According to Nyrud and Bringslimark (
2010), several factors affect the visual impression of wood, including species, number of knots, colour, structure and surface treatment. In particular, people focus on a mixture of five surface features when they look at wood: texture, knots, coloration, contrasts, and other properties (e.g., pitch wood, pitch pockets, and bark pockets) (Broman
1995). Generally, homogenous visual properties and surface harmony (e.g., only a few evenly dispersed knots on wood surface are perceived as desired aesthetic properties) (Broman
2001; Hoibo and Nyrud
2010).
People commonly have positive attitudes towards wood and perceive it as a natural material that evokes feelings of comfort, relaxation, and warmth (Rice et al.
2006; Burnard and Kutnar
2015; Watchman et al.
2017). The perception of warmth has been associated with the yellow-red colour hue, while the knots create a natural and rustic appearance (Rice et al.
2006). Solid wood samples (with stone and brick) are consistently considered more natural than engineered wood-based products or building materials with greater degrees of transformation, such as metal, plastic and fabric (Burnard et al.
2017). Furthermore, wood is generally preferred when compared with other building materials (Rice et al.
2006; Spetic et al.
2007; Zhang et al.
2016; Watchman et al.
2017; Dematte et al.
2018) and wood-based products (Jonsson et al.
2008; Lindberg et al.
2013). However, the results of some studies indicate that the level of used interior wood is an important factor and that rooms with intermediate levels of wood are preferred over rooms with no wood or extensive use of wood (Tsunetsugu et al.
2007; Nyrud et al.
2014; Dematte et al.
2018).
Even though most information about the physical environment is gathered through vision, in indoor environments, people frequently touch wooden surfaces and other materials, for example, interior applications and furniture (Lindberg et al.
2013), and this tactile sensation of wood has been observed in a few studies. In one survey, participants evaluated different floorings with their hands and feet: Parquet flooring with ‘natural’ oiled surface was perceived as warm, rough and fairly soft, and it generally was preferred over laminate flooring and parquet flooring with lacquer (Berger et al.
2006). Bhatta et al. (
2017) studied how the sensory and emotional aspects of touch with fingertip are related to eight different types of surfaces from Scots pine and oak wood boards with four types of treatments (sanding with sandpaper, brushing with metal brush, varnish and wax) applied to each species. Natural (uncoated) wood surfaces were rated significantly higher in descriptors that feature positive aspects of emotional components and least irritating and uncomfortable in descriptors that feature the negative aspects of affective touch. Similar results were attained in a study by Lindberg et al. (
2013), in which tactile sensations from solid wood samples from different species were perceived as natural and eco-friendly. A greater variation in terms of perceived attributes existed among the studied wood-composite materials, but they generally scored low on naturalness and exclusivity.
3.5.1 Physiological effects from wood derived stimuli
Psychophysiological responses are physiological responses (such as stress responses) to external stimuli, and by measuring these responses, it is possible to evaluate outcomes on psychological and physical well-being from encounters with wood (Nyrud and Bringslimark
2010). Physiological responses act as indicators of human stress, and common indices used to evaluate these responses include brain activity, autonomic nervous activity, endocrine activity, and immune system activity (Burnard and Kutnar
2015; Ikei et al.
2017). Research on the physiological effects of wood is relatively new, but a growing number of studies related to the topic now exist. Physiological effects from wood-derived stimulation studied through physiological indices have concentrated on olfactory, visual, auditory and tactile sensations (Ikei et al.
2017). In this section, the studies related to tactile and visual sensations are reviewed, and a summary of findings is presented in Table
3. Studies related to physiological effects from stimulations of olfactory sensation are presented in the section of this review about chemical emissions. The two studies found to examine the stimulation of auditory sensation mainly concerned physiological responses related to floor-impact sound insulation and were left out of this review (Sueyoshi et al.
2004a,
b).
