Annual assessment of the active plant wall on indoor environment from summer, transition season and winter

Abstract

Plants not only enhance the aesthetic appeal of indoor spaces, but also contribute to the regulation of the indoor thermal environment. In this study, an active plant wall (APW) integrated with air-conditioning system to investigate its influence on the indoor thermal conditions, as well as examine participants′ skin temperature and subjective perceptions. In transition season and winter, the results demonstrated that APW led to a decrease in indoor temperature by 1.35℃ and 1.03℃, respectively. The mean relative humidity (RH) enlarged by 11.6% and 20.76%. In summer, APW caused a rise of 0.18℃ in indoor temperature and led to a decline of 2.7% in RH. Throughout the year, APW controlled air speed at 0.2–0.3 m/s, reducing the CO₂ concentration by 42.35ppm, 43.83ppm and 46.83ppm, respectively. APW brought the mean skin temperature (MST) in Room B closer to neutral skin temperature of 33.2℃ throughout the year. Additionally, APW raised overall air fresh and thermal comfortable levels throughout the year to around “Fresh (+ 1)” and “Slightly comfortable (+ 1)”, respectively. The findings suggested that APW can enhance indoor air quality and thermal comfortable levels throughout the year.

Introduction

Energy and the environment are two prominent global challenges faced by humanity^1–4. As urban areas continue to grow in density across the globe, the area of green spaces in cities is gradually decreasing, leading to a worsening of environmental pollution issues, including challenges such as urban thermal island effect and high energy consumption in high-density buildings^5–7. Some studies suggested that indoor pollution is more severe than outdoor pollution⁸. The health, well-being, and productivity of residents were negatively impacted by indoor air pollution and inadequate thermal comfortable⁹, especially considering that urban dwellers spend over 80% of their time indoors¹⁰. Therefore, improving the quality and comfort of the indoor environment has attracted widespread attention, which has further stimulated many scholars to deeply explore the impact of the indoor thermal environment on the physical health of occupants and the assessment of thermal comfort, prompting them to carry out a large number of experimental research works¹¹. Numerous studies have shown that introducing indoor plants has positive effects on human health¹². Additionally, the active plant wall (APW) not only increase indoor greening area, but also serve as an important element of environmentally friendly sustainable design in green building architecture.

Plants in an APW can regulate indoor temperature and humidity levels through photosynthesis and transpiration, thereby improving thermal comfortable in indoor environments¹³. Mangone et al.¹⁴ evaluated the thermal comfort of 67 office workers. The experiments were conducted for one month each in summer, transition season and winter to explore the effect of indoor plants on thermal comfort in different seasons. The results showed that adding a large number of plants in office buildings can reduce the set temperature values, thereby lowering the set temperature values in summer and winter, and thus reducing the energy consumption and carbon emission rate of buildings. Pérez-Urrestarazu et al.¹⁵ preliminarily evaluated the effect of APW in indoor air conditioning. The results showed that at different distances near APW, the extent of temperature reduction presents significant changes, ranging from 0.8℃ to 4.8℃. Especially when the initial state of the room is relatively dry and warm, the cooling effect is particularly significant. Fernández-Cañero et al.¹⁶ established the APW system to explore the regulatory potential of APW for indoor temperature and RH under warm climate conditions. The results showed that APW has a cooling effect, reducing temperature by an average of 4℃, and the maximum reduction can reach 6℃ under warmer conditions. The air humidity level near APW is higher, which helped increase the overall indoor humidity.

The placement of indoor plants mitigated the requirement for ventilation and effectively decreased CO₂ concentration¹⁷. Tudiwer et al.¹⁸ conducted a controlled experiment to continuously monitor the CO₂ concentration in two classrooms. The study showed that the classroom with APW experienced a significant reduction of 3.5% in CO₂ concentration, compared to indoor environments with the same initial CO₂ levels. Pegas et al.¹⁹ explored the ability of plants to improve indoor air quality in schools. They introduced three common plants into classrooms and conducted a nine-week monitoring. The results showed that after hanging the plants, the average CO₂ concentration was significantly reduced from 2004ppm to 1121ppm. Shao et al.²⁰ studied the impact of vertical farming on indoor CO₂ concentration and found that vertically grown vegetables have a higher net photosynthetic rate and absorb CO₂ at a rate approximately 9.2 times greater than shade-loving landscape plants. When accommodating 1–3 people in a 30m² office, plants can effectively reduce indoor CO₂ concentrations by approximately 25.7-34.3%. Zhang et al.²¹ conducted an experimental study on the removal efficiency of APW for CO₂ concentration in an office environment and precisely simulated the energy-saving effect of APW on fresh air under different scenarios using EnergyPlus software. The results indicated that implementing plant wall in a 30m² office space capable of accommodating two or three people could reduce fresh air demand by 13.9-38.5% and fresh air energy consumption by 11.2-28.2%, respectively. Dominici et al.²² selected two plants (Chlorophytum comosum and Spathiphyllum wallisii), for the experiment, and set different light intensities and angles to evaluate the effects of these conditions on the CO₂ removal efficiency of the plants. Studies showed that optimizing light conditions, especially increasing light intensity and adjusting light angles, can significantly improve the CO₂ removal efficiency of plants.

