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The Effects of The Number of Indoor Foliage Plants on Productivity, Stress and Attention

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The number of researches on the effect of indoor plants to improve the quality of the workplace has increased. However, the results were varied. These differences in results might be caused by differences in the method adopted by each researcher, such as differences in the task, in acclimatization time, in work duration, in the number of plants, the size of plants and also the arrangement of plants. The objective of the present study is to investigate the effects of the number of plants on human productivity, stress and attention. In the present study, subjects were instructed to conduct a word searching task in a room with different numbers of plants on each condition. The results showed that there was no significant difference in productivity, stress and attention among four conditions. Significant differences were found in the subject's perception of friendliness, comfort, freshness, and cleanliness of the workplace.
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CLIMA 2013, Prague, Czech Republic, June 16-19, 2013
The Effects of The Number of Indoor Foliage Plants on
Productivity, Stress and Attention
Desto Jumeno1,2, Hiroshi Matsumoto3
1,3 Department of Architecture and Civil Engineering, Toyohashi University of
Technology, Japan
2 Department of Industrial Engineering, Andalas University, Indonesia
d119405@edu.tut.ac.jp
Abstract
The number of researches on the effect of indoor plants to improve the quality of the
workplace has increased. However, the results were varied. These differences in
results might be caused by differences in the method adopted by each researcher,
such as differences in the task, in acclimatization time, in work duration, in the
number of plants, the size of plants and also the arrangement of plants. The
objective of the present study is to investigate the effects of the number of plants on
human productivity, stress and attention. In the present study, subjects were
instructed to conduct a word searching task in a room with different numbers of
plants on each condition. The results showed that there was no significant difference
in productivity, stress and attention among four conditions. Significant differences
were found in the subject’s perception of friendliness, comfort, freshness, and
cleanliness of the workplace.
Keywords: indoor foliage plant; productivity; stress; attention
1. Introduction
People spend a large amount of time in their workplace. In order to
make their workplace more pleasant, people put plants, such as flowers or
foliage plants in rooms. The availability of plants in a room not only makes
the room more visually attractive, but also makes the air in the room fresher
by lowering the level of pollutants, such as formaldehyde [1], benzene [2]
and toluene [3]. Contact with plants is believed to be restorative to the
human spirit [4]. Therefore, human attitudes, behaviors and physiological
responses were affected by interaction with plants [5].
Number of researches on the benefits of indoor plants to improve
indoor air quality and its effect on humans has been increased. However, the
results were varied. Some positive results were found, such as an increase in
productivity [4], reduction in stress of the employee [4, 6, 7], improvement
of employee morale [8], reduction in health complaint [9], reduction in sick
leave [10], increase in satisfaction [11], and improvement on quality of
working life [11]. On the other hand, some negative results were also found
such as decrease in productivity [12] and increase in the level of perceived
stress [10]. [12] showed that subjects working in a room with no plants had
higher productivity than working in a room with 10 plants and a room with
CLIMA 2013, Prague, Czech Republic, June 16-19, 2013
22 plants. Therefore, there might be an optimal number between two
extreme numbers of plants to be placed in a room. Moreover, [10] showed
that an increase in the number of plants placed 1 meter from the subject
would increase the level of perceived stress.
These differences in results might be caused by differences in the
method adopted by each researcher, such as differences in the task, in
acclimatization time, in work duration, in the number of plants, in the size
of plants and also the arrangement of plants. A study on task design,
acclimatization time and work duration of indoor plant research has been
shown by [13].
The objective of the present study is to investigate the effects of the
number of plants on human productivity, attention and level of stress. In the
present study, a laboratory experiment was conducted where subjects were
instructed to conduct a word searching task in a climatic room with different
numbers of plants in each condition. To measure productivity, the number
of forms and the number of words found were used as the dependent
variable. To measure attention, a secondary task reaction time (STRT) was
applied. [14] suggested that STRT can be used to measure attention. To
measure the level of stress, heart rate change between heart rate before the
task and heart rate after the task was used. To understand further the effect
of the number of plants on productivity, attention and stress, a questionnaire
was given to the subjects.
