This study tried to generate consistent results by using the FAIR questionnaire and applying multiple regression analysis; these factors improve the limitation of conventional tests for attention capacity evaluation which measure only a single function at a time. However, the present study also had certain limitations such as a lack of control of some individual conditions that could influence indoor environment quality and occupants’ attention capacity. From the results above, the authors found that indoor occupants were greatly influenced by indoor airborne contaminants. Especially, individuals in the growing phase are highly sensitive to environmental factors (Faustman et al., 2000). For that reason, appropriate management should be established for indoor environment quality for individuals in the growing phase to maintain their attention capacity with appropriate standards.