The benefits of plants extend beyond enhancing indoor air quality and increasing indoor thermal comfortable. Numerous research studies have demonstrated that being in natural surroundings has beneficial impacts on individuals’ mental and physical well-being²³. Berger et al.¹² shown that the presence of plants not only affects participants’ perception of indoor air quality and humidity, but also significantly impacts on their reactions, aesthetic preferences, and the effect of plants on subjective well-being. Oh et al.²⁴ measured the psychophysiological effects of these visual stimuli on students using electroencephalogram (EEG) technology through four visual stimuli (actual plants, artificial plants, plant photos and no plants). The research results showed that, compared with artificial plants, plant photos or environments without plants, actual plants can significantly improve the attention level of primary school students and promote psychological relaxation. In addition, the visual stimulation of actual plants performs better in inducing positive emotions, such as enhancing students’ sense of comfort and naturalness. Meanwhile, Van Den Berg et al.²⁵ evaluated the impact of the APW on the cognitive performance, well-being and classroom evaluation of primary school students through attention tests. The results show that children in classrooms equipped with APW perform better in selective attention tests. In addition, the APW has also positively influenced children’s evaluation of the classroom, making them more satisfied with the classroom environment. Hähn et al.²⁶ introduced plants in the workspace and rest area and conducted a questionnaire survey to evaluate the perceived changes of participants regarding health, well-being and performance indicators. The results showed that introducing plants into the office has significantly improved employees’ perceived attention, creativity and productivity. And when plants were removed from the office, there were significant negative changes in employees’ perceived attention, productivity, stress and efficiency. Ma et al.¹⁰ recruited 144 college students to study the effects of green plant doses on emotional and physiological characteristics by testing neurobehavioral responses, measuring physiological indicators, monitoring electroencephalogram signals and conducting questionnaires. It is further pointed out that there is a correlation between the increase in the size of the plant wall and the enhancement of emotional relaxation and pleasure among individuals.

In summary, the APW effectively regulates the indoor temperature and RH through photosynthesis and transpiration, reduces the concentration of CO₂, and decreases the demand for ventilation, thereby enhancing the indoor thermal comfort and air quality. Moreover, plants significantly enhance people’s attention, psychological relaxation, and positive emotions, benefiting mental health. Introducing plants into working and learning environment boosts employees’ and students’ attention, creativity, and productivity while improving their satisfaction with the environment. Additionally, optimizing lighting conditions further enhances plants’ CO₂ removal efficiency and strengthens APW’s environmental regulation capacity.

Despite existing studies on APW’s indoor environment across different seasons, current research still has some limitations. Firstly, most studies focus on summer or winter, with relatively few focusing on the transition season. However, changes in the indoor environment during the transition season are equally important for the comfort and health of residents. Secondly, existing studies focus on the changes of indoor environmental parameters in a single season, but lack systematic studies on the comprehensive impact of indoor environmental parameters from the perspective of the whole year. In this study, not only a single season was focused on, but also a systematic analysis of indoor environmental parameters was conducted from an annual perspective. In addition, combined with the survey method, a comprehensive evaluation is conducted on the subjective perception and expectations of the indoor environment.

Methodologies

Experimental system description

As shown in Fig. 1, the APW was mainly composed of air-conditioning system, a sealed box body, vertical planting slots, and plants (Epipremnum aureum). Its dimensions were 1000 mm (Length) × 300 mm (Width) × 2000 mm (Height). The growth medium was sterile nutrient soil, mainly composed of coconut husk, garden soil, vermiculite, and perlite. This soil is loose, breathable, and has strong drainage performance, making it more suitable for plant growth. Ding et al.¹³ demonstrated Epipremnum aureum was the optimal choice indoor plant for cultivation, regardless of lighting conditions. Moreover, it exhibited adaptability to both dry and wet indoor environments. Therefore, in summer, soil moisture was regulated at 10-20% using an automated watering system to facilitate absorption of moisture from the indoor air. In winter, an automated watering system should maintain soil moisture levels between 50% and 70% to supplement the indoor air with the required moisture. In transition season, soil moisture was controlled between 30% and 50% through an automatic watering system. This not only effectively promoting plants’ healthy growth in response to seasonal changes while simultaneously regulating indoor humidity levels. In summer and winter, the room was supplied with air through an air conditioning system set at 25℃ for cooling and 20℃ for heating, respectively. In transition season, mechanical ventilation was utilized.

Fig. 1.

(a) Schematic of APW system and (b) a real shot of APW.

The sparsity of plants does affect their transpiration, photosynthesis and regulation of indoor environment, thus affecting the accuracy of experimental results. In order to ensure the consistency of plant density in different seasons, firstly, the position of plants is regularly adjusted to maintain an even distribution. Secondly, plants are regularly pruned to remove wilted and excessively long branches and leaves to maintain the healthy growth of plants. Thirdly, new plants will also be added and replaced in time to ensure the integrity and function of the APW.

Description of experimental platform

The experiment took place in the laboratory of Qingdao University of Technology. As shown in Fig. 2, two identical laboratories with the same south-facing layout and dimensions were selected, with room sizes of 3000 mm (Length) × 3000 mm (Width) × 2800 mm (Height). The laboratories were constructed using steel frame structures, that feature walls and roof structures composed of 2.5 mm steel layers, 90 mm rock wool insulation material, and an additional layer of 2.5 mm steel. In the experiment, Room B with an APW was designated as the experimental group, while Room A without an APW serves as the control group. Apart from the presence or absence of an APW, all other conditions in both rooms remain identical. Hence, the disparities observed between Room A and B can be attributed solely to the presence or absence of an APW.

Fig. 2.

Schematic layout of laboratory.

