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sustainability
Article
Effects of Indoor Plants on the Physical Environment
with Respect to Distance and Green Coverage Ratio
Ke-Tsung Han
Department of Landscape Architecture, National Chin-Yi University of Technology, No.57, Sec. 2,
Zhongshan Rd., Taiping Dist., Taichung 41170, Taiwan; kthan@ncut.edu.tw
Received: 28 May 2019; Accepted: 1 July 2019; Published: 4 July 2019
Abstract:
Few studies have conducted experiments in daily living environments to examine the
effects of indoor plants on objective aspects of the physical environment. This study examined the
effects of plant distance and green coverage ratio on the objective physical environment and subjective
psychological perceptions, along with the correlation between the objective physical environment and
subjective psychological perceptions regarding indoor plants. A randomized control trial of plant
distance and green coverage ratio was conducted in a room located in the basement of a university
building in Taiwan. Aspects of the objective physical environment were measured using air quality
detectors. Subjective psychological perceptions were evaluated based on the questionnaire responses
of 60 undergraduates. The results revealed that (1) regardless of number of plants, the closer the plant,
the higher the CO
2
level; (2) more indoor plants resulted in higher CO
2
and humidity and lower
PM
2.5
, PM
10
, and temperature; and (3) the lower the levels of fine and suspended particles in the air
were, the stronger were the feelings of preference, naturalness of the environment, and pleasure in
participants. Indoor plants that can regulate indoor air quality and microclimates without consuming
energy warrant greater attention and wider application.
Keywords:
Radermachera hainanensis Merr.; carbon dioxide; humidity; fine particulate matters;
suspended particles; temperature
1. Introduction
People in contemporary society spend 80% to 90% of their time indoors every day [
1
]. Thus, having
a comfortable indoor environment with favorable indoor air quality is imperative, as indicated by the
increasing prevalence of sick building syndrome [
2
]. In the United States alone, roughly 27 million office
workers are at risk of sick building syndrome, and 30% of new buildings worldwide are associated
with indoor air pollution problems [
3
]. Moreover, indoor air pollution is generally 2 to 5 times worse
(sometimes up to 100 times worse) than outdoor air pollution [
4
]. This may be one of the reasons
that people in contemporary society are experiencing increasingly severe physical and mental health
problems, along with declining general wellbeing [5].
By contrast, exposure to nature is considered beneficial to physical and mental health [
6
]. This is
why people often grow plants indoors to improve the quality of their living environment and
workspace [
7
], because plants represent nature [
8
,
9
]. Empirical studies have also demonstrated that
indoor plants have positive effects on physical and mental health as well as general wellbeing [
10
,
11
].
Therefore, the beneficial effect of indoor plants on public health is a topic requiring exploration.
Research has shown that having even a few indoor plants is beneficial to the general wellbeing and
physical and mental health of humans. For example, covering 6% of an indoor floor area with indoor
plants can elicit significantly stronger feelings of preference, comfort, and friendliness in students and
reduce their misbehavior and number of sick leave days [
12
]. A study conducted in real offices showed
that the presence of plants has a significant effect on reducing short-term sickness absence, compared to
Sustainability 2019,11, 3679; doi:10.3390/su11133679 www.mdpi.com/journal/sustainability
Sustainability 2019,11, 3679 2 of 19
the absence of plants [
13
]. Similarly, having a green space comprising 2% of a classroom floor area can
promote feelings of wellbeing in students [
14
]. Considering the extremely low number of indoor plants
used, the differences in participant responses were attributed to the possible effects of novelty (adding
“something new” to the classroom perceived visually, olfactorily, auditorily and/or tactually) and not
necessarily to the positive effects of the indoor plant itself. Furthermore, the distance from indoor
plants appears to have distinct effects on the subjective feelings of humans [
14
–
16
]. Another possibility
is that people’s responses were affected by the visibility of the plant rather than its distance from them
because greater proximity to a plant corresponds to greater plant visibility. Studies have reported a
positive correlation between the visible density of urban tree coverage with stress recovery [
17
] and
landscape preference [18].
Plants are the most common element of nature and are also often regarded as the most representative
of nature [
8
,
9
]. This is true even within manmade structures. However, a room with plants differs from
the natural outdoor environment in that the plants have been separated from their natural habitat.
Therefore, the role of indoor plants is ambiguous: they can be perceived as a symbol of nature or as a
result of human intervention, interference, and even control over nature [
8
]. Research has yet to explore
the effects of indoor plants with respect to novelty and perceived naturalness. This study is the first to
examine the psychological effects (i.e., novelty and perceived naturalness) of bringing plants, which
symbolize nature, from outdoor (natural environment) to indoor settings (artificial environment).
Furthermore, the objective effects of indoor plants on the physical environment (e.g., air quality,
temperature, and humidity) should also be examined. Because mechanical ventilation and
air-conditioning generally consume energy, research should focus on energy-free methods of regulating
indoor air quality and microclimate [
19
]. Studies conducted since 1989 have reported that indoor
plants can significantly reduce urban air pollution [
20
–
26
]. Nevertheless, little research has conducted
experiments in a daily living environment to examine the effects of indoor plants on air quality,
temperature, and humidity. The majority of existing experiments of indoor plants have involved
highly concentrated pollutants in small closed fumigation chambers, and these experiments have
not investigated the effects of distance from indoor plants. Greater proximity to the plant should
yield more favorable effects, considering that air purification and microclimate regulation rely
on plant photosynthesis, adsorption, respiration, transpiration [
27
], and soil microbes [
22
,
25
].
Additionally, objective conditions of the physical environment (i.e., air quality, temperature,
and humidity) should also influence subjective psychological perceptions (e.g., preference, emotions,
and comfort).
The primary objective of this study was, therefore, to investigate the effects of the distance
from indoor plants on the objective aspects of the physical environment (i.e., air quality, temperature,
and humidity) and subjective psychological perceptions (i.e., novelty, perceived naturalness, preference,
emotions, and environmental comfort). Plant effects were further manipulated with the distance and
green coverage ratio using the experimental method. The green coverage ratio refers to the ratio of
floor area covered by the vertical projection of plants to the total surface area. This ratio is an objective
measure of the number of plants in a two-dimensional plane. Stapleton and Ruiz-Rudolph [
28
] revealed
a linear correlation between the number of plants and the percentage of ultrafine particle reduction.
The secondary objective was to examine the correlation between the objective physical environment
and the subjective psychological perceptions regarding indoor plants.
The hypotheses proposed in this study are as follows:
Hypothesis 1 (H1)
.Plants at different distances and with the same green coverage ratio have different effects
on the objective physical environment in terms of air quality, temperature, and humidity.
Hypothesis 2 (H2)
.Plants at the same distance but with different green coverage ratios have different effects on
the objective physical environment in terms of air quality, temperature, and humidity.
