Investigation of Humidity Regulation and Heart Rate Variability in Indoor Environments with Larix kaempferi Wood Interiors

Abstract

Wood, as a natural material that stores carbon, is gaining increasing attention and has potential for use in interior architectural applications. Given the long indoor stay time characteristic of modern society, it is important to scientifically understand the effects of indoor wood application on the occupants. In this study, three residential buildings with an identical area and structure were constructed with different degrees of wood coverage (0%, 45%, 90%) using Larix kaempferi. Subsequently, indoor air quality (IAQ) evaluations and relative humidity measurements were conducted to assess the physical and chemical changes in each environment. The IAQ in wooden and non-wooden environments met the recommended IAQ standards established in South Korea. The results of the 8-month observation showed that, the higher the wood coverage ratio, the more the indoor humidity fluctuations were alleviated, and, in the case of the 90% wood coverage ratio condition, the humidity was maintained 5.2% lower in the summer and 10.9% higher in the winter compared to the 0% condition. To further assess the physiological responses induced by the wooden environment, the heart rate variability (HRV) was measured and compared for 26 participants exposed to each environment for two hours. In environments with a 0% and 90% degree of wood coverage, no statistically significant differences were found in the participants’ HRV indicators. But, in the group exposed to the 45% wooden environment, the results showed an increase in HRV indicators, natural logarithm of high frequency power (lnHF): 4.87 → 5.40 (p < 0.05), and standard deviation of normal-to-normal intervals (SDNN): 30.57 → 38.48 (p < 0.05), which are known indicators of parasympathetic nervous system activation.

1. Introduction

Wood has long been used by humans for diverse purposes, including housing, musical instruments, furniture, and fuel. Recently, it has gained renewed attention as a key material for building a sustainable society. This is because wood can store carbon and, when used as a construction material, generally has lower environmental impacts such as energy consumption and carbon emissions compared to other materials [1]. Recognizing these advantages, many countries including Canada, Finland, Sweden, Japan, China, and the United States have adopted wood as a primary material in construction [2].

Concurrently, contemporary architectural trends have moved toward an integrative approach that considers the psychological, physical, and behavioral needs of the occupants [3,4,5]. Therefore, to facilitate the development of wooden environments that align with the occupants’ needs, it is essential that we expand the research on the impacts of wooden interiors on human health and well-being, as well as the associated factors, across a range of environmental conditions. Recent field-based investigations have begun to address this need. Muilu-Mäkelä et al. compared two full office rooms, one with pine wood surfaces and another with conventional materials. In a crossover test with 61 participants, the wooden room was perceived as more pleasant and natural, with low VOC emissions. These finding show that wooden interiors can improve indoor environmental quality and occupant perceptions [6].

In South Korea, various policies are being supported to expand the use of wood to achieve the 2050 carbon neutrality goal. However, there is relatively little research verifying the impact of the actual wooden environment on humans’ physiological well-being. This study aimed to analyze how constructing indoor spaces with wood and varying the degree (0%, 45%, and 90%) of wood coverage would affect the physicochemical properties of the indoor environment and the physical responses of occupants. The Larix kaempferi used as the interior finishing material is considered one of the most economically valuable domestic tree species and is currently a key material in the South Korea timber industry [7].

RH (relative humidity) and IAQ (indoor air quality) were measured to characterize the physicochemical alterations associated with wooden indoor environments. The humidity regulation and function of wood in indoor environments have been reported to influence occupants’ thermal and respiratory comport [8]. Wood in service is exposed to both long-term (seasonal) and short-term (daily) changes in the relative humidity and temperature of the surrounding air, which induce changes in the wood moisture content [9]. A series of experiments were conducted in real indoor environments using various ventilation rates, hygroscopic-material-loading rates, and moisture loads to investigate the moisture buffering effects of three types of hygroscopic materials, including wood [10]. Simonson et al. reported that the use of hydroscopic materials, such as wood, increases residents’ thermal comfort by approximately 10% and enhances air quality satisfaction by 25%, ultimately improving overall health satisfaction [11]. Several studies have investigated the moisture-buffering capacity of L. kaempferi as interior materials [12], as well as the water sorption characteristic related to anatomical differences such as earlywood and latewood [13]. However, quantitative evaluations of the optimal composition ratios are required for actual indoor applications and their effects on occupants’ physiological responses. The physiological responses of the participants were indirectly assessed by examining autonomic nervous system activity based on changes in HRV, thereby indicating the level of physical relaxation.

