Air Purifi cation Capability of Potted Phoenix Roebelenii and Its Installation Effect in Indoor Space

Phoenix roebelenii (commonly called Roebelenii palm or dwarf date palm) is a wellknown potted plant, which is endemic to Hachijyo Island in Japan. It is commonly called “Roebe.” Roebe production is an important industry on the island. Most products of this industry are exported to Europe. The export volume has been decreasing since 2001 and the producers of Roebe are badly affected by this decrease. The reason for the decrease is that cheap Roebes from South America are now exported to Europe. Roebe is placed in indoor spaces, namely, offi ces and homes, because it has a healing effect and an air-purifying function. It is necessary to revitalize the agricultural industry in Hachijyo by characterizing the positive features of the Hachijyo Roebe. In this study, the air purifi cation capability (Pa) considering environmental factors (temperature and light intensity) and the healing effect of Roebe were examined with the goal of increasing the export volume of this plant. Results showed that Pa increased with an increase in room temperature from 21 to 26°C, reaching a range of 15–35. It became clear that Roebe has a substantial purifi cation function. It also became clear that it has a healing effect to some degree. The effect was evaluated using an instrument that measures the amylase activity in the saliva of subjects. The relationship between Pa and the molecular weight of a polluting chemical was also examined. Results showed that Pa decreased as the weight increased.


Introduction
Several hundred millions of years have elapsed since terrestrial plants emerged. Humans have been living with plants throughout their history and plants have made various contributions to human life. Humans desire emotional recovery and take great comfort from the presence of plants. (1) Plants degrade air pollutants and greenhouse gases discharged by humans. (2) Therefore, plants have been placed in indoor spaces, such as in living and working environments. It has been clarifi ed that workplace productivity increases with the presence of potted plants. (3) Work and living spaces can be improved by taking advantage of plant functions (contribution degree). Humans can easily place plants in most types of working environments. Humans spend much time in enclosed environments including those in ships, cars, and airplanes. Plants are even needed for humans to live in space shuttles. (4) Gaseous air pollutants are generated in a superinsulated space. These pollutants are emitted by humans and building materials. Plants can absorb these gases and emit oxygen when there is suffi cient light intensity. The concentration of carbon dioxide in the presence of plants does not reach excessive levels even in darkness. (5)(6) Phoenix roebelenii (popularly known as "Roebe") is grown on Hachijyo Island in Japan. The market share of producers on the island exceeds 97% in Japan. It is exported to the EU as well as to other prefectures in Japan. It is placed indoors. On Hachijyo Island, the agricultural production of Roebe is the key industry. The industry on the island has recently been hit by the increasing production of this plant in Guatemala and Costa Rica. These countries also export Roebe to the EU. A potted Roebe produced on Hachijyo costs about 10,000 to 20,000 Yen, but those produced in the above countries cost about 3,500 Yen. However, the Japanese type is superior in terms of brightness of leaf refl ection and leaf shape: the leaves are slender. In this study, the air purifi cation capability of the Hachijyo Roebe was examined to identify any distinctive positive characteistics. It is thought that the industry can recover its once increasing export volume of Roebe. For this reason, the capability of Roebe to absorb formaldehyde (HCHO) was investigated as a function of temperature and light intensity. This chemical is the main causative substance of sick house syndrome. The results show clearly that Roebe has a relatively high absorption capability in addition to exhibiting a healing effect to some extent. It is thought that Roebe is effective indoors.

