Interaction with indoor plants may reduce psychological and physiological stress by suppressing autonomic nervous system activity in young adults: A randomized crossover study

Abstract

Developments in information technology cause a great deal of stress to modern people, and controlling this stress now becomes an important issue. The aim of this study was to examine psychological and physiological benefits of interaction with indoor plants.
The study subjects were 24 young male adults at the age of 24.9 ± 2.1 (mean ± SD). The crossover experimental design was used to compare the differences in physiological responses to a computer task and a plant-related task. Subjects were randomly distributed into two groups. The first group (12 subjects) carried out transplanting of an indoor plant, whereas the second group (12 subjects) worked on a computer task. Then, each subject switched activities. The psychological evaluation was carried out using the semantic differential method (SDM) and physiological evaluation using heart rate variability (low-frequency (LF) and high-frequency (HF) components) and blood pressure.
Analysis of the SDM data showed that the feelings during the transplanting task were different from that during the computer task: the subjects felt more comfortable, soothed, and natural after the transplanting task than after the computer task. The mean value of total log[LF/(LF + HF)] (sympathetic activity) increased over time during the computer task but decreased at the end of the transplanting task, and the differences were significant. Furthermore, diastolic blood pressure was significantly lower after the transplanting task.
Our results suggest that active interaction with indoor plants can reduce physiological and psychological stress compared with mental work. This is accomplished through suppression of sympathetic nervous system activity and diastolic blood pressure and promotion of comfortable, soothed, and natural feelings.

Full text

O RIG INAL AR TIC L E Open Access
Interaction with indoor plants may reduce
psychological and physiological stress by
suppressing autonomic nervous system activity in
young adults: a randomized crossover study
Min-sun Lee
1
, Juyoung Lee
2
, Bum-Jin Park
3
and Yoshifumi Miyazaki
4*
Abstract
Background: Developments in information technology cause a great deal of stress to modern people, and
controlling this stress now becomes an important issue. The aim of this study was to examine psychological and
physiological benefits of interaction with indoor plants.
Methods: The study subjects were 24 young male adults at the age of 24.9 ± 2.1 (mean ± SD). The crossover
experimental design was used to compare the differences in physiological responses to a computer task and a
plant-related task. Subjects were randomly distributed into two groups. The first group (12 subjects) carried out
transplanting of an indoor plant, whereas the second group (12 subjects) worked on a computer task. Then, each
subject switched activities. The psychological evaluation was carried out using the semantic differential method
(SDM) and physiological evaluation using heart rate variability (low-frequency (LF) and high-frequency (HF)
components) and blood pressure.
Results: Analysis of the SDM data showed that the feelings during the transplanting task were different from that
during the computer task: the subjects felt more comfo rtable, soothed, and natural after the transplanting task than
after the computer task. The mean value of total log[LF/(LF + HF)] (sympathetic activity) increased over time during
the computer task but decreased at the end of the transplanting task, and the differences were significant.
Furthermore, diastolic blood pressure was significantly lower after the transplanting task.
Conclusions: Our results suggest that active interaction with indoor plants can reduce physiological and
psychological stress compared with mental work. This is accomplished through suppression of sympathetic nervous
system activity and diastolic blood pressure and promotion of comfortable, soothed, and natural feelings.
Keywords: Indoor plant, Technostress, Psychological and physiological effects, Heart rate variability, Sympathetic
nervous system
Background
The living space of modern people ha s moved from out-
doors to indoors - more than 85% of a person’s daily life
is spent indoors. Developments in information technology
have allowed people to connect and remain connected to
the computer environment. However, this diffusion of in-
formation technology causes a great deal of stress, such as
technostress [1], which is a modern disease of adaptation
caused by an inability to cope with the new computer
technologies in a healthy manner. Many studies have been
carried out to evaluate various ways to control this psy-
chological stress; for example, the effect of a natural en-
vironment on h uman beings has been actively studied
since t he 1980s [2-4]. A n umber of studies are also un-
derway concerning the physiological and psychological
effect of interacting with plants. Plants relieve physio-
logical stress and negative psycholog ical symptoms
[5-8]. This finding has important i mplications because
* Correspondence: ymiyazaki@faculty.chiba-u.jp
4
Center for Environment, Health and Field Sciences, Chiba University, 6-2-1
Kashiwanoha, Kashiwa, Chiba 277-0882, Japan
Full list of author information is available at the end of the article
© 2015 Lee et al.; licensee BioMed Central. This is an Open Access article distributed under the terms of the Creative
Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain
Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article,
unless otherwise stated.
Lee et al. Journal of Physiological Anthropology (2015) 34:21
DOI 10.1186/s40101-015-0060-8