2. Methods

2.1. Real-Size Indoor Setting

This study targeted residential-type indoor spaces and selected three units of 15-pyeong accommodations with identical structures at a resort located in Taean-gun, Chungcheongnam-do as the experimental spaces (Figure 1). Each experimental space was constructed based on different degree of wood coverage, with flooring uniformly finished using vinyl sheeting to maintain consistent conditions. The indoor coverage rate is defined as the proportion (%) of the total interior surface area (wall and ceilings) that is finished with wood panels. An indoor environment with a wood coverage of 0% has no wood used in the wall and ceiling at all. For the experimental groups, flame-retardant-treated domestic L. kaempferi purchased through the Korea Forest Service Cooperative was applied to the walls and ceilings, resulting in wood usage ratios of 45% and 90% relative to the total interior surface area, respectively. These wooden materials refer specifically to processed L. kaempferi wood used for interior finishing, and are distinct from living plants.

2.2. Participant and Procedure

Participants were recruited through informational materials placed at the Korea Wood Culture Promotion Association and Seoul St. Mary’s Hospital, targeting healthy individuals aged 18 to 65 with no current physical illnesses or history of psychiatric diagnosis or treatment.

This study targeted individuals aged 18 to 65 who were not undergoing treatment for any physical illness and had no history of psychiatric diagnosis or treatment. Participants were recruited via research participation flyers placed in the Korea Association of Wood Culture and Seoul St. Mary’s Hospital, and a total of 26 individuals were ultimately enrolled. A total of 26 participants were enrolled, comprising 15 males and 11 females. Participants were randomly assigned into three groups based on wood usage ratio (0%, 45%, and 90%), consisting of 9 individuals in the 0% group, 9 in the 45% group, and 8 in the 90% group.

The average age of each group was 35.50 years in the 90% group, 37 years in the 45% group, and 37.11 years in the 0% group. By age distribution, the 90% group included 2 participants in their 20s, 4 in their 30s, and 2 in their 40s; the 45% group had 3 in their 20s, 2 in their 30s, and 4 in their 40s; the 0% group consisted of 2 in their 20s, 5 in their 30s, and 2 in their 40s.

Each participant was then assigned to their randomly designated experimental space (0%, 45%, or 90% of wood coverage), and underwent the first heart rate measurement prior to indoor exposure. After staying in the assigned environment for two hours, a second heart rate measurement was conducted. The physiological response data were analyzed by categorizing participants into two groups—non-wooden (0%) and wooden (45% and 90%) environments—for comparative analysis. This study was approved by the Institutional Review Board of the Seoul St. Mary’s Hospital.

2.3. Indoor Air Quality Assessment

Indoor air quality was measured in accordance with official South Korean testing standards. The measured parameters included TVOC, HCHO, PM10, and background factors such as CO₂, CO, temperature, and relative humidity. CO₂ levels were measured using the Falcon-2 air quality monitor (Critical Environment Technologies Canada, Delta, Canada). HCHO and VOCs were measured using the integrated flow sampling method. The equipment used included an SIBATA pump (Soka-shi, Japan). Airborne HCHO and VOCs were collected using Tenax tubes. Total airborne bacteria were quantified by collecting samples via an inhalation pump followed by culturing for colony counting. Radon was measured using the RAD7 (DURRIDGE COMPANY Inc., Billerica, MA, USA), and NOx was measured using the DA-2300 (KEMIK Corporation, Seongnam, Republic of Korea). In accordance with the guidelines of the Korean Ministry of Environment, the external weather conditions at the time of measurement were recorded, and all experiments were conducted under controlled indoor environments.

2.4. Indoor Humidity Measurement

Temperature and humidity sensors were installed in all three experimental zones for each space. Except for the subjects staying in the study building for the experiment, the building was empty and no additional heating or cooling equipment was operated. In each space, two sensors were installed on interior bedroom walls not directly exposed to outdoor air, and an additional two sensors were placed on both the interior and exterior surfaces of the living room and exterior walls, using the MX2301A (Onset Computer) sensor capable of simultaneously measuring and recording temperature and humidity in real time (Figure 2).