Indoor environment parameter test description

This study employed both objective measurement methods and subjective questionnaire surveys to investigate the impact of a composite air conditioning system with an APW on indoor thermal environment and comfortable throughout the year. The indoor environmental parameters of Room A and B were continuously monitored in summer, transition season and winter in this study. These parameters included indoor air temperature, RH, air speed and CO₂ concentration. The round table in the middle of the room held all the instruments, and data was collected from them at 5-minute intervals. The experiment periods were from July 24th to July 31st, 2023 (summer), September 17th to September 30th, 2023 (transition season), and December 26th, 2023 to January 2nd, 2024 (winter), respectively. During the experiment, a time frame from 8:30 − 17:30 was selected to align with typical office hours. The indoor environment’s air temperature, RH, air speed, and CO₂ concentration were assessed as four environmental factors impacting human thermal comfortable. These variables were measured and analyzed during the study. Additionally, to maintain experimental data integrity and account for the airtightness of Room A and B, both rooms were kept sealed during the experiment.

Description of the questionnaire test method

The study revealed that there are individual differences among people, and even in the same environment, individuals will have varying reactions to hot environments. Considering these individual differences, questionnaires were essential and indispensable for evaluating the indoor thermal conditions in academic research. Through questionnaire survey, we can understand participants’ satisfaction with environmental parameters such as temperature, humidity, and air velocity, which can help us quantify these individual differences, so as to evaluate the indoor thermal environment more comprehensively. By conducting questionnaire surveys, researchers can collect subjective feelings and feedback from participants regarding thermal environments to understand their comfortable level, satisfaction level, and possible discomfortable. The participants recruited for this study included 64 participants in summer, 60 participants in transition season, and 64 participants in winter. The participants across these three conditions are essentially the same with some individual differences, all of whom are healthy students at school. The basic information for participants was shown in Table 1.

Table 1.

The basic characteristics of participants.

Variables	Summer		Transition season		Winter
	Number	Percentage (%)	Number	Percentage (%)	Number	Percentage (%)
Gender
Male	32	50	30	50	32	50
Female	32	50	30	50	32	50
Age
20–25	48	75	46	76.7	48	75
26–30	16	25	14	23.3	16	25
Weight status (BMI)
BMI < 18.5	13	20.3	11	18.3	14	21.9
18.5 ≤ BMI ≤ 23.9	40	62.5	37	61.7	38	59.4
BMI > 23.9	11	17.2	12	20	12	18.7

The questionnaire survey was conducted from August 1, 2023 to August 12, 2023 (summer), October 5, 2023 to October 16, 2023 (transition season), and January 4, 2024 to January 15, 2024 (winter), respectively. During the experiment, participants real-time perceptions of the indoor thermal conditions were collected via questionnaire surveys. Simultaneously, the skin temperature of eight parts of the participants, including forehead, chest, back, upper arm, forearm, hand, thigh and shank, was monitored by skin temperature monitors. The instrument was programmed to record data at every 5 s. The mean skin temperature (MST) was calculated by the formula²⁷:

T_sk = 0.07T₁ + 0.175T₂ + 0.175T₃ + 0.07T₄ + 0.07T₅ + 0.05T₆ + 0.19T₇ + 0.2T₈ (1).

In the formula, T_sk denote MST, in ℃, and the body parts represented by T₁, T₂, T₃, T₄, T₅, T₆, T₇, and T₈ are forehead, chest, back, upper arm, fore-arm, hand, thigh, and shank, respectively. In the course of the experiment, we utilized breathable medical tape to affix the surface temperature probe to the skin with the aim of reducing any local discomfortable caused by the tape. The range of measurement and precision of the devices in this experiment were detailed in Table 2.

Table 2.

The range of measurement and precision of the devices.

Instruments	Type	Environmental parameter	Range	Accuracy
Indoor heat environment tester	JT-IAQ-50	Air temperature	− 20–120 ℃	± 0.3℃
		RH	0–100% RH	± 2%RH
		Air speed	0.05–2 m/s	± 0.03 m/s
Indoor environmental monitor	YELI-SN	CO2 concentration	400-5000ppm	± 2%
Skin temperature monitor	ME103(0.1)3977V3B3000	Skin temperature	0–60℃	± 0.1℃

The experiment required a total of 40 min, with each group consisting of two participants. During this period, the laboratory was only accessible to the researchers and participants, with no one else permitted to enter or exit. The experimental procedures conformed to the Declaration of Helsinki and were approved by the ethics committee of Qingdao University of Technology (approval No. QUT-HEC-2024023) and it was confirmed that informed consent was obtained from all participants. Figure 3 illustrated the flowchart of the questionnaire survey process. The experimental procedure included the following detailed steps:

According to Table 2, Experimental system uncertainty was determined by the uncertainty analysis method and the total uncertainty of the experimental system was calculated as 5.25% from Eq. (1).

1

• To minimize interference from the outdoor environment, participants were asked to rest in the lounge for 10 min. During this time, participants were allowed to randomly choose either Room A or Room B. Then, they were briefed on relevant experimental precautions and had some skin temperature monitors attached to their respective body parts.

• According to their choice, participants entered Room A (or Room B) and sat quietly for 30 min. Throughout this period, participants’ skin temperatures were monitored continuously, and they filled out a survey questionnaire every 10 min. During the survey process, participants may read or use their mobile phones as desired.

Fig. 3.

Schematic diagram of the questionnaire survey process.

Figure 4 gave the indoor environmental survey questionnaire utilized in this experiment. The questionnaire encompassed three main aspects: participants’ basic information, evaluation of the indoor environment, and assessment of APW acceptability. The evaluation of the indoor environment included considerations such as thermal sensation, wet sensation, airflow sensation, air fresh, and thermal comfortable level. As indicated in Table 3, in accordance with ASHRAE 55-2023 standards, thermal sensation, wet sensation, and thermal comfortable were rated on a 7-point scale. Indoor air freshness and acceptability were rated on a 5-point scale. Airflow sensation was rated on a 4-point scale.