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Hypothesis 3 (H3)
.Plants at the same distance but with different green coverage ratios have different effects on
the subjective psychological perceptions of participants, including novelty, perceived naturalness, preference,
emotions, and environmental comfort.
Hypothesis 4 (H4)
.Air quality, temperature, and humidity correlate to novelty, perceived naturalness,
preference, emotions, and environmental comfort.
2. Materials and Methods
2.1. Research Design
This study, which was part of a series of experiments for a research project, conducted a randomized
controlled trial. To examine the effects of indoor plants on the physical environment, we constructed
control conditions and experimental treatments. The order of the experimental treatments was randomly
decided. To examine the effects of indoor plants on the perceptions of participants, participants were
randomly assigned to different experimental treatments. All participants gave informed consent for
inclusion before they underwent the experiment. The study was conducted in accordance with the
Declaration of Helsinki of 1975, and was approved by the Ministry of Science and Technology in
Taiwan, which did not require this study to be submitted to the Ethics Committee.
2.2. Experimental Setup
The experiment for this study was conducted in a room located in the basement of a technology
university in central Taiwan. The room (length 4.0 m
×
width 3.0 m
×
height 3.85 m) was newly constructed
in October 2018. During the experiment, the windows in the room were kept shut, the air-conditioner
was switched off but the air conditioning vents were not blocked, and the curtains were drawn to avoid
distractions from the view of the landscape outside. The experimental environment tended to be more like
an actual living or working space rather than a completely isolated laboratory unit. Except for participants,
no one entered or exited the room during the experiment. However, when the participants opened the
door and entered and exited the room, the physical environment of the room changed.
The Rural Development Administration of South Korea recommends installing one large potted
plant and one small potted plant per 6 m
2
floor area in a room [
29
]. US studies have also recommended
placing at least one 6-in (15.24 cm) potted plant per 9 m
2
floor area in a room [
30
] to purify indoor air.
Since the room in which our experiment was conducted had a floor area of 12 m
2
at least two large
potted plants (diameter of 24 or 30 cm) and two small potted plants (diameter of 18 cm) should be
installed to improve the indoor air quality. For simplicity, this study selected only large potted plants
(pot diameter 37 cm and plant height of around 1.3 m) because the eye of a sitting person is at a height
of approximately 1.1 m from the ground [
31
]. The indoor plant used was the evergreen Radermachera
hainanensis Merr. a shade-bearing tree whose brightly colored leaves give the plant its ornamental
value [
32
]. Though this tree has become popular for indoor use in Taiwan in recent years, its ability to
improve air quality and its influence on people’s responses has never been examined. Plants tested
in experiments for their effects should be used as plants are normally used in daily living. The plant
pots were made of plastic. The growing medium was potting soil. Since we viewed the plants, pots,
and potting soil as a whole unit, we did not distinguish the possibly different influences of the plants,
pots, and potting soil.
The experimental manipulation to achieve a low green coverage ratio (3.00 %) involved placing
one large potted plant against the wall farthest from the door at the midpoint between two corners of
the room. The experimental manipulation to achieve a high green coverage ratio (8.83 %) involved
placing three large potted plants against the wall so that the tree canopies of all three plants were
touching. The low and high green coverage ratios were intentionally limited because previous research
has found that even a few indoor plants are beneficial to human health [
12
,
14
]. Further, we followed
the recommendations for the minimum number of indoor plants for improving air quality [
29
,
30
].
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Table 1details the plant distance and green coverage ratio of the experimental treatments. Before each
of the experimental treatments, a control condition was conducted with no plants in the room.
Table 1. Experimental treatment.
Experimental
Treatment
Green Coverage
Ratio Code Plant-Measuring
Instrument Distance
Plant–Participant
Distance
Experimental
Period
High Green
Coverage Ratio 8.83 % HC 0.25 m, 1.5 m 1.5 m End of
November 2018
Low Green
Coverage Ratio 3.00 % LC 0.25 m, 1.5 m 1.5 m Beginning of
December 2018
Note: The distances of the plant(s) to the measuring instrument was from the bottom of the potted plant(s) to the
side of the two air quality detectors.
The participants of this experiment were 60 students studying in daytime programs at a technology
university in central Taiwan. For all experiments, participants sat on a chair placed in the center of the
room at a distance of 1.5 m from the potted plant(s) (Figure 1). In the experiment, participants were asked
to sit down, observe the surrounding environment for 1 min, and ignore the two measuring instruments
as much as possible. After observing the room, participants began to fill out the questionnaire, and were
given as much time as they required, provided that they spent at least 5 min on it.
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Further, we followed the recommendations for the minimum number of indoor plants for
improving air quality [29,30]. Table 1 details the plant distance and green coverage ratio of the
experimental treatments. Before each of the experimental treatments, a control condition was
conducted with no plants in the room.
The participants of this experiment were 60 students studying in daytime programs at a
technology university in central Taiwan. For all experiments, participants sat on a chair placed in
the center of the room at a distance of 1.5 m from the potted plant(s) (Figure 1). In the experiment,
participants were asked to sit down, observe the surrounding environment for 1 minute, and ignore
the two measuring instruments as much as possible. After observing the room, participants began
to fill out the questionnaire, and were given as much time as they required, provided that they
spent at least 5 minutes on it.
Table 1. Experimental treatment.
Experimental
Treatment
Green
Coverage
Ratio
Code
Plant-Measuring
Instrument
Distance
Plant–
Participant
Distance
Experimental
Period
High Green
Coverage Ratio
8.83 % HC 0.25 m, 1.5 m 1.5 m
End of
November 2018
Low Green
Coverage Ratio
3.00 % LC 0.25 m, 1.5 m 1.5 m
Beginning of
December 2018
Note: The distances of the plant(s) to the measuring instrument was from the bottom of the potted
plant(s) to the side of the two air quality detectors.
High Green Coverage Ratio
Low Green Coverage Ratio
Figure 1. Experimental treatment.
2.3. Objective Measurement of the Physical Environment
Figure 1. Experimental treatment.
2.3. Objective Measurement of the Physical Environment
The physical environment was objectively measured using two indoor air quality detectors, which
were both placed slightly to the left of the chair. The two detectors were placed 1.1 m above the
ground and 0.25 and 1.5 m away from the bottom of the potted plant(s), respectively. The indoor air
quality detectors used in this study (iAeris 14, Kaoten Scientific Co., Ltd., Kaohsiung, Taiwan) can
measure changes across eight aspects of the physical environment: temperature, humidity, carbon
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monoxide (CO), carbon dioxide (CO
2
), coarse particulate matter (PM
10
), fine particulate matter (PM
2.5
),
formaldehyde (HCHO), and total volatile organic compound (TVOCs) levels (Table 2for specifications).