The temperature range of the sensor is −40 to 70 °C with an accuracy of ±0.25 °C and a resolution of 0.02 °C, while the relative humidity range is 0–100% within the same temperature range, with an accuracy of ±1%. Measurements were taken over the course of 8 months by recording average values at 4 h intervals. The indoor humidity sensor was positioned at the center of the wall. The outdoor humidity was also measured at the center of the wall with a window. The temperature and humidity measurement sensors were placed at heights of 1.2 and 1.8 m (ASHRAE 2023). Based on the measured temperature and humidity data, the indoor and outdoor equilibrium moisture content (EMC) was calculated using the Hailwood–Horrobin Equation (1), where T is temperature (℃), h is relative humidity (%/100), EMC is moisture content (%), and W, K, K₁, and K₂ are coefficients of an adsorption model developed by Hailwood and Horrobin [14]. This allowed for the evaluation of hygrothermal stability of indoor air under changing outdoor conditions and an analysis of occupant comfort in relation to indoor humidity.

$$\mathrm{E}\mathrm{M}\mathrm{C}={\displaystyle \frac{1800}{\mathrm{W}}}({\displaystyle \frac{\mathrm{K}\mathrm{h}}{1-\mathrm{K}\mathrm{h}}}+{\displaystyle \frac{{\mathrm{K}}_1\mathrm{K}\mathrm{h}+2{\mathrm{K}}_1{\mathrm{K}}_2{\mathrm{K}}^2{\mathrm{h}}^2}{1+{\mathrm{K}}_1\mathrm{K}\mathrm{h}+{\mathrm{K}}_1{\mathrm{K}}_2{\mathrm{K}}^2{\mathrm{h}}^2}})$$

(1)

W = 349 + 1.29T + 0.0135T²

K = 0.805 + 0.000736T − 0.00000273T²

K₁ = 6.27 − 0.00938T − 0.000303T²

K₂ = 1.91 + 0.0407T − 0.000293T²

2.4.1. Heart Rate Variability (HRV) Monitoring

In this study, heart rate variability (HRV) signals were measured using a non-invasive method with the Brainno device (SOSO Company, Seoul, Korea) via photoplethysmography (PPG). Participants were seated comfortably and allowed to rest sufficiently before wearing a sensor band, and real-time heart rate data were collected using an optical sensor attached to the ear. For frequency domain analysis, TP (total power), lnLF (natural logarithm of low frequency 0.004–0.15 Hz), InHF (natural logarithm of high frequency 0.15–0.4 Hz), SDNN (standard deviation of all normal-to-normal intervals), and SI (stress index) were analyzed and used as indicators of parasympathetic nervous system activity.

2.4.2. Statistical Analysis of Data

When the two groups met the assumptions of normal distribution and homogeneity of variance, parametric tests such as the paired or independent t-test were used; when these assumptions were not met, non-parametric tests such as the Wilcoxon signed-rank test or the Wilcoxon rank-sum test were employed to enhance the reliability of the results. For all statistical analyses, a significance level of p < 0.05 was used.

3. Results and Discussion

3.1. IAQ Assessment

After the experimental indoor environments was established, IAQ was measured. IAQ can directly affect the health and living conditions of occupants who spend extended periods indoors, necessitating thorough analysis.

Considering the potential side effects of IAQ on both the experimental environments and human health, this study conducted a comparative analysis based on the guidelines stipulated by the Korean Ministry of Environment’s Indoor Air Quality Control Act. The parameters and threshold values specified in the recommended guidelines are listed below (

Table 1. Mean values of IAQ indicators in wooden and non-wooden environments.

Factor	Measurement Range *	Wood Cover Rate (%)
		0	45	90
PM10 (µg/m3)	75	18.7	18.4	17.6
PM2.5 (µg/m3)	35	7.2	5.1	4.3
CO (ppm)	10	2.2	2.1	1.7
HCHO (µg/m3)	80	18.6	16.1	17.4
CO2 (ppm)	1000	546	548	512
Total airborne bacteria (CFU/m3)	800	150	83	43
VOC (µg/m3)	400	349.3	159.9	61.7
NOx (ppm)	0.05	0.014	0.015	0.001
Fungi (CFU/m3)	500	31	33	25
Rn (Bq/m3)	148	13.0	14.0	17.0

* Thresholds stipulated in the IAQ Control Act by the Korean Ministry of Environment.