Fig. 4.

Comparison of indoor air temperature in Room A and B throughout the year.

Table 3.

The scale of the questionnaire parameters.

Parameters	Thermal sensation	Wet sensation	Airflow	Air freshness	Thermal comfortable level	Acceptance level
− 3	Cold	Very wet	−	-	Very uncomfortable	-
− 2	Cool	Wet	−	Very stale	Uncomfortable	Very unwilling
− 1	Slightly cool	Slightly wet	−	Stale	Slightly uncomfortable	Unwilling
0	Moderate	Moderate	No airflow	Moderate	Moderate	Moderate
+ 1	Slightly warm	Slightly dry	Slight airflow	Fresh	Slightly comfortable	Willing
+ 2	Warn	Dry	Obvious airflow	Very fresh	Comfortable	Very willing
+ 3	Hot	Very dry	Strong airflow	-	Very comfortable	-

Data statistical analysis

Descriptive statistical analysis was conducted to initially understand the basic characteristics of environmental parameters, physiological parameters and questionnaire data. Physiological responses and subjective responses are expressed as mean ± standard deviation (mean ± SD)²⁸.

To ensure the accuracy and consistency of the data, Microsoft Excel is used for classification and preprocessing. Statistical analysis was conducted using Origin 2022 software to evaluate the influence of APW on the indoor thermal environment and human thermal sensation. In the study, the paired sample T-test was used to compare the differences in indicators such as skin temperature and thermal sensation voting between Room B (with APW) and room A (without APW) at different time points. The significance test is two-tailed, and the p value is used to determine the statistical significance of the result. The significance levels were set as “p < 0.05”, “p < 0.01” and “p < 0.001”.

Results and discussion

Analysis of indoor environment

Indoor temperature played a crucial role as an indicator and influencing factor in the assessment of indoor thermal conditions and comfortable. Figure 4 depicted the distribution of indoor temperatures in summer, transition season and winter, accompanied by a statistical significance analysis. After statistical analysis, there was a significant difference in temperature between Room A and B in three seasons (P < 0.01). Overall, when the indoor temperature is maintained at 20℃-25℃, it is a more comfortable state. In summer, the temperature range of Room A and B was 22.5℃-26.5℃ and 23.7℃-26℃, respectively. The mean temperature in Room A measured 24.48℃, whereas the mean temperature in Room B was recorded at 24.66℃, indicated that the mean temperature of Room A was slightly lower than Room B. In transition season, Room A had a temperature range of 22.5℃-32.5℃, while Room B had a temperature range of 21.5℃-30℃. The mean temperature in Room A measured 27.19℃, while the mean temperature in Room B was recorded at 25.74℃, indicated that the mean temperature in Room B was 1.35℃ less than Room A. In winter, the temperature range of Room A was 18℃-22℃, while Room B had a temperature range of 15.5℃-22.5℃, respectively. The mean temperature in Room A measured 20.23℃, whereas the mean temperature in Room B was recorded at 19.20℃, indicated that the temperature in Room B was lower on mean compared to Room A by 1.03℃. The findings suggested that APW can effectively lower indoor air temperature in winter and transition season by removing thermal from the air through transpiration. Conversely, in summer, the mean temperature in Room A was 0.18℃ higher than Room B, attributed to the thermal inertia of the nutrient soil in APW which regulated indoor air temperature. Plants release water and absorb heat through transpiration, thereby reducing the temperature of the surrounding environment. The soil and plants in the APW themselves have a certain thermal inertia, which can absorb and store some heat, reduce rapid temperature changes, and make the indoor temperature more stable. This indicated that APW has diverse effects on indoor temperature throughout the year. It not only reduced thermal loss through the insulation effect of the soil, but also regulated indoor temperature through transpiration, thereby providing residents with a more comfortable indoor environment.

The thermal comfortable in indoor environment was not only influenced by temperature, but also by indoor RH. Figure 5 gave the distribution of indoor RH in summer, transition season and winter, and conducted statistical significance analysis. After statistical analysis, there was a significant difference in RH between Room A and B in three seasons (P < 0.01 or P < 0.001). Overall consideration, when the air conditioner is running, the indoor relative humidity is more comfortable in the range of 40-70%. As shown, in summer, the RH of Room A and B ranged of 67.5-90% and 62.5-85%, respectively. Moreover, the mean RH of Room A was 79.77% throughout the duration of the experiment, while that of Room B was 77.07%. The humidity level in Room B was significantly lower compared to Room A. In transition season, the RH of Room A and B ranged of 35-70% and 50-75%, respectively. The humidity level in Room B was significantly higher compared to Room A. The mean indoor RH in Room A was 51.32%, while the mean RH in Room B was 62.92%. The mean RH in Room B was 19.1% greater than Room (A) In winter, the indoor RH in Room A and B ranged of 27.5-42.5% and 50-62.5%, respectively. The mean indoor RH was measured at 34.92% in Room A, while it reached 55.68% in Room (B) The RH in Room B was greater than Room A by 20.76%. The findings showed that the RH in Room B was significantly greater than Room A in winter and transition season. This was due to the transpiration of APW, which increased the moisture content in the air. However, in summer, contrary to what was observed in winter and transition season, this could be explained by Qingdao’s high atmospheric humidity levels. The vigorous vitality of plants can be utilized to reduce watering frequency and amount, allowing indoor air moisture to naturally absorb and alleviate indoor humidity levels. This indicates that the transpiration of plants can be utilized to increase indoor humidity. By regulating the indoor temperature and humidity, APW can enhance thermal comfort. Especially in extreme seasons (such as high temperatures in summer and dryness in winter), the effect of APW is more obvious. This also demonstrated that APW had potential for regulating indoor temperature and humidity, thereby improving indoor comfortable.