The air quality detectors had quality control and quality assurance, which were calibrated again
before the experiment. The air quality detectors were switched on throughout the experiment and
uploaded the data measurements to the cloud every 6 min. Therefore, there was no repetition in the
measurements of the physical environment.
Table 2. Specifications of the indoor air quality detector.
Measurement
Factors CO CO2PM10 PM2.5
Detection range 0–1000 ppm 400–20,000 ppm 0–500 µg/m30–500 µg/m3
Resolution 1 ppm 1 ppm 1µg/m31µg/m3
Method of
Detection Electrochemical
Nondispersive infrared
Optical Optical
Reaction Time <30 s <120 s <10 s <10 s
Sampling Real time Real time Real time Real time
Measurement
Factors HCHO TVOC Temperature Humidity
Detection range 0–5 ppm 0.13–2.5 ppm −40–125 ◦C 0%–100% RH
Resolution 0.01 ppm 0.01 ppm 0.1 ◦C 1% RH
Method of
Detection Electrochemical
Microelectromechanical
systems chip
Microelectromechanical
systems chip
Microelectromechanical
systems chip
Reaction Time 90 s <10 s <30 s <30 s
Sampling Real time Real time Real time Real time
2.4. Subjective Measurement of Psychological Perceptions
This study employed a questionnaire to measure the subjective perceptions of the participants.
The questionnaire contained six sections: (1) preference, (2) environmental comfort, (3) perceived
naturalness, (4) novelty, (5) the pleasure–arousal–dominance (PAD) emotional state scale, and (6)
participant demographic information. The participants selected their answer on a 9-point Likert
scale on which a higher score indicates a greater level of agreement with the questionnaire item.
Participant demographics consisted of sex, year of birth, college, department, and year of study.
2.4.1. Preference
Preference reflects an immediate and direct response to or assessment of the environment
in general [
33
,
34
] and can be either task-dependent, which is related to a specific activity,
or task-independent, which is unrelated to any particular activity [
35
]. The survey of preference
for indoor plants in this study comprised two items: “mere liking,” which is task-independent,
and “whether (the participants) liked to study in the space,” which is task-dependent [
36
,
37
]. The total
average score for these two items constituted the index score for the preference.
2.4.2. Environmental Comfort
Few studies have investigated the effects of indoor plants on comfort level.
Nevertheless, one literature review derived four dimensions of indoor environmental quality: thermal
comfort, visual comfort, acoustic comfort, and indoor air quality [
38
]. Therefore, the environmental
comfort examined in this study also covered these four items: How do you evaluate the indoor
thermal environment? How do you evaluate the indoor visual comfort? How do you evaluate the
indoor acoustic comfort? How do you evaluate the indoor air quality [
39
]? The index score for the
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environmental comfort was obtained by calculating the average of the total scores for these four items.
A higher score indicates greater comfort.
2.4.3. Perceived Naturalness
Perceived naturalness refers to the subjective perception of the closeness of a physical environment
to a natural state [
40
,
41
]. Higher levels of perceived naturalness of a landscape or environment indicate
that a landscape or environment is perceived as closer to its natural state [
42
]. Ho and Chang [
43
]
developed a 16-item perceived naturalness scale consisting of two constructs: richness of natural elements
and environmental wildness. The items used in this study for perceived naturalness were also adapted
from other studies to comprise a total of three dimensions (naturalness, richness of natural elements,
and environmental wildness), each of which consisted of two items (e.g., Do you think this environment is
natural? Is this environment full of vitality? Is this environment without a trace of human destruction?).
A higher score indicates a greater level of naturalness. The index score for the perceived naturalness was
calculated by first determining the composite scores of the three dimensions (i.e., the total score of all items
in each dimension divided by the number of items) and then averaging the composite scores.
2.4.4. Novelty
A novelty scale suitable for this study has not yet been developed. Therefore, this study adopted
two related scales that have been used in Taiwan and overseas. The first scale was obtained from
Lee and Crompton [
44
], who argued that novelty has four dimensions: change from routine, thrill,
surprise, and boredom alleviation. The second is the familiarity scale developed by Cheng, Shen,
and Chon [
45
], given that familiarity is the opposite of novelty [
46
]. Cheng et al. [
45
]. classified
familiarity of a place into four categories based on Baloglu [
47
]: overall familiarity, having seen it,
having the ability to recognize it, and having visited it. The novelty items in this study were adapted
from the two aforementioned scales and adjusted to include two items from each of the four dimensions
specified by Lee and Crompton [
44
] and the three types of familiarity (not including overall familiarity)
proposed in Cheng et al. [
45
], which were reverse-worded. Therefore, the novelty items proposed in
this study comprised a total of five dimensions and eleven questions (e.g., There are new things in
this environment that I can explore. This environment feels a bit dangerous to me. This environment
makes me feel a little surprised. This environment can reduce my boredom. I have never seen this
environment or a similar environment.). A higher score denotes greater novelty. The index score
of the novelty was calculated by first determining the composite scores of the five dimensions (i.e.,
the total score of all items in each dimension divided by the number of items) and then averaging the
composite scores.
2.4.5. Emotions
Emotion is considered to be an innate human quality, as evidenced by the emotional expressions
of newborn infants [
48
,
49
]. Emotions are widely believed to be the fundamental factor influencing
environmental perception [
50
–
52
]. This study adopted the PAD emotional state scale [
53
], which consists
of three dimensions: pleasure–displeasure, arousal–nonarousal, and dominance–submissiveness.
Each dimension has six adjectives (e.g., “happy-unhappy; excited-calm; in control-cared for”), and the
scores on the scale ranges from 4 to
−
4. The scores of the PAD scale were calculated by determining
the composite scores of the three dimensions, which represent the average score for each adjective.
A higher score denotes a higher level of perception toward the dimension.
2.5. Statistical Analyses
The statistical analyses were conducted using IBM SPSS Statistics 22.0 (IBM, Armonk, NY, USA).
The data on the physical environment and the psychological perception were first analyzed for their
normality and homogeneity of variance, which are the premise of parametric statistical tests such as
analysis of variance [
54
]. The data on the physical environment failed to pass the tests of normality and
Sustainability 2019,11, 3679 7 of 19
homogeneity of variance and were, thus, analyzed using nonparametric statistical tests for detecting
difference between conditions. The data on the psychological perceptions passed the tests of normality
and homogeneity of variance and were analyzed using parametric statistical tests for detecting difference
between conditions. In addition, when the focus of interest was the difference within the same condition
(controls or treatments), tests with related samples were conducted (e.g., the Wilcoxon tests). When the
focus of interest was the difference between different conditions, tests with independent samples were
conducted (e.g., the Mann-Whitney U test) [
54
]. Since the indicators of the difference in the physical
environment and the psychological perceptions were the means of the measurement data, the means were
below the detection resolution of the air quality detector and the digits of the scale, respectively.