Table 3
Summary of the physiological effects of wood on visual and tactile sensations
Autonomic nervous activity | Pulse rate and systolic blood pressure: Small fluctuation with silk and sawn wood Large fluctuation with steel and vinyl bag | Japanese cypress (sawn), Japanese cedar (sawn, planed), silk, denim, stainless steel, vinyl bag Tactile sensation (60 s) | Female students n = 19 | |
Autonomic nervous activity | Pulse rate: Decrease in standard room Increased in designed room Diastolic blood pressure: Decreased in standard room | “Standard” room with wood and “designed” room with added wooden elements (90 s) | Male students n = 10 | |
Autonomic nervous activity | Pulse rate: Decrease in standard room Increased in designed room Diastolic blood pressure: Decreased in standard room | “Standard” room with wood and “designed” room with added wooden elements (90 s) | Male students n = 15 | |
Brain activity | Regional cerebral blood flow (rCBF): Increased in standard and designed room |
Autonomic nervous activity | Pulse rate: Increased in 45% room No change in 0% and 90% rooms Diastolic blood pressure: Decreased in all rooms Systolic blood pressure: Decreased in 90% room | Rooms with surface wood ratios: 0%, 45%, 90% (90 s) | Male students n = 15 | |
Brain activity | Changes in total haemoglobin concentration (tHb): Increased in all rooms |
Autonomic nervous activity | Systolic blood pressure: Exposure to cypress panels Decreased in “like” group n = 5 No change in “dislike” group n = 5 Exposure to steel panels Increased in “dislike” group n = 9 | Full sized wall panels: Japanese cypress and white steel (90 s) | Male students n = 14 | |
Autonomic nervous activity | Systolic blood pressure: Increased with aluminium and cold plastic No change with cypress, cedar and oak | Japanese cypress, Japanese cedar, oak, acrylic plastic, aluminium (cool, room temp., hot) Tactile sensation (60 s) | Male students n = 13 | |
Endocrine activity | Plasma cortisol levels: Decreased in redecorated room | Hospital isolation rooms: "standard" and redecorated with wood panels and rice paper (26 h) | Male students n = 7 | |
Autonomic nervous activity | Frequency of non-specific skin conductance responses (F-NS-SCR): Decreased in wood environments No effects regarding plants | Four office environments: no plants and no wood, plants with no wood, no plants and wood, plants with wood (40 min) | University students n = 119 | |
Autonomic nervous activity | Systolic blood pressure: Lower in wooden rooms Ratio of heart rate variability: Lower in wooden rooms Oxyhemoglobin saturation SpO2: Higher in wooden rooms | Five test rooms: non-wooden preparation and test rooms, and 3 wooden rooms with different contrast (60 min) | Adult subjects n = 20 | |
Autonomic nervous activity | Salivary free cortisol concentration: Decreased in oak environment | Four test settings: offices with oak and walnut furniture and control offices with no wood (75 min) | Adult subjects n = 61 | Burnard and Kutnar ( 2019) |
Autonomic nervous activity | Ratio of heart rate variability: Decreased in wooden environment | Before and after stay in hospital waiting room with pine walls and ceilings, and larch furniture | Adult subjects n = 40 | |
Brain activity | EEG (α), EEG (β), and SMR waves: Initially decreased, and after a while EEG (β) waves increased | | n = 4 |
Morikawa et al. (
1998) studied the influence of contact with wood (Japanese cypress (
Cryptomeria japonica) with a sawn surface and Japanese cedar (
Chamaecyparis obtusa) with a planed and sawn surface), silk, denim, a stainless-steel board and a vinyl bag filled with cold water. Nineteen female subjects touched the materials for 60 s while their pulse rate and systolic blood pressure were measured. The study showed that contact with silk and wood with a sawn surface caused small variations in pulse rate and systolic blood pressure. In contrast, fluctuations were wide for both measures during contact with the stainless steel and the vinyl bag filled with water.
Sakuragawa et al. (
2008) examined the effects from contact with wood (Japanese cypress and Japanese cedar), plastic, and aluminium using subjective evaluation and blood pressure as an indication of physiological stress responses. They found that contact with wood produced safe/comfortable and coarse/natural sensations, and that contact with cooled wood produced similar coarse/natural sensation with a subjectively dangerous/uncomfortable sensation. Contact with wood caused no increase in blood pressure, but contact with aluminium or cold acrylic plastic produced flat/artificial and dangerous/uncomfortable sensations with increased systolic blood pressure.