2. Methods
Materials
Seven plants were used in the experiment. The seven plants were
aglaonema commutatum, calathea concinna, spathyphyllum, epiremnum
aureum, benjamin, schefflera arboricola, and areca palm. All plants were
potted and were placed on the table and on the floor. Plant heights were
varied from 20 cm to 120 cm.
Numerous papers containing numerous alphabet characters were used
for the word-search task. The objective of the task was to finish as many
papers as the subject could do.
Facility
The experiment was conducted in Toyohashi University of
Technology, in the climatic chamber of the Building Environment
Laboratory, from 09:00 until 20:00, from June 6 to July 14, 2012. The space
of the chamber was 3.57 meters in length, 2.67 meters wide, and 2.44
meters high. The illuminance on the desk was 408 Lux, from two
fluorescent lamps and without daylight.
CLIMA 2013, Prague, Czech Republic, June 16-19, 2013
Subjects
18 subjects participated in the study, with volunteer or unpaid basis.
Among the 18 subjects, 10 were male and 8 were female, with age ranged
from 22-30 years old with average 23.5 years old. All participants were
university students from various departments.
Experimental conditions
Four conditions were tested during the experiment. The four
conditions were a room with no plant, one pot of plant, 3 pots of plant, and
7 pots of plant in the room. The four conditions were depicted in Fig. 1.
Measurements
Heart rate before and after performing the primary task, secondary task
reaction time (STRT), and productivity (number of forms and number of
words) were measured during the experiment. Air temperature, air
humidity, CO2 compound, and lighting condition were measured during the
experiment.
Experimental procedure
A within subject design was selected as the experimental design due to
the limited number of participants, in which all 18 subjects participated in
all conditions. However, to overcome learning bias, the sequence of
conditions which followed by the subjects was randomized.
Before performing the task, the subject was given 10 minutes for
acclimatization with room condition. After 10 minutes, the heart rate of the
subject was measured. Then, the subject performed a word search on a
form. Words on the form were in English. When a word was found, the
subject then highlighted the word. The subject was instructed to find as
many words as he/she could within 10 minutes. During the experiment, the
subject was presented with a sound signal from a laptop. When the subject
heard the signal, the subject was instructed to react to the signal by pressing
the “Enter” key. The time between the signal was launched and the “enter”
key was pressed was measured. After the task had been finished, the subject
was instructed to fill in the questionnaire for 5 minutes. Schedule of the
experiment is shown in Fig. 2.
CLIMA 2013, Prague, Czech Republic, June 16-19, 2013
a) No plant condition
b) 1 plant condition
c) 3 plants condition
d) 7 plants condition
Fig. 1. Conditions of the experiment
Fig. 2. Schedule of the Experiment
Statistical Analysis
Data for subjects tested in all conditions were compared. A repeated
measures test of the general linear model was performed to compare means
of heart rate, secondary task reaction time, and productivity of all
conditions. On the other hand, a nonparametric test of related samples
Friedman’s Two Ways Analysis of Variance by Ranks was performed to
compare data of subjects perception of the conditions. Statistical tests were
performed using IBM SPSS Statistics Release 20.0.0.
min
0
10
20
25
a
b
c
a = acclimatization
b = word search task
c = questionnaire
CLIMA 2013, Prague, Czech Republic, June 16-19, 2013
3. Results
Physical measures of indoor environment were presented in Table 1.