Air purifi cation capability
The air purifi cation capability of Roebe was evaluated in an enclosed chamber made of acrylic. The chamber is 575 mm long, 510 mm wide, and 1,000 mm high, and has a capacity of about 300 L. A potted Roebe was placed in the chamber and each test chemical was injected into the chamber using a microsyringe. The chemicals were formaldehyde, toluene, xylene, and styrene, which cause sick house syndrome. The concentration of each injected chemical decreased owing to the purifi cation function of Roebe. The concentration was measured using a gas sensor (TGS #826; Figaro Co., Osaka). The sensor is a type of alarm detector. The resistance of the sensor decreases when a reducing gas (the above-mentioned chemicals) is present in the atmosphere. The resistance is also affected by atmospheric temperature, humidity, and pressure to some degree. Temperature markedly affects the resistance. Therefore, the temperature in the experimental room was controlled using an air-conditioner. The humidity in the room was also kept low. The purifi cation capability was evaluated using the sensor output.
The basic measurement circuit of the output is indicated in Fig. 1. The sensor resistance R s and the load resistance R out were connected in series and the dc voltage V c was applied to the circuit. R s decreases when the concentration of a reducing gas increases, and the output V out at both ends of R out increases. The output is determined using eq. (1).
The sensor output V out was converted into a digital signal through an A-D converter and the signal was inputted into a data logger. Subsequently, the signal was inputted into a mobile computer. The sampling interval was 1 min. Sunlight was blocked and daylight fl uorescent lamps were used in the experimental room at one stage. Sunlight through paned windows was adopted at another stage. Light intensity was controlled using the above-mentioned lights. Atmospheric temperature was controlled using an air conditioner, but the temperature control accuracy was not good. The Roebe was watered one day before the measurement using 100 cc of water. Photographs of the Roebe and the experimental chamber in which the Roebe was placed are shown in Fig. 2. The height of the Roebe was 93 cm from the pot bottom. The pot was 20 cm in diameter and 26 cm in height. The soil in the pot was a planter mix.
The typical purifi cation characteristic is shown in Fig. 3. The base level (offset level) was measured for 1 h and a chemical was injected into the chamber, as shown in the fi gure. After the chemical was injected, sensor output (V out ) increased rapidly with the chemical concentration, namely, the sensor resistance decreased when the chemical was injected. The sensor output returned to its base level as a result of the purifi cation capability of the Roebe. The capability (P a ) is derived using a maximum V out (symbol h is adopted in the fi gure) from the base level and the half-value width t w . P a is defi ned in eq. (2). It becomes large when the rate of decrease is larger than the pollutant concentration. The capability can be compared with other plant purifi cation capabilities. Fig. 1. Basic measuring circuit for sensor output.

Stress-relieving capability
Stress has become an increasingly important issue because of changing working environments and lifestyles. Humans easily get sick with stress. Fatigue sometimes causes stress. Selye used the term "stress" in connection with a living organism for the fi rst time in 1953. Rest and healing are necessary to avoid the accumulation of stress. (7) Healing is very diffi cult to defi ne and there is as yet no appropriate defi nition. There are various types of approaches to reduce stress, for instance, music, travel, and aroma. It is often said that forest therapy and horticulture also have a marked effect. (8) Humans have had a close relationship with plants over a long period of time and have experienced emotional recovery by living with plants.  The amylase activity of humans increases with the excitation of the sympathetic system by a stress from outside the body as part of a self-defense reaction. (9) It can be determined whether or not humans are stressed by measuring amylase activity. Equipment for analyzing saliva amylase activity (Nipro Co., COCORO Meter) was used, and the healing effect of Roebe was measured. (7) The activity is measured using a measuring sheet that can sample the saliva of a subject. Namely, the sheet is placed in the mouth and then saliva is analyzed using the equipment. It takes about 1 min to measure and analyze the saliva sample.
The measurement steps are as follows. Each step was carried out in the presence and absence of Roebe. The data were compared and the healing effect of Roebe was determined. The suffi x n denotes the absence of Roebe. The suffi x r denotes the presence of Roebe, for example, h bn and h br . The difference between the factors is dealt with as a mitigation grade (healing grade). Base levels (h bn and h br ) are compared with levels after stress by the following procedure.