Fig. 5.

Comparison of indoor RH in Room A and B throughout the year.

The evaluation of indoor thermal comfortable not only considers indoor temperature and RH, but also requires the inclusion of indoor air speed as a crucial indicator. The distribution of indoor air speed in Room A and B in summer, transition season and winter was compared in Fig. 6, with a subsequent analysis conducted to determine the statistical significance. After statistical analysis, there was a significant difference in air speed between Room A and B in three seasons (P < 0.01). According to Indoor Air Quality Standard (GB/T 18883 − 2002), the indoor air speed range was recommended to be 0.20 m/s-0.30 m/s²⁹. The air speed fluctuation range of Room B was more consistently distributed and less variable, fluctuating around 0.3 m/s. The air speed fluctuation range in Room A was higher. In summer, transition season and winter, mean air speed in Room B were 0.32 m/s, 0.37 m/s and 0.29 m/s, respectively. The mean air speed in Room A were 1.05 m/s, 0.86 m/s and 0.64 m/s, respectively. By conducting a comparative analysis of indoor air speed throughout the year, it can be inferred that the APW has a certain adjustment effect on indoor air speed, which can make the air speed more uniform, so as to improve indoor thermal comfort to a certain extent. The specific reason is that the porous structure of APW and the shading effect of plants can improve the air distribution of the air conditioning system, making the air speed more uniform, and avoiding local excessive cold or heat. However, the ability of the APW to adjust is limited, and the final state of the indoor air speed is also dominated by the air conditioning system setting and operation mode.

Fig. 6.

Comparison of indoor air speed in Room A and B throughout the year.

The concentration of CO₂ is a significant factor that impacts the quality of indoor air, with high concentrations having adverse effects on people’s health and leading to the presence of sick building syndrome. However, introducing an APW, it is possible to utilize plants to absorb CO₂ and release O₂, thereby enhancing indoor photosynthesis of plants and improving indoor air quality. Figure 7 compared the distribution of CO₂ concentration in summer, transition season and winter, and conducted statistical significance analysis. After statistical analysis, there was a significant difference in CO₂ concentration between Room A and B in three seasons (P < 0.01 or P < 0.001). Generally, an indoor CO₂ concentration of 450ppm is a more desirable level, indicating good air quality. 450 ppm CO₂ concentration contributes to the comfort and health of the living environment. In summer, the CO₂ levels in Room A ranged from 465ppm to 512ppm, with the mean CO₂ concentration of 486.19ppm. CO₂ concentration range in Room B was 420ppm-470ppm, with the mean CO₂ concentration of 443.84ppm, which was 42.35ppm lower than Room A. In transition season, the CO₂ levels in Room A ranged from 470ppm to 550ppm, with the mean CO₂ concentration of 508.81ppm. The CO₂ levels in Room B ranged from 430ppm to 510ppm, with the mean CO₂ concentration of 464.98ppm, which was 43.83ppm lower than Room A. In winter, the CO₂ levels in Room A ranged from 490ppm to 560ppm, with the mean CO₂ concentration of 523.69ppm. The CO₂ levels in Room B ranged from 450ppm to 510ppm, with the mean CO₂ concentration of 476.83ppm, which was 46.83ppm lower than Room A. The experimental results indicated that, throughout the year, the concentration of CO₂ in Room B was notably lower compared to Room A. This was attributed to the APW can reduce indoor CO₂ concentration by facilitating photosynthesis. The reason why APW can reduce the indoor CO₂ concentration is that plants absorb CO₂ and release O₂ through photosynthesis, and this process reduces the indoor CO₂ concentration. The density and layout of plants can also affect their effect on regulating indoor air quality. To a certain extent, increasing the plant density and optimizing the layout can enhance the photosynthetic efficiency of the plant wall and further reduce the indoor CO₂ concentration. In addition, the porous structure of APW and the shading effect of plants can improve indoor air speed, making the airflow more uniform. This uniform air speed distribution helps to reduce the accumulation of local CO₂ concentration, thereby lowering the overall indoor CO₂ concentration.

Fig. 7.

Comparison of indoor CO₂ concentration in Room A and B throughout the year.

Analysis of skin temperatures

Skin temperature refers to the temperature of the body’s surface and is usually affected by factors such as the ambient temperature. The measurement of skin temperature is a widely used method for monitoring physiological parameters, allowing for the evaluation of individuals’ thermal adaptability and comfortable in various environmental conditions. Figure 8 compared the MST of human body in Room A and B in summer, transition season and winter, and conducted statistical significance analysis. After statistical analysis, there was a significant difference in MST between Room A and B in three seasons (P < 0.01). In summer, the MST for participants was 31.91℃ in Room A and 32.21℃ in Room B. In transition season, the MST for participants was 33.33℃ in Room A and 33.13℃ in Room B. In winter, the MST for participants was 33.43℃ in Room A and 33.19℃ in Room B. As shown, the MST of participants in Room B closely approximated the neutral skin temperature of 33.2℃. The red dotted line in the figure represents the neutral skin temperature of 33.2℃, which refers to the specific skin temperature when thermal sensation is perceived as neutral³⁰.

Fig. 8.

Comparison of MST of human body in Room A and B throughout the year.