3. Results
3.1. Objective Physical Environment
3.1.1. Control Conditions
The control conditions were that the room did not have any plants inside. To match the total
number of the data points of the control conditions to that of the experimental treatments, 30 data
points from the physical environments were randomly selected for the control condition before each of
the experimental treatments. The results showed that although the room had been recently constructed,
the air pollution levels of the control conditions were still much lower than the standards mandated
under the Indoor Air Quality Act (Table 3). Newly constructed rooms and/or buildings usually have
high air pollution levels because of the Volatile Organic Compounds (VOCs) emitted from the building
materials such as paint, flooring, caulking, sealants, and engineered wood [55].
Table 3. Means and standards for the air quality during the control.
Environmental Factor Control Mean Indoor Standards a
0.25 m 1.5 m
PM2.5
(µg/m3)HC 8.673 19.963 35
LC 23.863 27.977
CO2
(ppm) HC 396.833 391.400 1000
LC 473.733 479.767
PM10
(µg/m3)HC 13.187 22.713 75
LC 33.680 32.207
HCHO
(ppm) HC 0.016 0.010 0.08
LC 0.010 0.010
CO
(ppm) HC 0.000 0.000 9
LC 0.000 0.000
TVOC
(ppm) HC 0.225 0.196 0.56
LC 0.206 0.205
Note: HC denotes high green coverage ratio and LC denotes low green coverage ratio;
a
refers to the Indoor Air
Quality Act.
Furthermore, there were some significant differences in the physical environment between 0.25 m
and 1.5 m, as indicated by the results of the Wilcoxon tests with two dependent samples using the
physical environmental factors as dependent variables and the distance as the independent variable.
For the control condition of the high green coverage ratio treatment, PM
2.5
and PM
10
at 1.5 m distance
were significantly greater than at 0.25 m, while CO
2
, HCHO, TVOC, temperature, and humidity at the
0.25 m distance were significantly greater than at the 1.5 m distance (Table 4). However, for the control
condition of the low green coverage ratio treatment, only the temperature at 0.25 m was significantly
greater than at 1.5 m (Table 5).
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Table 4.
Results of the Wilcoxon tests of different distances for the control condition of the high green
coverage ratio treatment.
Environmental Factor Distance Mean S.D. Z-value p-Value
PM2.5
(µg/m3)0.25 8.673 3.709 −4.782 0.0000
1.5 19.963 2.197
CO2
(ppm) 0.25 396.833 8.429 −2.780 0.0054
1.5 391.400 4.917
PM10
(µg/m3)0.25 13.187 6.080 −4.433 0.0000
1.5 22.713 2.139
HCHO
(ppm) 0.25 0.016 0.005 −4.243 0.0000
1.5 0.010 0.000
CO
(ppm) 0.25 0.000 0.000 0.000 1.0000
1.5 0.000 0.000
TVOC
(ppm) 0.25 0.225 0.017 −4.638 0.0000
1.5 0.196 0.015
Temperature
(◦C)
0.25 26.063 0.110 −4.810 0.0000
1.5 25.530 0.106
Humidity
(%)
0.25 56.467 0.507 −2.840 0.0045
1.5 56.100 0.305
Table 5.
Results of the Wilcoxon tests of different distances for the control condition of the low green
coverage ratio treatment.
Environmental Factor Distance Mean S.D. Z-value p-Value
PM2.5
(µg/m3)0.25 23.863 17.045 −1.214 0.2249
1.5 27.977 9.662
CO2
(ppm) 0.25 473.733 25.874 −0.995 0.3198
1.5 479.767 24.555
PM10
(µg/m3)0.25 33.680 20.606 −0.134 0.8936
1.5 32.207 12.116
HCHO
(ppm) 0.25 0.010 0.000 0.000 1.0000
1.5 0.010 0.000
CO
(ppm) 0.25 0.000 0.000 0.000 1.0000
1.5 0.000 0.000
TVOC
(ppm) 0.25 0.206 0.053 0.000 1.0000
1.5 0.205 0.057
Temperature
(◦C)
0.25 25.283 0.226 −4.490 0.0000
1.5 24.840 0.230
Humidity
(%)
0.25 61.200 3.547 −0.927 0.3538
1.5 60.300 3.476
3.1.2. Difference between High Green Coverage Ratio Treatment and Its Control
The results of the Mann-Whitney U tests with two independent samples using the physical
environmental factors as dependent variables and the condition as the independent variable showed
that PM
2.5
, PM
10
, TVOC, and temperature were significantly greater for the control than for the high
green coverage ratio treatment at 0.25 m, while CO
2
, HCHO, and humidity were significantly greater for
the high green coverage ratio treatment than for the control at 0.25 m (Table 6). Further, the results of
the Mann-Whitney U tests with two independent samples using the physical environmental factors as
dependent variables and the condition as the independent variableshowed that TVOC and temperature
were significantly greater for the control than for the high green coverage ratio treatment at 1.5 m, while
CO
2
and humidity were significantly greater for the high green coverage ratio treatment than for the
control at 1.5 m (Table 7).
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Table 6. Results of the Mann-Whitney U tests of the high green coverage and control at 0.25 m.
Environmental Factor Condition Mean S.D. Mann-Whitney U Z-Value p-Value
PM2.5
(µg/m3)Control 8.673 3.709 290.500 −2.359 0.0183
HC 6.807 4.517
CO2
(ppm) Control 396.833 8.429 0.000 −6.656 0.0000
HC 555.700 51.848
PM10
(µg/m3)Control 13.187 6.080 311.000 −2.055 0.0399
HC 10.977 6.569
HCHO
(ppm) Control 0.016 0.005 159.000 −4.581 0.0000
HC 0.031 0.012
CO
(ppm) Control 0.000 0.000 450.000 0.000 1.0000
HC 0.000 0.000
TVOC
(ppm) Control 0.225 0.017 45.000 −6.019 0.0000
HC 0.166 0.029
Temperature
(◦C)
Control 26.063 0.110 0.000 −6.710 0.0000
HC 25.603 0.217
Humidity
(%)
Control 56.467 0.507 0.000 −6.787 0.0000
HC 65.333 1.539
Note: HC denotes high green coverage ratio.
Table 7. Results of the Mann-Whitney U tests of the high green coverage and control at 1.5 m.
Environmental Factor Condition Mean S.D. Mann-Whitney U Z-Value p-Value
PM2.5
(µg/m3)Control 19.963 2.197 430.500 −0.288 0.7731
HC 19.677 4.709
CO2
(ppm) Control 391.400 4.917 0.000 −6.658 0.0000
HC 558.900 51.575
PM10
(µg/m3)Control 22.713 2.139 423.000 −0.399 0.6897
HC 22.480 4.633
HCHO
(ppm) Control 0.010 0.000 450.000 0.000 1.0000
HC 0.010 0.000
CO
(ppm) Control 0.000 0.000 450.000 0.000 1.0000
HC 0.000 0.000
TVOC
(ppm) Control 0.196 0.015 221.500 −3.409 0.0007
HC 0.171 0.029
Temperature
(◦C)
Control 25.530 0.106 74.500 −5.647 0.0000
HC 25.227 0.230
Humidity
(%)
Control 56.100 0.305 0.000 −7.020 0.0000
HC 65.133 1.737
Note: HC denotes high green coverage ratio.