Purifi cation capability of Roebe for formaldehyde
Formaldehyde is one of the main causative substances of sick house syndrome. Its concentration indoors is restricted to 0.08 ppm (100 μg/m 3 at 25°C) by the Ministry of Health, Labor and Welfare of Japan. The value is a guideline. There are twelve additional restricted substances. Formaldehyde was injected into the chamber at a concentration 100-fold that of the guideline (8 ppm) in this experiment and the purifi cation capability was examined. Plants absorb substances through their stomata under ordinary circumstances. However, the stalk of Roebe is porous and can absorb some pollutants. Namely, its capability may be slightly high. Equation (2) is, however, adopted to evaluate the purifi cation capability of Roebe under normal conditions. The purifi cation characteristic of Roebe for formaldehyde at 8 ppm is indicated in Fig. 4. The vertical axis denotes the concentration change and the horizontal axis denotes the elapsed time. The characteristic was measured at room temperature (26°C) and a light intensity of 1,000 lx. The characteristic increased rapidly after the injection of formaldehyde and decreased according to Roebe's purifi cation function. The level returned to the base level. In the fi gure, h is 0.36 [V], t w is 1.05 [h], and P a is 34. The P a of Roebe is relatively high compared with those of other plants. As an example, pothos (Epipremnum aurum) has a higher purifi cation capability. The P a of pothos is shown in Fig. 5. The experimental conditions are similar to those in Fig. 4. It is thought that indoor air quality has been considerably improved in Europe owing to Roebe. Note that Europe is the main destination of exported Roebe. The purifi cation capability changes with light intensity and atmospheric temperature. It becomes higher when the room temperature is in the range of the growing temperature.

Purifi cation capability dependence on temperature
Room temperature is usually about 20 to 26°C in homes and offices. This temperature range is good for indoor plants. In the homes of elderly people, the optimum temperature is slightly higher. Therefore, the purifi cation capability was examined at temperatures of 21 to 26°C. The results are shown in Fig. 6. P a increases as atmospheric temperature increases. The optimum growing temperature of Roebe is 16 to 24°C. The differential coeffi cient D is also indicated in the fi gure. D is defi ned as follows. Formaldehyde concentration decreases and sensor output also decreases depending on plant purifi cation function after pollutant injection. D is defi ned as the absolute value of the maximum differential coeffi cient of the P a curve. The capability is high when D is large. P a and D increase as temperature increases and there is a positive correlation between the two. The correlation is shown in Fig. 7. The correlation coeffi cient of the plots is about 0.86, but it is low at low temperatures. It is thought that the coeffi cient D denotes a real capability, but common people cannot grasp such a concept. The correlation coeffi cient means that P a is also effective for judging the green plant purifi cation capability.

Purifi cation function dependence on light intensity
Illumination is an important environmental factor when humans use a personal computer and carry out clerical work. The intensity is specifi ed by Japanese Industrial Standards (JIS) in each environment, for example, 750-1,500 lx in offi ces, and 750-1,000 lx in computer rooms. The range is aimed at 20 to 40-year-olds. The optimum light intensity at an offi ce desk is about 750 lx. P a was analyzed as a function of light intensity. The results are shown in Fig. 8. P a increases as light intensity increases. It is about 15 at 0 to 500 lx and exceeds the value of 25 above 700 lx. Roebe has a high capability at the optimum temperature and optimum light intensity in an offi ce environment. The plant indicates photosynthesis at about 350 lx and emits oxygen above 700 lx. Roebe also has a purifi cation function even at a light intensity below 500 lx. This function is derived from microorganisms living in the potted soil and leaf surfaces. Gaseous pollutants can diffuse into the soil, where they are decomposed by many types of microorganisms.