the cardiovascular system can be damaged by overacti-
vation of the sympa the tic nerv ous syste m a s a result of a
stressful situat ion [9,10].
In rece nt years, the comforting effect of a natural en-
vironment has been verified, and further evidence-based
studies are underway. Various experimental approaches
have been attempted in regard to physiological measures,
which can verify the beneficial effects of natural stimuli
quantitatively. A contact with plants is an intuitive and
nonverbal activity that can provide psychological stability
and comfort by stimulating four senses in various ways.
Indoor plants have drawn the attention of the scientific
community because of their various benefits: they enhance
job satisfaction in office workers [11], reduce psychological
stress [12], improve mood states [13-16], and enhance
cognitive health [17-19]. These effects can positively affect
resistance to diseases and chronic stress [20,21], but rigor-
ous evidence is lacking. With the present methods of
psychological assessment, health benefits of indoor plants
cannot be sufficiently explained. Furthermore, few studies
have investigated the physiological mechanism underlying
the health benefits due to indoor plants.
Therefore, in this study, we attempted to examine the
physiological benefits of indoor plants in mod ern people.
We focused on cardiovascular changes when a person
makes a contact with foliage plants: we measured the auto-
nomic nerve system activity. In addition, we attempted to
quantify the psychological changes during the contact with
plants as well.
Methods
Subjects and the protocol
We enrolled 24 young male adults at the age of 24.9 ±
2.1 (mean ± SD). None of the subjects reported a history
of physical or psychiatric disorders. The study lasted
3 days. Alcohol and tobacco were prohibited, and caf-
feine intake was controlled. Prior to the start of the ex-
periments, the subjects were fully informed of the aims
and procedures of the experiments, and informed consent
was obtained. This study was conducted in compliance
with regulations of the Clinical Trial Center, Chungnam
National University Hospital, Korea, and the Ethics
Committee of the Center for Environment, Health and
Field Sciences , Chiba University, Japan.
The crossover experimental design was used t o com-
pare differences in ph ysiolog ical respo nses to the two
tasks. Twenty-four subject s were randomly distributed
into two groups. On the first day o f the experiments,
the first group (12 subjects) tended to indoor plant s
while t he second group (12 subje cts ) worked on a docu-
ment in a word processor, one of the most typical com-
puter ta sk s, which needs continuous physical activity,
like the transplanting task. On the se cond day, the s ub-
jects switched a ctivities. Each subject performed each
taskatthesametimeofthedaytoreducetheeffectsof
diurnal variation.
Materials
Peperomia dahlstedtii, a common indoor plant, was used
for the transplanting work. The transplanting method
was taught to each subject prior to the experiments so
that they could work more comfortably. The experiment
was carried out in a greenhouse room, where the wall
was covered with a black curtain (Figure 1), and envir-
onmental conditions were maintained relatively consist-
ent (temperature 20.8°C ± 1.4°C, mean ± SD; humidity,
57.7% ± 6.6%; illuminance 1,365.5 ± 327.9 lux).
The temperature was set at 22°C, and humidity was
controlled so that it would not decrease from 50%. The
lighting con dition was controlled at comfortable level by
hanging curtains from the ceiling and on walls to protect
from direct sunlight.
Measurements
An electrode was attached to the subjects’ chest in the wait-
ing room, and they moved to the experimental room. After
a 2-min rest in a seated position, they performed the given
tasks, that is, transplanting houseplants or computer work,
for 15 min. Heart rate variability (HRV) was measured con-
secutively during the task using a portable electrocardio-
graph (Activtracer AC-301A; GMS, Tokyo, Japan). Blood
pressure and pulse rate data were collected before and after
the tasks using a digital blood pressure monitoring device
(HEM-1000; OMRON, Kyoto, Japan).
Data analys is
HRV data were calculated by averaging 1-min inter-beat
(R-R) data and analyzed by means of maximum entropy
methods (MemCalc, GMS, Tokyo, Japan) using the low-
frequency (LF; 0.04 to 0.14 Hz) and high-frequency (HF;
0.15 to 0.40 Hz) components of the power spectrum.
The HF component reflects activity of the parasympa-
thetic nervous system, which increases in a rela xed state,
and LF/(LF + HF) reflects activity of the sympathetic ner-
vous system, which increases in a stressed state. All
HRV values were log-transformed (base 10).
The feelings that the subjects experienced during the
test were measured using the semantic differential method
(SDM), which is a self-rating assessment. The subjects
rated their feelings on a seven-point scale for three test
items - ‘Comfortable’, ‘Relaxed ’ ,and‘Natural’ -bywrit-
ing down their fe elings at the moment be fore and after
the tasks.
Apairedt test was used to compare the differences in
HRV values and blood pressure between the two tasks.
Wilcoxon signed-rank test was used for the analysis of psy-
chological data. Statistical analysis was carried out using
the SPSS software, version 21.0 (IBM Corp., Chicago, IL,
Lee et al. Journal of Physiological Anthropology (2015) 34:21 Page 2 of 6