The most important factors affecting the skin temperature are indoor temperature and air speed. Take summer as an example, when the air conditioning equipment is turned on, the temperature of room A may be uneven due to the cold air directly blown by the air conditioning, which may lead to a decrease in local skin temperature. In room B, due to the shielding effect of APW plants, the air speed is adjusted to make the air flow to the subjects more uniform. This uniform airflow distribution avoids local supercooling or overheating, thereby improving overall comfort. This results further demonstrates that the presence of APW can enhance indoor thermal conditions when combined with air conditioning system.

Analysis of survey questionnaires

The subjective questionnaire enables researchers to gain real-time insights into participants’ perceptions and experiences with APW. Figure 9 compared the thermal sensation voting of participants in Room A and B, and the overall thermal sensation voting in summer, transition season and winter. After statistical analysis, there was a significant difference in indoor thermal sensation voting between Room A and B in three seasons (P < 0.01 or P < 0.001). In summer, the overall thermal sensation voting for Room A was − 0.35 (near to “Moderate (0)”) and the median was 0 (“Moderate (0)”), while the overall thermal sensation voting for Room B was − 0.16 (near to “Moderate (0)”) and the median was 0 (“Moderate (0)”). In transition season, the overall thermal sensation voting for Room A was 0.68 (ranging from “Moderate (0)” to “slightly warm (+ 1)”) and the median was + 1 (“Slightly warm (+ 1)”), while the overall thermal sensation voting for Room B was − 0.2 (near to “Moderate (0)”) and the median was 0 (“Moderate (0)”). In winter, the overall thermal sensation voting for Room A was 1.13 (near to “Slightly warm (+ 1)”) and the median was + 1 (“Slightly warm (+ 1)”), while the overall thermal sensation voting for Room B was 0.23 (near to “Moderate (0)”) and the median was 0 (“Moderate (0)”). The results indicated that, during a half-hour experimental process, the thermal sensation of Room B was closer to “Moderate (0)” throughout the year. This suggested that the thermal environment of Room B better meets the participants’ needs, demonstrating the potential of APW to regulate the thermal environment.

Fig. 9.

Comparison of (a) summer, (b) transition season, (c) winter indoor thermal sensation voting and (d) overall thermal sensation voting throughout the year in Room A and B.

Figure 10 compared the wet sensation voting and overall wet sensation voting of Room A and B in summer, transition season and winter. After statistical analysis, there was a significant difference in wet sensation voting between Room A and B in three seasons (P < 0.01 or P < 0.05). In summer, the overall wet sensation voting for Room A was − 1.09 (near to “Slightly wet (-1)”) and the median was − 1 (“Slightly wet (-1)”), while the overall wet sensation voting for Room B was − 0.22 (near to “Moderate (0)”) and the median was 0 (“Moderate (0)”). In transition season, the overall wet sensation voting for Room A was 0.57 (ranging from “Moderate (0)” to “Slightly dry (+ 1)”) and the median was + 1 (“Slightly dry (+ 1)”), while the overall wet sensation voting for Room B was − 0.13 (near to “Moderate (0)”) and the median was 0 (“Moderate (0)”). In winter, the overall wet sensation voting for Room A was 1.02 (near to “Slightly dry (+ 1)”) and the median was + 1 (“Slightly dry (+ 1)”), while the overall wet sensation voting for Room B was 0.17 (near to “Moderate (0)”) and the median was 0 (“Moderate (0)”). The results indicated that throughout the year, the wet sensation voting of Room B was closer to “Moderate (0)” during a half-hour experimental process. The voting results for the wet perception in summer were different from those in transition season and winter, as Qingdao experiences higher wet levels in summer, which is intended to alleviate excessive moisture. In contrast, humidification was achieved through the transpiration of plants in winter and transition season. This also indicated the potential of APW to regulate the wet environment, which were in line with the RH outcomes depicted in Fig. 5.

Fig. 10.

Comparison of (a) summer, (b) transition season, (c) winter indoor wet sensation voting and (d) overall wet sensation voting throughout the year in Room A and B.

The perception of airflow is also an evaluative parameter for expressing individuals’ thermal comfortable in indoor environment. Figure 11 compared airflow sensation voting and overall airflow sensation voting of Room A and B in summer, transition season and winter. After statistical analysis, there was a significant difference in airflow sensation voting between Room A and B in three seasons (P < 0.01 or P < 0.05 or P < 0.1). In summer, the overall airflow sensation in Room A was 0.97 (near to “Slightly airflow (+ 1)”) and the median was + 1 (“Slightly airflow (+ 1)”), while the overall airflow sensation in Room B was 0.63 (ranging from “No airflow (0)” to “Slightly airflow (+ 1)”) and the median was + 1 (“Slightly airflow (+ 1)”). In transition season, the overall airflow sensation in Room A was 1.13 (slightly above the level of than “Slightly airflow (+ 1)”) and the median was + 1 (“Slightly airflow (+ 1)”), while the overall airflow sensation in Room B was 0.73 (ranging from “No airflow (0)” to “Slightly airflow (+ 1)”) and the median was + 1 (“Slightly airflow (+ 1)”). In winter, the overall airflow sensation in Room A was 1.52 (ranging from “Slightly airflow (+ 1)” to “Obvious airflow (+ 2)”) and the median was + 2 (“Obvious airflow (+ 1)”), while the overall airflow sensation in Room B was 1.12 (near to “Slightly airflow (+ 1)”) and the median was + 1 (“Slightly airflow (+ 1)”). The result was consistent with Fig. 7, indicated that the APW evenly disperses the air from the air conditioning system outlet, achieving an optimal air speed range to minimize the discomfortable and alleviate thermal discomfortable.