): particulate matter with a diameter of 10 µg or less (PM10 75 µg/m³), particulate matter with a diameter of 2.5 µg or less (PM2.5 35 µg/m³), carbon monoxide (CO 10 ppm), carbon dioxide (CO₂ 1000 ppm), formaldehyde (HCHO 85 µg/m³), total airborne bacteria (800 CFU/m³), volatile organic compounds (VOCs 400 µg/m³), nitrogen oxide (NOx 0.05 ppm), fungi (500 CFU/m³), and radon (148 Bq/m³).

As seen in the results, all measured values of three indoor environments satisfied the threshold stipulated in the Indoor Air Quality Control Act by the Ministry of Environment. Meanwhile, the VOC levels in the wooden indoors (61.7–159.9 µg/m³) were lower than in the non-wood indoors (349.3 µg/m³) (Table 1). Volatile organic compounds can be classified into naturally occurring and artificially generated substances. Although wood is generally known to emit natural volatile organic compounds (VOCs) such as terpenes, the higher VOC concentrations were observed in the non-wooden indoors (0%) in this study. We hypothesize that this is attributable to the adhesives used in the finishing materials, such as the wallpaper, of the non-wooden indoors.

Currently, in Korea, the “interior wood-based remodeling” policy initiative is being promoted to encourage the use of wood in welfare facility interiors such as daycare centers and hospitals. Kim et al. evaluated 12 IAQ indicators at the wood-based remodeling project sites and reported that the measured values satisfied the international IAQ guidelines. They suggested that the establishment of wooden indoor environments contributes to the improvement of IAQ [15].

The BTEXs listed in

Table 2. Mean values of BTEXS in wooden and non-wooden environments.

Factors	Measurement Range *	Degree of Wood Coverage (%)
		0		45		90
Benzene (µg/m3)	30	4.3	7.4	6.2	3.9	5.2	4.0
Toluene (µg/m3)	1000	34.5	37.2	36.1	22.4	33.8	26.1
Ethylbenzene (µg/m3)	360	5.7	6.5	6.4	5.5	7.2	5.6
Xylene (µg/m3)	700	9.6	8.4	11.6	10.7	14.8	12.1
Styrene (µg/m3)	300	1.5	3.9	2.7	0.9	2.1	1.3

* Thresholds stipulated in the IAQ Control Act by the Korean Ministry of Environment.

are particularly hazardous VOCs that are separately regulated due to their high toxicity, and are included in the recommended IAQ standards for newly built residential buildings in Korea. The results of this study showed that the BTEX concentrations in all three environments met the recommended thresholds.

3.2. Observation of RH Changes

RH of indoor air not only affects occupants’ thermal comfort but also plays a key role in a building’s energy consumption. Wood exposed to indoor environments absorbs moisture when the relative humidity rises, and conversely releases stored moisture into the air when the humidity drops [16,17]. The moisture buffer capacity of the wall-covering materials, as well as furniture and textiles inside the buildings, define the hygroscopic inertia level of a room, which can play an important role in the reduction in RH peaks.

In terms of RH variation, the building with a 90% wood interior composition showed a monthly average that was 5.2% lower (77.3% → 72.1%) than the non-wood building during the humid summer month of August, and 10.9% higher (25.4% → 36.3%) during the dry winter month of January (Figure 3). The wood coverage at the 45% coverage one exhibited a humidity control performance comparable to that of the 90% coverage one under moderate humidity conditions (40~60%). The difference in the average relative humidity between 45% and 90% in intermediate humidity conditions was less than 1%, which was smaller than the difference in humidity between 0% and 45%, which was 2–3%. However, under low and high humidity conditions, its performance was similar to that of the 0% wood coverage. Wood is a natural hygroscopic material that is mainly composed of cellulose, hemicelluloses, and lignin. The water adsorption capacity of the amorphous part of cellulose is similar to that of hemicellulose, where an increase in crystallinity decreases the moisture adsorption capacity [18] (Hou et al., 2022). It has the characteristic of trying to balance the temperature and humidity changes in the air. When the humidity in the air is high, it absorbs moisture, and, when the humidity in the air is low, it releases moisture. As a result, it showed contrasting results in summer when the humidity is high and winter when the humidity is low. For example, hygroscopic building materials can absorb moisture in a high humidity environment and desorb it in a low humidity environment, and, thus, they can adjust the indoor relative humidity without energy consumption. It is essential that we maintain the indoor humidity environment steady within an acceptable range of 40–70% [19]. This indicates that wooden indoors contribute to controlling the indoor humidity levels close to the optimal range for human comfort, even without additional humidity control equipment.