USA). In both cases, we applied one-sided tests because of
the hypothesis that humans would feel more relaxed after
the transplanting task. In all cases, the differences were
considered statistically significant at P < 0.05.
Results
Analysis of the SDM data showed that the feelings dur-
ing the transplanting task were different from that dur-
ing the computer task. The subjects felt comfortable,
soothed, and natural after the transplanting task, whereas
they felt uncomfortable, awakened, and artificial after the
computer task. There were significant differences between
the two for the three feelings tested after the 15-min tasks,
despite the absence of significant differences in these feel-
ings before the tasks (Figure 2) when the subjects showed
generally neutral responses for these three feelings.
The changes of log[LF/(LF + HF)] reflecting the sym-
pathetic nervous system activity during the tasks are
shown in Figure 3. Although there were no significant
differences in the mean value of total log[LF/(LF + HF)]
for the 15-min period, these values changed in ways that
differed between the two tasks. The log[LF/(LF + HF)]
value increased over time during the computer task but
decreased at the end of the transplanting task. Therefore,
the data from the last 3 min were compared; they showed
significant differences in log[LF/(LF + HF)] (transplanting
Figure 1 Photographs of (A) Peperomia dahlstedtii, (B) a computer, (C) a subject transplanting indoor plants, and (D) a subject performing a
computer task.
Figure 2 Comparison of psychological assessments between plant and computer stimuli. (A) Feelings of comfort, (B) the feeling of relaxation,
and (C) the feeling of naturalness. N = 24, mean ± SD, **P < 0.01 according to the Wilcoxon signed-rank test.
Lee et al. Journal of Physiological Anthropology (2015) 34:21 Page 3 of 6