Fig. 11.

Comparison of (a) summer, (b) transition season, (c) winter indoor airflow sensation voting and (d) overall airflow sensation voting during different seasons in Room A and B.

Figure 12 gave indoor air fresh voting and overall air fresh voting of Room A and B in summer, transition season and winter. After statistical analysis, there was a significant difference in air fresh voting between Room A and B in three seasons (P < 0.01). The air fresh assessment for Room B consistently exceeded Room A throughout the year. The evaluation results for Room B exhibited an upward trend as the duration of stay in Room A and B increased. However, the evaluation result of Room A exhibited a downward trajectory. In summer, the overall air fresh voting in Room A was − 0.19 (lower than “Moderate (0)”) and the median was 0 (“Moderate (0)”), while the overall air fresh voting in Room B was 1.05 (slightly above the level of “Fresh (+ 1)”) and the median was + 1 (“Fresh (+ 1)”). In transition season, the overall air fresh voting in Room A was − 0.31 (ranging from “Moderate (0)” to “Stale (-1)”) and the median was 0 (“Moderate (0)”), while the overall air fresh voting in Room B was 0.83 (near to “Fresh (+ 1)”) and the median was + 1 (“Fresh (+ 1)”). In winter, the overall air fresh voting in Room A was − 0.63 (ranging from “Moderate (0)” to “Stale (-1)”) and the median was − 1 (“Stale (-1)”), while the overall air fresh voting in Room B was 0.74 (ranging from “Moderate (0)” to “Fresh (+ 1)”) and the median was + 1 (“Fresh (+ 1)”). The reason for this was that in Room B, the APW absorbed CO₂ through photosynthesis and released O₂, which improved indoor air quality and achieved the function of repairing and purifying indoor air. Additionally, the APW incorporated natural elements into the interior design to create a comfortable atmosphere and enhance participants’ overall experience.

Fig. 12.

Comparison of (a) summer, (b) transition season, (c) winter indoor air fresh sensation voting and (d) overall air fresh sensation voting throughout the year in Room A and B.

Figure 13 compared indoor thermal comfortable sensation voting and overall indoor thermal comfortable sensation voting of Room A and B in summer, transition season and winter. After statistical analysis, there was a significant difference in thermal comfortable sensation voting between Room A and B in three seasons (P < 0.01). The thermal comfortable sensation in Room B was consistently higher than that of Room A throughout the year. As the participants spent more time, the evaluation of Room B remained stable, while their evaluation of Room A decreased over time. In summer, the overall thermal comfortable voting in Room A was − 0.34 (ranging from “Moderate (0)” to “Slightly comfortable (-1)”) and the median was 0 (“Moderate (0)”), while the overall thermal comfortable voting in Room B was 1.19 (slightly above the level of “Slightly comfortable (+ 1)”) and the median was + 1 (“Slightly comfortable (+ 1)”). In transition season, the overall thermal comfortable voting in Room A was − 0.64 (ranging from “Moderate (0)” to “Slightly comfortable (-1)”) and the median was − 1 (“Slightly uncomfortable (-1)”), while the overall thermal comfortable voting in Room B was 0.99 (near to “Slightly comfortable (+ 1)”) and the median was + 1 (“Slightly comfortable (+ 1)”). In winter, the overall thermal comfortable voting in Room A was − 0.92 (near to “Slightly comfortable (-1)”) and the median was − 1 (“Slightly uncomfortable (-1)”), while the overall thermal comfortable voting in Room B was 0.89 (near to “Slightly comfortable (+ 1)”) and the median was + 1 (“Slightly comfortable (+ 1)”). This was attributed to the ability of the APW to increase negative oxygen ions concentration in indoor air, thereby contributing to the improvement of people’s mood and mental state, and ultimately enhancement of comfortable. Secondly, the incorporation of APW can cultivate a bio-friendly environment and foster a stronger connection to nature for individuals. Additionally, the visual appeal of APW contributes to heightened indoor comfortable, thereby enriching the overall living and working experience. In conclusion, APW have a positive impact on indoor comfortable through enhanced air quality, the establishment of a natural ambiance, and elevated aesthetics.

Fig. 13.

Comparison of (a) summer, (b) transition season, (c) winter indoor thermal comfortable sensation voting and (d) overall thermal comfortable sensation voting throughout the year in Room A and B.

When introducing APW into indoor environments, it is crucial to assess the willingness of participants to accept APW. Figure 14 compared participants’ acceptance of an APW in summer, transition season and winter. According to the acceptance voting results, 80.1%, 86.7%, and 85% of the participants expressed that their willingness to introduce the APW into indoor environments in summer, transition season and winter, respectively, with ratings of “Willing (+ 1)” or “Very willing (+ 2)”. This indicated a predominantly positive attitude among participants towards the APW and were willing to accept it, so the effect of introducing the APW will be better and the improvement of the indoor environment will be more significant.

Fig. 14.

Comparison of acceptance of an APW throughout the year.

The correlation analysis

In this study, the main focus was on the influence of APW on indoor physical environment parameters (such as temperature, RH, air speed and CO₂ concentration), as well as the impact of these changes on the subjective thermal sensation of the participants. Although we did not directly calculate the correlation between physical environment parameters and subjective voting, the preliminary analysis indicates that the changes of these parameters have a certain association with the thermal sensation of the participants.

In experiments conducted in different seasons, we found that there were significant differences in the neutral temperatures of the subjects. For example, in summer, the neutral temperature of the subjects in Room B was close to 33.2℃, while in winter, the neutral temperature was slightly lower. This difference is related to the changes in environmental conditions caused by seasonal variations.