From June 2021 to January 2022, the measurement results showed significant changes in the equilibrium moisture contents and RH values in indoor environments based on the degree of wood coverage. In the outdoor environment, EMC showed large fluctuations due to the significant seasonal changes in temperature and humidity. In contrast, indoor EMC varied according to the degree of wood coverage: in August, the 90% condition showed an approximately 2% lower EMC compared to the 0% condition, while, in January, it was approximately 2% higher. Thus, a higher degree of wood coverage was associated with increased moisture absorption, resulting in lower EMC values (0%: 13.2, 45%: 13.9, and 90%: 11.5). During winter seasons, a higher wood coverage led to more moisture release into the air, resulting in higher EMC values (0%: 5.7, 45%: 6.3, and 90%: 7.7), showing an inverse pattern to that observed in the high-humidity summer season (Figure 4). As a result, the wooden indoor environment enables the modulation of EMC by approximately 4%. Maintaining a stable EMC through wood interiors contributes not only to occupant comfort but also to a reduced risk of material deformation caused by thermal-moisture fluctuations, ultimately lowering building maintenance costs.

3.3. HRV Analysis

HRV refers to the variation in time intervals between consecutive heartbeats and is recognized as a key physiological indicator that indirectly reflects activity changes in the autonomic nervous system, including both sympathetic and parasympathetic systems [20]. In this study, the HRV of participants exposed to wooden and non-wooden environments was measured simultaneously. For a comparative analysis, the non-wooden environment and the wooden environments (45% and 90%) were grouped for evaluation. The results showed no statistically significant pre–post changes in any HRV indicators for participants exposed to the non-wooden environment.

The 45% wooden environments group showed statistically (p < 0.05) significant changes in HRV indicators such as SDNN, lnHF, and SI pre- and post-exposure (

Table 3. Changes in HRV of the participants in wooden indoor environments.

Indicators	45%			90%
	Pre M (SD)	Post M (SD)	t (p)	Pre M (SD)	Post M (SD)	t (p)
SDNN	28.85 (9.02)	41.78 (16.12)	−3.351 (0.010 **)	33.16 (14.28)	33.54 (5.39)	−0.074 (0.944)
RMSSD	25.37 (14.08)	30.54 (17.17)	−1.676 (0.132)	21.46 (10.26)	23.76 (7.74)	−1.155 (0.300)
lnLF	5.05 (1.08)	5.39 (1.23)	−0.905 (0.392)	5.31 (1.08)	5.26 (1.22)	0.107 (0.919)
lnHF	4.77 (1.42)	5.31 (1.10)	−2.776 (0.024 *)	5.02 (1.07)	5.53 (0.90)	−1.728 (0.145)
SI	2.42 (2.06)	1.50 (0.78)	(0.05 *)	2.14 (1.20)	1.39 (0.70)	1.491 (0.196)

* p < 0.05, ** p < 0.01; M: Mean, SD: Standard Deviation.

). SDNN increased from 28.85 ± 9.02 pre-exposure to 41.78 ± 16.12 after exposure. An increase in SDNN is known to be associated with enhanced parasympathetic nervous system activity, reflecting a physiological state of relaxation [21]. lnHF increased from 4.77 ± 1.42 to 5.31 ± 1.10 following exposure to the wooden environment. In the frequency domain analysis, the HF component (0.15–0.4 Hz) reflects parasympathetic nervous system activity and is generally associated with relaxation or rest states [22]. SI is a stress index associated with the pressure exerted on the heart. In the wooden environment, SI decreased from 2.31 ± 1.77 to 1.45 ± 0.75 after exposure. The observed increase in SDNN and lnHF values, along with a decrease in SI, suggests enhanced parasympathetic nervous system activity and a corresponding relaxation response in the 45% wooden environment. Several previous studies have reported changes in HRV among participants in wood-based interior environments. Kotradyova et al. measured the HRV of patients by installing a pine wood wall and arch wood seats in the waiting area of the hospital. The LF/HF ratio decreased by 26.4% in a wood-finished waiting room, indicating autonomic stabilization and relaxation effects [22]. The LF/HF ratio is used to assess the balance between sympathetic and parasympathetic nervous activity, with lower values indicating a more relaxed state. Lipovac and Burnard reported that SDNN, RMSSD, and LF/HF values tended to decrease after contact with wood. They suggest that contact with wood promotes parasympathetic activation and overall relaxation, shifting the autonomic balance toward parasympathetic dominance [23]. However, parasympathetic dominance in wooden environments does not always appear consistently, and some studies have reported opposing results. Miyazaki et al. observed a gradual increase in heart rate during short-term exposure to wooden environments, attributing this to a “wow effect” associated with initial wood exposure [24].