3.1.3. Difference between Low Green Coverage Ratio Treatment and Its Control
The results of the Mann-Whitney U tests with two independent samples using the physical
environmental factors as dependent variables and the condition as the independent variable showed that
PM2.5 and PM10 were significantly greater for the control than for the low green coverage ratio treatment
at 0.25 m, while CO
2
, HCHO, temperature, and humidity were significantly greater for the low green
coverage ratio treatment than for the control at 0.25 m (Table 8). Further, the results of the Mann-Whitney
U tests with two independent samples using the physical environmental factors as dependent variables
and the condition as the independent variable showed that PM
2.5
and PM
10
were significantly greater for
the control than for the low green coverage ratio treatment at 1.5 m, while CO
2
, temperature, and humidity
were significantly greater for the low green coverage ration treatment than for the control at 1.5 m (Table 9).
Sustainability 2019,11, 3679 10 of 19
Table 8.
Results of the Mann-Whitney U tests of the low green coverage ratio treatment and control at
0.25 m.
Environmental Factor Condition Mean S.D. Mann-Whitney U Z-Value p-Value
PM2.5
(µg/m3)Control 23.863 17.045 130.000 −4.732 0.0000
LC 7.250 5.414
CO2
(ppm) Control 473.733 25.874 111.500 −5.006 0.0000
LC 544.533 49.515
PM10
(µg/m3)Control 33.680 20.606 150.00 −4.436 0.0000
LC 12.227 8.675
HCHO
(ppm) Control 0.010 0.000 0.000 −7.145 0.0000
LC 0.036 0.011
CO
(ppm) Control 0.000 0.000 450.000 0.000 1.0000
LC 0.000 0.000
TVOC
(ppm) Control 0.206 0.053 393.500 −0.839 0.4013
LC 0.194 0.051
Temperature
(◦C)
Control 25.283 0.226 186.000 −3.924 0.0001
LC 25.850 0.522
Humidity
(%)
Control 61.200 3.547 288.500 −2.418 0.0156
LC 63.667 1.470
Note: LC denotes low green coverage ratio.
Table 9.
Results of the Mann-Whitney U tests of the low green coverage ratio treatment and control at
1.5 m.
Environmental Factor Condition Mean S.D. Mann-Whitney U Z-Value p-Value
PM2.5
(µg/m3)Control 27.977 9.662 196.000 −3.756 0.0002
LC 19.973 6.005
CO2
(ppm) Control 479.767 24.555 179.000 −4.008 0.0001
LC 521.033 42.808
PM10
(µg/m3)Control 32.207 12.116 198.500 −3.718 0.0002
LC 23.040 6.591
HCHO
(ppm) Control 0.010 0.000 450.000 0.000 1.0000
LC 0.010 0.000
CO
(ppm) Control 0.000 0.000 450.000 0.000 1.0000
LC 0.000 0.000
TVOC
(ppm) Control 0.205 0.057 386.500 −0.942 0.3462
LC 0.191 0.053
Temperature
(◦C)
Control 24.840 0.230 143.000 −4.561 0.0000
LC 25.493 0.509
Humidity
(%)
Control 60.300 3.476 193.500 −3.830 0.0001
LC 63.900 1.583
Note: LC denotes low green coverage ratio.
3.1.4. Difference between High and Low Green Coverage Ratio Treatments
The results of the Mann-Whitney U tests with two independent samples using the physical
environmental factors as dependent variables and the experimental treatment as the independent
variable showed that humidity was significantly greater for the low green coverage ratio treatment
than for the high green coverage ratio treatment at 0.25 m (Table 10). Further, the results of the
Mann-Whitney U tests with two independent samples using the physical environmental factors as
dependent variables and the experimental treatment as the independent variable showed that CO
2
and humidity were significantly greater for the low green coverage ratio treatment than for the high
green coverage ratio treatment at 1.5 m (Table 11).
Sustainability 2019,11, 3679 11 of 19
Table 10.
Results of the Mann-Whitney U tests for the high and low green coverage ratio treatments at
0.25 m.
Environmental Factor Condition Mean S.D. Mann-Whitney U Z-Value p-Value
PM2.5
(µg/m3)LC 6.807 4.517 439.500 −0.155 0.8766
HC 7.250 5.414
CO2
(ppm) LC 555.700 51.848 420.500 −0.436 0.6627
HC 544.533 49.515
PM10
(µg/m3)LC 10.977 6.569 417.500 −0.481 0.6308
HC 12.227 8.675
HCHO
(ppm) LC 0.031 0.012 364.500 −1.325 0.1852
HC 0.036 0.011
CO
(ppm) LC 0.000 0.000 450.000 0.000 1.0000
HC 0.000 0.000
TVOC
(ppm) LC 0.166 0.029 323.000 −1.892 0.0585
HC 0.194 0.051
Temperature
(◦C)
LC 25.603 0.217 334.000 −1.725 0.0845
HC 25.850 0.522
Humidity
(%)
LC 65.333 1.539 201.500 −3.772 0.0002
HC 63.667 1.470
Note: HC denotes high green coverage ratio and LC denotes low green coverage ratio.
Table 11.
Results of the Mann-Whitney U tests for the high and low green coverage ratio treatments at
1.5 m.
Environmental Factor Condition Mean S.D. Mann-Whitney U Z-Value p-Value
PM2.5
(µg/m3)LC 19.677 4.709 448.500 −0.022 0.9823
HC 19.973 6.005
CO2
(ppm) LC 558.900 51.575 258.000 −2.839 0.0045
HC 521.033 42.808
PM10
(µg/m3)LC 22.480 4.633 438.500 −0.170 0.8650
HC 23.040 6.591
HCHO
(ppm) LC 0.010 0.000 450.000 0.000 1.0000
HC 0.010 0.000
CO
(ppm) LC 0.000 0.000 450.000 0.000 1.0000
HC 0.000 0.000
TVOC
(ppm) LC 0.171 0.029 367.000 −1.235 0.2169
HC 0.191 0.053
Temperature
(◦C)
LC 25.227 0.230 322.500 −1.895 0.0581
HC 25.493 0.509
Humidity
(%)
LC 65.133 1.737 266.500 −2.784 0.0054
HC 63.900 1.583
Note: HC denotes high green coverage ratio and LC denotes low green coverage ratio.