Tsunetsugu et al. (
2002,
2005,
2007) investigated physiological responses to wood in three studies using actual-size model rooms. In the first study, one of the rooms was a ‘standard’ Japanese living room with a wooden floor and papered walls and ceiling. The second room was identical except for wooden beams and columns that were added. Ten male subjects were exposed to test rooms for 90 s while their blood pressure and pulse rate were measured. In addition, the subjects evaluated the rooms subjectively and their temporal mood states were examined. Decreased blood pressure was detected in participants in the ‘standard’ room, whereas it increased in the room with added wood. Additionally, the diastolic blood pressure tends to decrease in the ‘standard’ room, but no significant differences were reported between the two rooms in other measurements.
In the second study, Tsunetsugu et al. (
2005) used the same test settings as in the first study with 15 male subjects. The same measures were used, but regional cerebral blood flow (rCBF) measurement was added. The results were similar to those from the first study: Pulse rate and diastolic blood pressure decreased in the ‘standard’ room, while pulse rate increased and diastolic blood pressure did not change in the other room. Furthermore, rCBF increased in both rooms, but no significant differences existed between the two rooms in terms of blood flow, mood, or subjective evaluation.
The third study by Tsunetsugu et al. (
2007) investigated the physiological responses of 15 male subjects to three different surface wood ratios (0%, 45% and 90%) of actual-size living rooms. The measures used otherwise were the same as in the second study, but changes in total hemoglobin concentration (tHb) were measured as an index of central nervous activity instead of rCBF. In the subjective evaluation, the 45% room tended to be evaluated as the most comfortable and restful. The 90% and 45% rooms were evaluated as natural, while the 0% room was evaluated as the most artificial. Diastolic blood pressure decreased significantly in all three rooms. A significant decrease in systolic blood pressure was measured in the 90% room and pulse rate increased significantly in the 45% room, whereas these two indices did not change in the 0% room.
Sakuragawa et al. (
2005) studied the influence of visual stimulation from full-size wooden Japanese cypress wall panels and white steel wall panels with 14 male subjects. The subjects were exposed to different panels for 90 s while their blood pressure and pulse rate were measured, and after the exposure, subjective evaluation and mood test were performed. The measured mood scale scores on depression/dejection were significantly lower for the visual stimulation from the cypress wall panels than the control, and conversely, scores for visual stimulation by white wall panels were significantly higher. In physiological measurements, the researchers found that subjects who reported liking cypress panels had a significant decrease in systolic blood pressure during exposure to cypress wall panels. Subjects who reported liking steel wall panels maintained stable blood pressure when exposed to a steel wall, whereas the subjects who reported disliking the steel panel registered significant increase in blood pressure during the exposure.
Ohta et al. (
2008) studied the effects of redecorating a hospital isolation room on the stress level of seven male subjects. Two actual isolation rooms in a hospital were used, with one room redecorated with wood panelling and Japanese rice paper, and the other used as a conventional hospital room with white painted concrete walls and ceiling boards. The subjects stayed in the rooms for 26 h and their physiological responses were monitored during and after staying in the rooms. The investigated parameters included heart rate, blood pressure, arterial vascular compliance, and plasma levels of cortisol, antidiuretic hormone, oxytocin, adrenaline, noradrenalin and dopamine. The plasma cortisol levels remained significantly lower after staying in the redecorated room compared with the control room, which according to the authors suggests that natural materials may provide a less-stressful environment. No significant differences between the two rooms were found with other studied parameters.
Fell (
2010) investigated the stress responses of 119 subjects in four different office-like environments before, during, and after a stressful mental test. During the study, sympathetic nervous system activity was monitored by measuring the subjects’ skin conductivity. Measures for cardiovascular responses to stress included inter-beat interval and heart rate variability. The test-room setups were: no plants and non-wooden furniture, plants with non-wooden furniture, no plants and wooden furniture, and plants with wooden furniture. The values of frequency of non-specific skin conductance responses were significantly lower during the pre-test and post-test periods in the rooms with wooden furniture, indicating that the subjects in the presence of wood were less-stressed than subjects in the rooms without wooden furniture. Similar effects for indoor plants were not found.