Healing effect of plant
The term "healing effect" is used frequently nowadays because humans are often tired both mentally and physically. The term is diffi cult to defi ne and there are many things that exert the healing effect, for example, music, scent, plants, and pets. In this study, the healing effect was evaluated by measuring relief from stress. Alpha amylase is present in saliva as a stress marker. The evaluation was conducted by measuring alpha amylase concentration. Data entry work for a limited time was adopted as the source of stress. The work is an examination (the pre-level 1 Japanese test) for data entry carried out by The Association of Japan Information Processing Examination. The examination time is 10 min. The volume of amylase was analyzed and data for both the presence and absence of Roebe in the examination room were compared. The results are shown in Table 1. h r decreases in the presence of Roebe. Other values are almost the same P a Light intensity (lx) Fig. 8. P a as a function of atmospheric light intensity. (unit: kIU/L) between the two conditions. It is diffi cult to measure stress level because it depends on the subject's condition. Only one subject was used, but the measurement was carried out several times. The data were very much the same. However, it is necessary to study many subjects and to perform the measurement several times to obtain more conclusive results. Only one evaluation method is introduced in this investigation. In this method, we expect to evaluate stress using brain waves and other techniques. The results must be compared with each other. It is understood that stress is alleviated during work breaks. Thus, it is necessary to take appropriate breaks to decrease stress in a work environment.

Purifi cation capability for styrene
Roebe is capable of purifying many other airborne chemicals aside from formaldehyde. It takes a long time for this plant to purify large-molecular-weight substances, and in such circumstances, the P a of Roebe decreases with time. The P a for styrene was examined for reference in this study. The molecular weight of formaldehyde is 30.03 and that of styrene is 104.15. The purifi cation capability for styrene is indicated in Fig. 9. Styrene was injected into the chamber at a concentration of 1.5 ppm. The P a of Roebe for styrene was 15.3 in the experiment. The P a is inversely related to molecular weight and decreases compared with that for formaldehyde. Styrene is mainly emitted by offi ce machines. Toluene and xylene are emitted by building materials.

Purifi cation capability for toluene
Toluene (molecular weight: 92.14) is an organic compound belonging to the aromatic hydrocarbon family. It is a typical airborne chemical contained in coating compounds. Construction companies perform safety assessments by measuring toluene concentration when they construct a building. The concentration of toluene in an indoor environment is required to be below 0.07 ppm (0.26 mg/m 3 ), as specifi ed by the Ministry of Health, Labor and Welfare of Japan. The purifi cation capability of Roebe for toluene is shown in Fig. 10. The concentration of toluene in the chamber was 1.5 ppm at the beginning of the experiment. P a was 6.5, namely, it takes a relatively long time to remove the chemical. Toluene has a lower molecular weight than styrene, and its P a is low. The relationship between P a and molecular weight is discussed in the following section. Plants require a long time to purify large-molecular-weight chemicals. Thus, it is better to use smallmolecular-weight and low-risk substances as much as possible.

Relationship between purifi cation capability of plant and molecular weight of pollutant
It is diffi cult to evaluate a plant's purifi cation capability because plants are living things and their capability depends on environmental factors. However, people want to know the general capability of particular plants to degrade pollutants, and they also  often desire to live a simple lifestyle designed to reduce the earth's environmental load. The purifi cation capability of Roebe for some chemicals of different molecular weights was investigated in this study. The results are shown in Fig. 11. It took a long time for Roebe to purify and decompose large-molecular-weight chemicals. Thus, it is better to use small-molecular-weight chemicals from the viewpoint of the earth's environmental load.

Conclusions
The purifi cation capability and healing effect of Roebe, which is endemic to Hachijyo Island, were evaluated. Roebe has a purifi cation capability for airborne chemicals, for example, formaldehyde, toluene, and styrene. It also has a healing effect. However, the effect must be investigated in more detail. An increase in the export volume of Roebe can be achieved by promoting the positive characteristics of this plant obtained in this study. It is also important to control the growth environment of Roebe. Other positive features of the Roebe produced on Hachijyo Island are the slender leaf confi guration and refl ective leaf surface. These features could be promoted together with the newly found positive characteristics.