3.1.5. Difference between Different Distances
The results of the Wilcoxon tests with two dependent samples using the physical environmental
factors as dependent variables and the distance as the independent variable showed that HCHO
and temperature were significantly greater at 0.25 m than at 1.5 m for the high green coverage ratio
treatment, while PM2.5 and PM10 were significantly greater at 1.5 m than at 0.25 m for the high green
coverage ratio treatment (Table 12). Further, the results of the Wilcoxon tests with two independent
samples using the physical environmental factors as dependent variables and the distance as the
independent variable showed that CO
2
, HCHO, and temperature were significantly greater at 0.25 m
than at 1.5m for the low green coverage ratio treatment, while PM
2.5
and PM
10
were significantly
greater at 1.5 m than at 0.25 m for the low green coverage ratio treatment (Table 13).
Sustainability 2019,11, 3679 12 of 19
Table 12.
Results of the Wilcoxon tests of different distances for the high green coverage ratio treatment.
Environmental Factor Distance Mean S.D. Mann-Whitney U Z-Value
PM2.5
(µg/m3)0.25 6.807 4.517 −4.783 0.0000
1.5 19.677 4.709
CO2
(ppm) 0.25 555.700 51.848 −0.638 0.5235
1.5 558.900 51.575
PM10
(µg/m3)0.25 10.977 6.569 −4.783 0.0000
1.5 22.480 4.633
HCHO
(ppm) 0.25 0.031 0.012 −4.654 0.0000
1.5 0.010 0.000
CO
(ppm) 0.25 0.000 0.000 0.000 1.0000
1.5 0.000 0.000
TVOC
(ppm) 0.25 0.166 0.029 −2.812 0.0049
1.5 0.171 0.029
Temperature
(◦C)
0.25 25.603 0.217 −5.002 0.0000
1.5 25.227 0.230
Humidity
(%)
0.25 65.333 1.539 −1.604 0.1088
1.5 65.133 1.737
Table 13.
Results of the Wilcoxon tests of different distances for the low green coverage ratio treatment.
Environmental Factor Distance Mean S.D. Mann-Whitney U Z-Value
PM2.5
(µg/m3)0.25 7.250 5.414 −4.783 0.0000
1.5 19.973 6.005
CO2
(ppm) 0.25 544.533 49.515 −2.892 0.0038
1.5 521.033 42.808
PM10
(µg/m3)0.25 12.227 8.675 −4.782 0.0000
1.5 23.040 6.591
HCHO
(ppm) 0.25 0.036 0.011 −4.820 0.0000
1.5 0.010 0.000
CO
(ppm) 0.25 0.000 0.000 0.000 1.0000
1.5 0.000 0.000
TVOC
(ppm) 0.25 0.194 0.051 −1.245 0.2131
1.5 0.191 0.053
Temperature
(◦C)
0.25 25.850 0.522 −4.916 0.0000
1.5 25.493 0.509
Humidity
(%)
0.25 63.667 1.470 −1.941 0.0522
1.5 63.900 1.583
3.2. Subjective Psychological Perceptions
3.2.1. Participants’ Demographic Information
This study had a total of 60 participants (12 men; 48 women), who had an average age of 20.92 years
(SD =1.44). A total of 30 participants (8 men; 22 women) were randomly assigned to receive the
high green coverage ratio treatment, and 30 participants (4 men; 26 women) were randomly assigned
to receive the low green coverage ratio treatment. Next, a chi-square test was performed with sex,
college, department, and year of study as the dependent variables and experimental treatment as
the independent variable. The results revealed no significant differences in sex, college, department,
and year of study (p>0.05). Additionally, the results of the independent sample ttest revealed a
nonsignificant difference between the experimental groups (p>0.05) in terms of age.
Sustainability 2019,11, 3679 13 of 19
3.2.2. Scale Reliability
This study adopted questionnaires for measuring preference, environmental comfort, perceived
naturalness, novelty, and PAD emotional state to investigate the effects of different experimental
treatments on the subjective psychological perceptions of participants to examine hypothesis 3. Table 14
provides the reliability of each scale. The Cronbach’s
α
of the environmental comfort scale was 0.67,
which is slightly less than ideal. The Cronbach’s
α
of all other scales was greater than 0.728, indicating
favorable internal consistency reliability [56].
Table 14. Reliability analysis of the subjective psychological perception scales.
Subjective Psychological Perception No. of Items Overall Cronbach’s α
Preference 2 0.859
Environmental Comfort 4 0.670
Perceived Naturalness 6 0.866
Novelty 11 0.728
Emotions 18 0.839
3.2.3. Difference in the Participant Perceptions between Different Experimental Treatments
The results of the independent sample ttest revealed a nonsignificant difference regarding preference
(index score) and environmental comfort (index score) between the experimental treatments (p>0.05),
and the difference contained a zero in the 95% confidence interval for the difference (Table 15). One-way
multivariate analyses of variance (MANOVA) using the perceived naturalness, novelty, PAD emotions
as dependent variables and the experimental treatment as the independent variable revealed that the
four variables of perceived naturalness (3 composite scores and index score), six variables of novelty
(5 composite scores and index score), and three variables of the PAD scale (3 composite scores) were not
significant (p>0.05), indicating no significant difference between the experimental treatments (Table 16).
Because more than one dependent variable was examined, MANOVA was used since it can
simultaneously consider the relationship of several dependent variables, particularly between the composite
scores and the index scores in this case, to test whether a significant difference exists in the experimental
treatment and analyze whether the experimental treatment exhibits significant differences across the
individual dependent variables. However, analysis of variance (ANOVA) hypothesizes that no relationship
exists between the dependent variables, and analyzes each individual dependent variable. By contrast,
MANOVA is a statistical test of all dependent variables based on their optimal linear combination [57].
Table 15.
Difference between different experimental treatment groups in preference and
environmental comfort.
Index Score Experimental
Treatment Mean SD T Value P Value Effect
Size η2Observed
Power
95% Confidence Interval for the Difference
Lower Bound Upper Bound
Preference HC 6.267
1.982
−0.458 0.649 0.004 0.074 −1.165 0.731
LC 6.483
1.674
Environmental
Comfort
HC 6.483
1.345
−0.253 0.801 0.001 0.057 −0.742 0.575
LC 6.567
1.198
Note: HC denotes high green coverage ratio and LC denotes low green coverage ratio.
Table 16.
Difference between different experimental treatment groups in perceived naturalness, novelty,
and PAD emotions.
Subjective Psychological Perception Independent Variable Dependent Variable F P Value Effect Size η2Observed Power
Perceived naturalness Experimental Treatment Composite Score (3)
Index Score (1) 0.575 0.634 0.030 0.161
Novelty Experimental Treatment Composite Score (5)
Index Score (1) 0.417 0.835 0.037 0.150
Emotions Experimental Treatment Composite Score (3) 1.433 0.243 0.071 0.359
Note: ( ) denotes the number of dependent variables.