task 0.57 ± 0.04, computer task 0.60 ± 0.05; P =0.021;
Figure 4). There were no significant differences in logHF
values of the last 3 min between the transplanting (1.94 ±
0.12) and computer task (1.84 ± 0.12). In the analysis of
diasto lic bloo d pressure, a significant differe nce was
obser ved after completion of a t ask (transplanting ta sk,
65.26 ± 0.14; compu ter ta sk, 71.75 ± 0.16; P = 0.001;
Figure 5).
Discussion
In this study, we examined the stress-reducing effects of
interaction with indoor foliage plants by measuring
physiological and psychological responses. The results of
HRV analysis indicate that indoor plants have positive
physiological effects on the autonomic nervous system
by suppressing sympathetic activity, which often increases
when a subject is exposed to a stressor. In this study, the
value of log[LF/(LF + HF)] (corresponds to sympathetic
activity) increases soon after the subjects start the tasks
(either the transplanting or computer task), then shows a
tendency for a slow decrease during the transplanting
task, despite a consistent increase during the computer
task. The stress-reducing effect was observed at the end of
the transplanting task (the last 3 min); this finding may be
partly consistent with that of a previous study [22]. How-
ever, this finding is on the basis of comparison with a
computer task, a type of mental task, which tends to in-
crease sympathetic nervous activity [23,24].
In the present study, the subje cts were found to have
positive feelings when intera cting with indoor plants. In
contrast, the computer task increased diastolic blood
pressure and sympathetic nervous system activity. The
self-rating SDM scores also indicated that working on a
computer may have negative effect s on the psychological
state. It was assumed that participants in this study were
familiar with the compu ter tasks in real life because we
recruited university students. Nonetheless, the results
showed that the subjects felt stressed when performing
0.52
0.54
0.56
0.58
0.6
0.62
0.64
12345678910111213141516171819
log[(LF/(LF+HF)]
Plant Computer
Time(min)
1234567891011121314150-3 -2 -1
Figure 3 Comparison of average log[LF/(LF + HF)] of HRV during the
plant and computer tasks. N = 24, mean ± SE. HF: high-frequency
component, LF: low-frequency component.
1.7
1.8
1.9
2.0
2.1
log[HF(msec
2
)]
0.55
0.56
0.57
0.58
0.59
0.60
0.61
0.62
log[LF/(LF+HF)]
*
Plant Com
p
uter Plant Com
p
uter
00
Figure 4 Comparison of average log[LF/(LF + HF)] and logHF of HRV during the last 3 min of plant and computer tasks. N = 24, mean ± SE,
*P < 0.05 (paired t test). HF: high-frequency component, LF: low-frequency component.
60
65
70
75
80
85
Diastolic blood
pressure(mmHg)
**
Plant Com
p
uter
0
Figure 5 Comparison of diastolic blood pressure after the plant and
computer tasks. N = 24, mean ± SD, **P < 0.01 (paired t test).
Lee et al. Journal of Physiological Anthropology (2015) 34:21 Page 4 of 6

the computer task compared to the transplanting task;
even though the latter was seemingly unfamiliar work to
our subjects.
Our data support the notion that active interaction
[25] with indoor plants can have positive effects on human
stress response mediated by cardiovascular activities.
These physiological benefits may result from multiple nat-
ural stimuli acting on the senses of vision, hearing, touch,
and smell; this effect is also seen in forest therapy research
[26-29]. Although many studies reported positive effects
of indoor plants, most of them have been focused on the
benefits of passive interaction [25] with indoor plants
[11,13,30,31]. Our study presents relevant data that can
explain the mechanism behind the health benefits of ac-
tive interaction with indoor plants, from the standpoint of
the stress response.
A possible limitation of this study is that our subject
group was limited to healthy young male university stu-
dents; more diverse subject groups should be tested in
the future to generalize the results. In the present work,
physiological responses to specific stimuli appeared at
the end of the 15-min experimental period task duration;
therefore, the task duration should be extended beyond
15 min in future studies. It is also recommended that fu-
ture research should use a control task more realistic and
practical with regard to the application of the results.
Conclusions
Our results suggest that active interaction with indoor
plants can reduce physiological and psychological stress
compared with mental work. This is accomplished through
suppression of sympathetic nervous system activity and
diastolic blood pressure and promotion of comfortable,
soothed, and natural feelings.
Abbreviations
HF: high-frequency; HRV: hea rt rate variability; LF: low-frequency;
SDM: semantic differential method.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
ML participated in the study design, carried out the data collection and
analysis, and drafted the manuscript. JL participated in the study design
and carried out the data collection and interpretation. BP participated
in the study design and carried out the data collection and analysis.
YM participated in the study design and data interpretation and edited
the manuscript. All authors read and approved the final version of
the manuscript.
Author details
1
Department of Horticulture Sciences, College of Agriculture and Life
Sciences, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon
305-764, Korea.
2
Korea Forest Service, Government Complex 1, 189
Cheongsa-ro, Seo-gu, Daejeon 302-701, Korea.
3
College of Agriculture and
Life Sciences, Chungnam National University, 99 Daehak-r o, Yuseonggu
Daejeon 305-764, Korea.
4
Center for Environment, Health and Field Sciences,
Chiba University, 6-2-1 Kashiwanoha, Kashiwa, Chiba 277-0882, Japan.
Received: 1 September 2014 Accepted: 7 April 2015
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