Zhang et al. (
2017) studied physiological responses to wooden indoor environment using five test rooms simulating an office environment. The test-room setups were: non-wooden preparation room, non-wooden test room, and three wooden rooms with different contrasts. Twenty adult subjects completed work tasks during 60-min exposure periods to different rooms, while physiological parameters such as electrocardiogram measurements, skin temperature, skin resistance, blood pressure, oxyhemoglobin saturation (SpO
2), and near-distance vision were monitored. The researchers found out that the mean value of systolic blood pressure was significantly lower during the test and SpO
2 values were slightly higher in the three wooden rooms compared with the non-wooden room. A similar trend was observed with the ratio of low frequency and high frequency heart rate variability, which was lower in wooden rooms than in the non-wooden room. Additionally, the mean value of near visual distance was slightly, but not significantly, higher in the wooden rooms in the beginning of the experiment, and it was significantly higher in one of the rooms at 52 min measurement compared to the non-wooden room. According to the authors, these results indicate that people felt less stress and tension in wooden environments. These results were supported by results from a simultaneous survey made by Zhang et al. (
2016), where they studied different psychological responses to the same rooms by systematic and quantitative tests. They found out that in the wooden rooms the subjects had more positive emotions and fatigue evaluation values were dramatically lower compared to non-wooden room.
Burnard and Kutnar (
2019) examined human stress responses to wood in two office-like environments with a total of four test settings. The test settings in both offices were a control environment with white furniture and no visible wood and a wooden environment with wood furniture. The wooden environment in one office was made with oak veneered furniture, and in the other with American walnut furniture. The study had a total of sixty-one subjects, both male and female aged between 18 and 52. During the 75-min test phase, stress was induced in subjects, so that they were able to observe and analyze stress responses and recovery. Salivary free cortisol was analyzed from seven saliva samples collected during the test phase and used as an indicator of stress responses. The cortisol concentration levels were significantly lower throughout the entire test period and during the response period (35–75 min) in the oak environment compared to the control environment. No significant differences were detected between the walnut environment and the control environment.
Kotradyova et al. (
2019) studied physiological responses to wood in a hospital waiting room. Experiments were executed on forty adult volunteers (both male and female) before entering, during, and after their stays in the waiting room. The waiting room was decorated with wooden wall panels and ceiling cladding made from solid pine wood, seating made of larch timber, and new warm white lighting. The subjects’ heart rate, heart rate variability, and respiration activity were recorded, and their cortisol concentration was measured before and after the stay in the room. Additionally, brain activity of four subjects was analyzed from recorded electroencephalograph (EEG). They found out that ratio low frequency and high frequency heart rate variability decreased in the wooden environment, and modest increase in heart rate and respiration frequency was measured. Cortisol concentration had modest tendency towards decreasing but no significant differences were found. Brain wave recordings showed decreased brain activity in the wooden waiting room compared to the former space. Time progress during the time in the waiting room showed that EEG (α) and SMR waves decreased, and EEG (β) waves increased, which according to the researchers indicate that the subjects’ brains became more active after the initial relaxation.
All in all, in the reviewed studies some differences were found with the used physiological indices between wooden and non-wooden environments for both visual and tactile stimulation. In eight out of the eleven studies, the results indicated that wood materials may provide less stressful environments. In two studies, the results were somewhat opposite but there were only small differences (wooden beams and columns) between the investigated rooms (Tsunetsugu et al.
2002,
2005). However, the reviewed studies had several limitations and observations which hinder the possibilities to draw concrete conclusions about the stress relieving effects of interior wood. With the exception of research made by Ohta et al. (
2008), Fell (
2010), Zhang et al. (
2017), Burnard and Kutnar (
2019), and Kotradyova et al. (
2019), the used exposure time was 90 s or less, which makes it difficult to confirm physiological stability (Zhang et al.
2017), and to translate such data to daily real-life situations and long-term effects. Further, the number of participants was 20 or less in most of the studies, comprising students in their 20s. However, positive physiological results were obtained as well in the studies with greater sample sizes and age variation. It was also noticed that different wood quantities (Tsunetsugu et al.
2007), contrasts of the used wood (Zhang et al.
2017; Burnard and Kutnar
2019), and personal preferences (Sakuragawa et al.
2005,
2008) affected the measured responses. Additionally, there exist preferences for different material combinations, and wood surfaces together with painted surfaces or other elements might add preferred complexity in the indoor environment (Nyrud et al.
2014; Burnard and Kutnar
2019), which may have been a significant factor in the majority of the reviewed studies. Therefore, it is difficult to separate whether these responses are directly from wood-derived stimulation and how to generalize the obtained results.