Sustainability 2019,11, 3679 14 of 19
3.3. Correlations between Objective Factors of the Physical Environment and Subjective Psychological Perceptions
Because the participants sat at a distance of 1.5 m from the plants, their subjective psychological
perceptions were subject to a correlation analysis with the objective factors of the physical environment
measured at a distance of 1.5 m from the plants. Given that the focus of interest was the relationships
between the participants’ subjective perceptions and the physical environment with respect to the
plants, the correlation analysis used the data on both the low and high green coverage ratio treatments.
The results are as follows. (1) Preference exhibited a significantly weak negative correlation with PM
2.5
and PM
10
. (2) Environmental comfort exhibited no significant correlation with air quality, temperature,
and humidity. (3) Perceived naturalness exhibited a significantly weak negative correlation with PM
2.5
and PM
10
. (4) The novelty index score exhibited a significantly weak negative correlation with TVOC
level. (5) Change in routine in novelty exhibited a significantly weak negative correlation with TVOC
level. (6) Pleasure in the PAD scale exhibited a significantly moderate negative correlation with PM
2.5
and a significantly weak negative correlation with PM10 (Table 17).
Table 17.
Correlations between objective factors of physical environment and subjective psychological
perceptions (N=60).
Subjective Perception Score PM2.5 CO2PM10 HCHO CO TVOC Temper–ature Humidity
Preference Index Score r=−0.294 * r=−0.033 r=−0.281 * c c r=−0.253 r=0.161 r=−0.146
(p=0.023) (p=0.800) (p=0.029) (p=0.051) (p=0.220) (p=0.266)
Environmental
Comfort Index Score r=−0.227 r=0.099 r=−0.204 c c r=−0.115 r=0.074 r=−0.164
(p=0.081) (p=0.451) (p=0.118) (p=0.381) (p=0.574) (p=0.211)
Perceived Naturalness
Naturalness r=−0.280 * r=0.017 r=−0.257 * c c r=−0.197 r=0.121 r=−0.100
(p=0.030) (p=0.895) (p=0.047) (p=0.131) (p=0.356) (p=0.445)
Richness of natural
elements
r=−0.089 r=0.131 r=−0.072 c c r=−0.168 r=0.110 r=−0.022
(p=0.499) (p=0.317) (p=0.584) (p=0.200) (p=0.403) (p=0.868)
Environmental
Wildness
r=−0.157 r=0.015 r=−0.148 c c r=−0.197 r=−0.019 r=−0.102
(p=0.230) (p=0.910) (p=0.258) (p=0.131) (p=0.885) (p=0.438)
Index Score r=−0.204 r=0.065 r=−0.185 c c r=−0.222 r=0.080 r=−0.088
(p=0.118) (p=0.621) (p=0.157) (p=0.089) (p=0.543) (p=0.503)
Novelty
Change in Routine r=−0.185 r=−0.052 r=−0.160 c c r=−0.361 ** r=0.094 r=−0.111
(p=0.157) (p=0.692) (p=0.222) (p=0.005) (p=0.473) (p=0.397)
Thrill r=0.064 r=0.099 r=0.062 c c r=−0.014 r=0.059 r=0.064
(p=0.629) (p=0.452) (p=0.639) (p=0.915) (p=0.655) (p=0.625)
Surprise r=−0.157 r=0.015 r=−0.148 c c r=−0.197 r=−0.019 r=−0.102
(p=0.230) (p=0.910) (p=0.258) (p=0.131) (p=0.885) (p=0.438)
Boredom
Alleviation
r=−0.074 r=−0.018 r=−0.032 c c r=−0.077 r=−0.052 r=−0.248
(p=0.572) (p=0.890) (p=0.808) (p=0.558) (p=0.695) (p=0.056)
Unfamiliarity r=−0.006 r=−0.084 r=−0.005 c c r=−0.077 r=−0.034 r=−0.027
(p=0.966) (p=0.524) (p=0.968) (p=0.561) (p=0.795) (p=0.838)
Index Score r=−0.133 r=−0.023 r=−0.106 c c r=−0.259*r=0.010 r=−0.159
(p=0.311) (p=0.863) (p=0.420) (p=0.046) (p=0.942) (p=0.226)
Emotions
Pleasure r=−0.423 ** r=0.028 r=−0.394 ** c c r=−0.234 r=0.146 r=−0.158
(p=0.001) (p=0.829) (p=0.002) (p=0.071) (p=0.267) (p=0.228)
Arousal r=−0.119 r=−0.070 r=−0.103 c c r=−0.241 r=0.183 r=0.031
(p=0.367) (p=0.596) (p=0.435) (p=0.064) (p=0.161) (p=0.816)
Dominance r=−0.154 r=0.001 r=−0.134 c c r=−0.099 r=0.146 r=−0.163
(p=0.241) (p=0.994) (p=0.309) (p=0.451) (p=0.266) (p=0.214)
Note: **(p<0.01) indicates significance (two-tailed); *(p<0.05) indicates significance (two-tailed);
c
denotes that the
factor was not calculable because at least one of the variables was a constant. Ndenotes sample size.
4. Discussion
The first hypothesis (H1) of this study postulates that different plant distances correspond to
different effects on the objective physical environment in terms of air quality, temperature, and humidity
level. The results showed that H1 could not be rejected. When the high green coverage ratio treatment
was compared to its control, the significantly greater CO
2
, TVOC, and humidity at 0.25 m than 1.5 m
were no longer significant. This may because the participants siting at 1.5 m exhaled CO2and TVOC
that was purified by the 3 plants via their respiration. However, the change in humidity was difficult to
Sustainability 2019,11, 3679 15 of 19
explain. It was expected that the humidity would be greater when closer to the plants than when farther
away due to the evapotranspiration of the plants. Nevertheless, some plants can reduce the humidity
in the air [
58
]. When the low green coverage ratio treatment was compared to its control, PM
2.5
and
PM
10
were significantly greater at 1.5 m than at 0.25 m, while CO
2
and HCHO were significantly
greater at 0.25 m than at 1.5 m. This may because (1) PM
2.5
and PM
10
were reduced by the single
plant via its respiration [
59
], (2) the plant itself exhaled CO
2
, though photosynthesis also reduced CO
2
,
and (3) HCHO was released by the newly painted wall. Although the control conditions of the two
treatments, which were different from each other, made the examinations of the effects of the plants
at different distances difficult, it was clear that regardless of number of plants, the closer the plant,
the higher CO
2
level. It is also suggested that closer to indoor plants there was less particulate matter.
The second hypothesis (H2) of this study postulates that the effects on the objective physical
environment in terms of air quality, temperature, and humidity level would differ when the distance
from the plant remains the same but the green coverage ratio varies. The results partially supported H2.