Table 1. Physical measures of indoor environment
No
plant
1
plant
3
plants
7
plants
Test of
ANOVA
Temp.,
:
SD :
24.3
0.6
24.3
0.6
24.2
0.6
24.1
0.8
F = 0.41
p = 0.70
Humid.,
:
SD :
82.9
6.5
83.4
6.5
82.1
7.5
83.3
6.4
F = 0.22
p = 0.86
CO2,
:
SD :
973.3
189.4
856.4
148.0
1017.4
228.0
933.8
251.9
F = 2.96
p = 0.06
: average SD: standard deviation
In Table 1, the results of the test of ANOVA for examining the
difference among conditions was also presented in the 6th column. There
were no significant difference in the temperature and humidity. A
significant difference was found on the CO2 level, with p value < 0.1. The
lowest CO2 was the room with 1 plant condition. However, the relationship
between the number of plants and CO2 level was unclear. From Table 1, it
also can be seen that rooms with 1 plant and 7 plants have lower amount of
CO2 than rooms with no plants and rooms with 3 plants.
Dependent variables measured in the experiment were presented in
Table 2. Averages of condition 1, 2, 3, and 4 were presented in 2nd, 3rd, 4th,
and 5th column respectively. The 7th column contained the results of the
ANOVA test, which is used for examining the averages among the 4
conditions.
Table 2. Physiological and productivity measures
No
plant
1
plant
3
plants
7
plants
Test of
ANOVA
Heart rate change,
:
SD:
2.0
3.7
3.3
6.4
3.9
5.3
2.9
5.3
F = 0.51
p = 0.67
Average of STRT,
:
SD:
1.5
0.2
1.7
0.6
1.5
0.2
1.4
0.2
F = 1.94
p = 0.18
STRT change,
:
SD:
0.0
0.3
-0.4
2.0
0.2
0.3
0.2
0.5
F = 0.94
p = 0.36
Number of papers,
:
SD:
4.9
1.6
5.0
2.7
4.8
1.7
5.3
2.2
F = 0.44
p = 0.67
Number of words,
:
SD:
42.0
14.8
45.7
26.34
42.7
17.3
44.4
20.8
F = 0.36
p = 0.72
CLIMA 2013, Prague, Czech Republic, June 16-19, 2013
During the experiment, STRT was measured 4 times, randomly. From
these measurements, the average value of STRT can be obtained. The
change of the STRT variable was used to detect any change of reaction time
during the experiment, which was obtained by subtracting the 4th
measurement by the 1st STRT measurement. Most of the averages of STRT
changes were positive, except for conditions with 1 plant. It means that the
average of the 4th STRT change was smaller than the average of the 1st
STRT change.
From Table 2, it can be seen that there was no significant difference in
the average of heart rate change, the average of STRT, the average of STRT
change, average number of paper and average number of words among
conditions, as the p values were larger than 0.05 or 0.1. Although the room
with 7 plants was better on several variables than the other conditions, it
was difficult to draw any conclusion based on the data on each condition
due to the large standard deviation.
Questionnaire results
Perceptions of the participant on their physical condition, motivation
and subjective feeling of the room are shown in Fig. 3.
Fig. 3. Subject’s Perceptions of the Room
0
1
2
3
4
Physical
condition
Motivation
Feeling of
Temperatur
e
Feeling of
Humidity
Thermal
Satisfaction
Air Quality
Friendliness
Comfort
Freshness
Cleanliness
No plant
1 plant
3 plants
7 plants
CLIMA 2013, Prague, Czech Republic, June 16-19, 2013
From Fig. 3, it can be seen that physical condition, motivation, feeling
of temperature, humidity, thermal satisfaction and air quality among
conditions were not significantly different (p values were larger than 0.05 or
0.1). It can also be seen from Fig. 3 that significant differences were found
in the perception of the subjects on friendliness, comfort, freshness, and
cleanliness of the conditions (p < 0.05). From Fig. 3, it can be seen that
rooms with plants were perceived better than no plant condition. It also can
be seen that the room with seven pots of plant has the largest area than the
other experimental conditions. It suggests that the room with seven pots of
plants was perceived best by the participants.