To further explore the relationship between subjective questionnaires and physiological parameters, correlation analyses were conducted on thermal sensation voting, Wet sensation Voting, Airflow voting, thermal comfort voting, and MST. There was a significant linear correlation between MST and thermal sensation, wet sensation, airflow voting and thermal comfort, indicating that MST can be used as an important physiological indicator for evaluating the level of thermal comfort.

Figure 15 a presents the fitting results of thermal sensation voting and the MST in summer, transition season and winter. The analysis revealed a significant positive correlation between MST and thermal sensation voting. When the thermal sensation is at a neutral state (0), the MST values for Room A and Room B are 31.95℃ and 32.26℃ in summer, respectively. In transition season, the MST values for Room A and Room B are 32.35℃ and 32.55℃, respectively. In winter, the MST values for Room A and Room B are 32.35℃ and 32.82℃, respectively. In Room B, the skin temperature of the participants was closer to the neutral skin temperature (33.2℃), which also explains why the thermal sensation was more comfortable in the subjective voting.

Fig. 15.

the correlations between (a) thermal sensation, (b) wet sensation, (c) airflow sensation, (d) thermal comfort and MST throughout the year in Room A and B.

Figure 15b presents the fitting results of wet sensation voting and the MST in summer, transition season and winter. With the increase of the MST, the wet sensation voting shows an upward trend and a significant positive correlation. Moreover, under the same MST, the wet sensation voting value of Room B is higher in transition season and winter, while it is lower in summer. This finding is consistent with the indoor RH measurements shown in Fig. 4.

Figure 15c presents the fitting results of airflow vote and MST in summer, transition season and winter. With the increase of the MST, the airflow voting shows a downward trend and a significant negative correlation. Moreover, at the same MST, the airflow voting is lower in Room B.

Figure 15d presents the fitting results of thermal comfort voting and MST in summer, transition season and winter. With the increase of the MST, the thermal comfort voting shows an upward trend and A significant positive correlation. Moreover, the thermal comfort voting value of Room B was higher than Room A.

Conclusions

In this study, objective measurement and subjective questionnaires were utilized to investigate the impact of APW on indoor environments throughout the year through comparative experiments.

1. Throughout the year, the APW played pivotal role in maintaining a healthy indoor environment. In transition season and winter, the APW decreased the mean indoor temperature by 1.35℃ and 1.03℃, while increasing the mean RH by 11.6% and 20.76% compared to room without APW, respectively. In summer, the mean temperature of room with APW was 0.18℃ exceeded room without APW, and the RH was reduced by 2.7%. In addition, in summer, transition season and winter, the mean air speeds in room with APW were 0.32 m/s, 0.37 m/s and 0.29 m/s, respectively. CO₂ concentration was reduced by 42.35ppm, 43.83ppm and 46.83ppm, respectively. These results show that APW can flexibly adjust indoor temperature and RH according to seasonal changes, thereby improving indoor thermal comfort, and APW has a significant effect on improving indoor air quality.

2. Throughout the year, APW improved the skin temperature of the participants. In summer, transition and winter, the MST of room without APW was 31.91℃, 33.33℃ and 33.43℃, respectively. While that of room with APW was 32.21℃, 33.13℃ and 33.19℃, respectively. In contrast, the participants’ MST in room with APW closely approximated the neutral skin temperature of 33.2℃. These results APW was able to improve thermal comfort by adjusting the indoor environment to bring participants’ skin temperature closer to a neutral value.

3. Throughout the year, the participants’ subjective assessment of room with APW was markedly superior to room with APW. In summer, transition season and winter, the overall thermal sensation voting scores of room with APW were − 0.16, -0.2 and 0.23 (near to “Moderate (0)”), and the overall wet sensation scores were 0.22. -0.13 and 0.17 (near to “Moderate (0)”), respectively. The overall airflow sensation scores of room with APW were 0.63, 0.73 and 1.12 (near to “Slightly airflow (+ 1)”), respectively. The overall air fresh scores of room with APW were near “Fresh (+ 1)”.

4. Throughout the year, the thermal comfortable of room with APW was markedly superior to room without APW. In summer, transition season and winter, the overall thermal comfortable scores of room without APW were − 0.34, -0.64 and − 0.92 (ranging from “Moderate (0)” to “Slightly uncomfortable (-1)”). The overall thermal comfortable scores of room with APW were 1.19, 0.99 and 0.89 (near to “Slightly comfortable (+ 1)”). The results show that APW can significantly improve indoor thermal comfort in different seasons and make the participants feel more comfortable.

5. The majority of participants exhibited a favorable attitude towards the APW and expressed their willingness to incorporate it. In summer, transition season and winter, 80.1%, 86.7%, and 85% of participants expressed their readiness as “Willing (+ 1)” or “Very willing (+ 2)” to integrate APW into indoor environments, respectively. This shows that APW is not only technically effective, but also has significant advantages in terms of user acceptance.

In summary, the APW had the potential to regulate indoor temperature and humidity, as well as improved CO₂ absorption. APW significantly improved indoor thermal and wet sensation, air fresh and thermal comfortable levels.

Author contributions

F.L., Q.Y., Y.R. and X.M. wrote the main manuscript text, and they prepared all figures. F.L., Q.Y. and Y.R. done the experimental design and completed the experiment. F.L. and X.M. supervised. All authors reviewed the manuscript.

Data availability

All data generated or analysed during this study are included in this published article.

Declarations

Competing interests

The authors declare no competing interests.