Regardless of the distance from the plant (1.5 m and 0.25 m), CO
2
and humidity were significantly greater
for the high green coverage ratio treatment than for its control, while TVOC and temperature were
significantly greater for the control than for the high green coverage ratio treatment. Further, regardless
of the distance from the plant, CO
2
, temperature, and humidity were significantly greater for the low
green coverage ratio treatment than for the control, while PM
2.5
and PM
10
were significantly greater for
the control than for the low green coverage ratio treatment. Therefore, regardless of the distance from
the plant, CO
2
and humidity were significantly greater for the presence of indoor plants (3 and 1 plants)
than for the absence of indoor plants. This may because the plant exhaled CO
2
and transpirated water.
In addition, it suggested that three indoor plants could reduce temperature at both 1.5 m and 0.25 m
because of transpiration, while one indoor plant could not reduce the temperature at both 1.5 m and
0.25 m. However, humidity was greater for the low green coverage ratio treatment than for the high
green coverage ratio treatment at both 1.5 m and 0.25 m, which was difficult to explain. It was expected
that more plants produce greater humidity unless the plants can reduce humidity [58].
The third hypothesis (H3) of this study postulates that the effects on subjective psychological
perceptions in terms of novelty, preference, perceived naturalness, emotions, and environmental comfort
would differ when the distance from the plant remains the same but the green coverage ratio varies.
The results did not support H3. Therefore, the green coverage ratio (8.83% or 3.00%) did not influence the
subjective psychological perceptions of participants. This result differs from Han [
12
], who reported that a
6% green coverage ratio could elicit significantly stronger feelings of preference and comfort in students.
However, this study adopted an experimental method to investigate the preferences (task-dependent and
task-independent) and environmental comfort (four items) of university students following brief exposure
to R. hainanensis Merr., whereas Han [
12
] conducted a field quasi-experiment to examine the preference (one
item) and comfort (one item) of students from junior high schools after prolonged exposure to Cinnamomum
kotoense Kanehira et Sasaki. Therefore, a direct comparison of our study with that of Han [
12
] may not
appropriate. In addition, this study discovered that the participants’ feelings of novelty were unaffected,
irrespective of the number of potted plants. Hence, the finding reported in other studies that only a few
indoor plants benefit peoples’ physical and mental health as well as overall wellbeing [
12
,
14
,
16
,
60
] is
unlikely attributable to the effect of novelty. In future investigations of the effects of indoor plants on
participants’ subjective psychological perceptions, researchers may need to further increase the green
coverage ratio to identify possible effects.
The fourth hypothesis (H4) of this study postulates that air quality, temperature, and humidity
correlate with novelty, perceived naturalness, preference, emotions, and environmental comfort.
The results partially supported H4. Preference, perceived naturalness, and pleasure exhibited a
significant negative correlation with PM
2.5
and PM
10
levels. Therefore, the lower levels of fine and
suspended particles in the air, the stronger the feelings of preference, naturalness of the environment,
and pleasure in participants. Furthermore, both change in routine in the novelty and the index
score for novelty exhibited a significant negative correlation with TVOC level, suggesting that the
Sustainability 2019,11, 3679 16 of 19
lower levels of TVOC in the air, the stronger the feelings of novelty and change from routine in
participants. This may because participants spend much time in TVOC-polluted rooms [
61
] and likely
have become accustomed to such environments. Finally, environmental comfort and objective physical
environmental factors were not significantly correlated, possibly because the air quality, temperature,
and humidity level of the experimental room did not vary considerably and were within standard
ranges defined by the Indoor Air Quality Act (Table 3).
This study examined the effects of indoor plants on both the physical environment and the
psychological perceptions, manipulating their distances and green coverage ratios. This made the
research design relatively complicated. A limitation of this study was that the participants were
situated only at 1.5 m but not at 0.25 m from the plant, which resulted in an unbalanced design.
The air quality detector at 1.5 m would be more affected by its proximity to the participant than the
plant. Although any person in the room would emit heat, VOCs, CO
2
, and would likely re-suspend
particulate matter, this was not the case in this study, except for CO
2
. Future studies may adopt
more straight forward approaches than this one. Moreover, the analyses of the data on the physical
environment relying on the nonparametric statistical tests could not take into account the effects of
both the plant distances and amount simultaneously. Therefore, the effects of the plant distances and
green coverage ratios on the physical environment were analyzed separately. When combining the
pieces of information together, the results showed a clearer picture: (1) regardless of number of plants,
the closer the plant, the higher the CO
2
level; (2) the major effects of more indoor plants were greater
CO
2
and humidity; and (3) the secondary effects of more indoor plants were less PM
2.5
and PM
10
as
well as lower temperature. Furthermore, given that the experiments of this study were conducted
in an actual environment, some variables such as weather could not be fully controlled. It rained
for two days and one day during the high and the low green coverage ratio treatment, respectively,
which may influence the physical environment (e.g., humidity) and the psychological perceptions (e.g.,
emotions) [62]. More replication studies are needed.
More studies on the physiology of R. hainanensis Merr are necessary. How much water the plant
needs for photosynthesis, how much it transpirates, how much CO
2
its exhales during its breathing,
and how much it absorbs during its photosynthesis remain unknown. Laboratory studies and field
studies are both needed.
5. Conclusions
This study was the first to verify that the distance from indoor plants and their green coverage
ratio in a real setting can significantly influence the physical environment. Simply placing one or three
large pots of R. hainanensis Merr. in a room with a floor area of 12 m
2
can increase the humidity levels
and reduce PM
2.5
and PM
10
. Further, three large pots of R. hainanensis Merr. can reduce temperature
more significantly than one large pot. These findings bridge the gap in research regarding experiments
that mostly involve highly concentrated pollutants in small and enclosed fumigation chambers, which
differ from people’s daily living environments. Furthermore, the literature still lacks investigations of
the distance and green coverage ratio of indoor plants. This study also demonstrated findings that
only a few indoor plants benefit people’s physical and mental health and wellbeing are probably not
attributable to the effect of novelty, and further revealed that even a few pots of indoor plants in a
minimally polluted room, which meets the Indoor Air Quality Act, can still effectively improve the air
quality and environmental comfort. Therefore, indoor plants that can regulate indoor air quality and
microclimates without consuming energy warrant greater attention and wider application.
Funding: Ministry of Science and Technology MOST 107-2410-H-167-008-MY2.
Acknowledgments:
This study represents the partial results of a research project sponsored by the Ministry of Science
and Technology in Taiwan (MOST 107-2410-H-167-008-MY2). The sponsors had no role in the design, execution,
interpretation, or writing of the study. The author thanks L.-W. Ruan for data collection, organization, and analyses.
Conflicts of Interest: The author declares no conflict of interest.
Sustainability 2019,11, 3679 17 of 19
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