On the other hand, although variations in the indicators were identified under the a 90% wooden environment, no statistically significant differences were found. The absence of significant changes in HRV under both the 0% and 90% wood coverage conditions may be attributed to different underlying mechanisms. In the 0% condition, the absence of natural material stimuli fails to induce physiological relaxation. Conversely, in the 90% condition, a saturation effect or psychological adaptation may occur, where excessive visual exposure to wood no longer enhances and may even diminish restorative responses. This phenomenon has been documented in previous studies on restorative environments, which indicate that including additional natural elements beyond an optimal level does not yield further benefits [25]. Tsunetsugu et al. created three test rooms with wood coverage rates of 0%, 45%, and 90%, and randomly exposed 15 participants to each room for 90 s. They found that the participants’ physiological relaxation effects did not necessary increase with a higher wood coverage. They suggested that the observed increase in heat rate was not interpreted as stress, but rather as a physiological response reflecting positive emotional states similar to those experienced in natural environments [26]. Several studies have reported that moderate levels of wood coverage in indoor environments may induce physiological relaxation effects. Visual stimulation by the 30% wood-covered room was found to reduce diastolic blood pressure and pulse rate, indicating a physiological relaxation effect [27]. It remains to be determined whether the results observed in the 90% wooden environments are due to a saturation effect or to positive psychological responses; this issue will be explored in further study.

Some researchers have pointed out that there are limitations in interpreting physiological changes using only a single or a few indicators. For this reason, there is a growing need to integrate a wider range of physiological indicators such as HRV, blood pressure, skin conductance, skin temperature, and salivary cortisol levels. Moreover, several studies have combined physiological indicators with psychological tools like POMS (Profile of Mood States) analysis to produce more comprehensive results [28].

However, since this study limited its physiological evaluation to a single indicator, utilized a small sample size, and had a short exposure duration, it has limitations in providing a comprehensive interpretation of the effects of wooden environments on the physiological response. In addition, the relatively short exposure period may have constrained the observation of longer-term autonomic adjustments. Further studies should consider longer exposure durations or repeated visits and include a broader range of physiological indicators to achieve more robust and comprehensive analyses of the impact of wooden environments.

4. Conclusions

This study examined how varying proportions of wood-based interiors affect the physicochemical properties of indoor environments and the physiological responses of the occupants. The results demonstrated that the use of hygroscopic materials like wood indoors contributes to maintaining a comfortable environment by mitigating abrupt humidity fluctuations without forced ventilation or additional energy input. As a result of observing the change in indoor humidity for eight months, the higher the proportion of the wood interior, the more likely it was that a high indoor humidity would be maintained through moisture proofing in relatively low-humidity indoor conditions and a low indoor humidity would be maintained through moisture absorption in high-humidity indoor conditions. In terms of RH variation, the building with a 90% wood interior composition showed that, monthly, the humidity was maintained 5.2% lower in the summer and 10.9% higher in the winter compared to the 0% condition.

Physiological relaxation effects were found to be statistically significant only under the 45% wood coverage condition. Based on this result, we suggest that a moderate wood coverage can be applied in residential indoor environments to achieve a balance of physiological benefits. Moreover, these finding provide practical design guidance for architects or interior designers. However, since humidity control performance tends to improve proportionally with increasing wood coverage, further research is needed in order to identify an optimal range—between 45% and 90%—that can simultaneously satisfy both physiological relaxation and humidity regulation requirements. Moreover, since this study relied on a single non-specific physiological indicator, further experiments are planned in order to enable a more comprehensive understanding of how wooden environments affect the human body. In addition, the study is limited by the relatively small size and the controlled experimental setting, which may restrict the generalizability of the finding. Future research involving larger and more diverse participant groups and a wider variety of indoor environments is needed in order to validate and extend these results.