4. Discussion
Productivity
The result showed that there was no significant difference in the
number of paper and the number of words among conditions. However,
from Figure 4, it is clear that the room with 7 indoor plants had higher
productivity than no plant condition. It seems that 7 was the optimal value
for the number of plants, as decreasing or increasing the value would reduce
the productivity. Larsen [7] suggested that the room with 10 plants and 22
plants had lower productivity than the room without any plant.
Stress
The result showed that there was no significant difference in heart rate
change among conditions. The insignificance might be caused by the high
standard deviation of data. However, it is difficult to understand the cause
of the high standard deviation of data of heart rate change. The high
variability in heart rate change data might be due to the fact that for some
participants the task was not so difficult, not fatiguing, and not stressful,
although the duration was appropriate. Some of the subjects said that the
task was interesting and challenging, therefore some of them wouldn’t look
stressed by the task. This finding confirms that plants had rather weak
effects on stress measures and not always statistically significant as
suggested by [4], [6], [8], and [10].
Attention
The result showed that although the room with seven plants condition
was better in the average of secondary task reaction time than the other
conditions (Table 2), this was not significant enough (p = 0.18). The result
suggested that this might be that the subject still have enough resources to
pay attention at the secondary task without being tied up to the primary task.
The small value of STRT change (from -0.4 to 0.2 second) suggests that
attention of the subject was not changed significantly during 10 minutes of
the experiment. Therefore, a further study with a longer working time is
required, so that attention change and fatigue can be observed.
CLIMA 2013, Prague, Czech Republic, June 16-19, 2013
Although not statistically significant, it can be seen from the mean
value in Table 2 that the room with seven plants was superior to the other
conditions. The room with seven plants condition has the lowest average of
secondary task reaction time, the highest number of forms finished, and the
second highest number of words found. Therefore, the room with seven
plants was best for participants productivity and attention This was also
supported by the fact that the room with seven plants condition was
perceived as the most friendly, the most comfort, the most fresh, and also
the most clean among the other conditions, as shown in Fig. 3. Compared to
the results of [4] and [12], our findings support the idea that the presence of
plants may increase worker productivity and worker attention in tasks
requiring moderate concentration, and improve the feeling on the
environmental quality.
However, due to relatively small sample size, determining whether
these differences in perception are as a result of the number of the plants or
random variation is difficult. The differences found in this study were from
hypothesis generating study. Therefore, a study with larger sample size is
required. Moreover, as the subjects in this study were mainly students, a
further study with more variations in the demography of the subjects is also
considered.
As the task carried out by the subject in the experiment is a visual
search, lighting factor is also important. Also, plants need light to grow and
for photosynthesis, so that they can absorb CO2 and release O2. In this
experiment, however, the level of light is almost the same throughout the
experiment days, as no daylight is used in the experiment, and the source of
light was only two fluorescent lamps, the same for every treatment.
Therefore, the results of the present study are not affected by lighting
condition.
5. Conclusion
In the present experimental study, the following question was
addressed: how is the effect of the number of the indoor foliage plants on
productivity, stress and attention? After carefully controlling other physical
factors and designing tasks appropriate for the study [13], we found that the
effects of the number of indoor foliage plants on productivity, stress and
attention were not statistically significant.
Subject’s perception in temperature, humidity, thermal satisfaction and
air quality among conditions was also not significantly different. On the
other hand, differences in subjects’ perceptions in the friendliness, comfort,
freshness, and the cleanliness of the room were statistically significant. The
best number of plants that should be placed in a room sized 9.53 meters
squared was seven. However, as it is not easy to determine whether these
CLIMA 2013, Prague, Czech Republic, June 16-19, 2013
differences are due to the number of plants or simply the chance, a further
confirmatory study with larger sample size is required.
6. Acknowledgement
This study was funded by Directorate General of Higher Education,
Ministry of National Education, Indonesia. All plants used in this study
were provided by PLANET Co. Ltd. Japan.
7. References
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