Study on the Relationship between Indoor Vertical Greening and Oxygen Content in High-Rise Buildings

Abstract

This article clarifies the quantitative relationship between vertical greening, indoor ventilation, and the oxygen content in high-rise buildings, with the aim of determining values for a high-oxygen-content threshold to assess the ventilation and greening of high-rise buildings. The quantitative index could be provided to architects to assist in the sustainable design of vertical greening in high-rise buildings. The quantitative index offers an effective, convenient, and environmentally friendly oxygen-content-testing method for interior spaces, while avoiding the air pollution caused by the current red phosphorus combustion method. Firstly, a floor of a high-rise building in Harbin was selected for on-site and fixed-point experiments. Secondly, through the design of a candle-burning experiment in a gas bottle, we measured the change in candle-burning time before and after installing vertical greening, as well as under different ventilation states. Finally, the changes in relative oxygen content in each functional space before and after vertical greening and under different ventilation states were statistically analyzed. The results showed that there was a potential correlation between indoor oxygen content and vertical greening placement in high-rise buildings; this correlation was found to be directly related to room orientation, the degree of the plants’ photosynthesis, and indoor airflow. In general, vertical greening should be placed in south-facing rooms. For daily ventilation, two or more windows should be opened to ensure convection in rooms, which can increase their oxygen content.

1. Introduction

With rapid urbanization occurring worldwide, high-rise buildings with more than 10 floors have become the main building type that urban residents live in [1]. People spend 90% of their time in an indoor environment [2,3]; since the onset of the COVID-19 pandemic in particular, people’s indoor activity time has increased significantly. Human activities produce a large amount of carbon dioxide (CO₂) indoors [4,5], which reduces the oxygen content in the air. Excessive CO₂ content in indoor air leads to “indoor air syndrome”, which causes a reduction in work efficiency [6] and results in symptoms such as fatigue, difficulty concentrating, and headache [7]. Long-term exposure to such an environment may even lead to respiratory diseases and decreased lung function, directly threatening people’s health. Thus, the study of oxygen content in high-rise buildings cannot be ignored. A common method currently used to increase indoor oxygen content is natural ventilation and indoor greening [8]. Studies have shown that natural ventilation can reduce the energy consumption of building air-conditioning systems and improve air quality [9,10]; in addition, by installing vertical greenery in interior spaces, indoor air pollution can be effectively reduced, and the oxygen content can be increased [11]. Placing green plants in indoor spaces can improve the air quality [12]. An experiment conducted at Australian National University confirmed that increasing green coverage could improve the environmental conditions of an indoor environment [13]. Indoor greenery can induce relaxation and positive emotions [14], and work efficiency can be improved by 15–20% when people are comfortable in their indoor environment [15,16,17]. Therefore, studying the quantitative relationship between vertical greening and indoor ventilation and oxygen content in high-rise buildings can provide architects with a reference index for the sustainable design of vertical greening curtain walls in high-rise buildings. Research on indoor oxygen content should also address the design of high-rise interior spaces. These are key issues that need to be addressed urgently.

At present, international research on the vertical greening of high-rise buildings mainly focuses on the research of “green wall” technology [18] and the function of green plants. Vegetation covers the curtain wall and the surface of the wall to achieve the purpose of beautifying the environment [19]. Research on the function of green plants mainly addresses the role of vertical greening in air purification, sterilization, and detoxification and the improvement of outdoor thermal comfort, as well as the construction of microclimates in the urban environment [20,21]. The research on green plants and indoor environments mainly focuses on the effect of green plants on indoor pollutants, heat and humidity, the acoustic environment, and human mental health and visual comfort [22,23,24]. Research on green plants and indoor oxygen content mainly focuses on the concentration of negative oxygen ions released by different types of green plants and their relationship with indoor air quality [25]; meanwhile, research on the relationship between indoor vertical greening and indoor oxygen content has especially focused on indoor natural ventilation conditions. This relationship needs to be explored further. Regarding research measuring indoor oxygen content, the traditional method is to indirectly measure the oxygen content in the air by burning red phosphorus. The experimental technique of red phosphorus combustion involves igniting red phosphorus in the air with an alcohol lamp, collecting it in a gas bottle, and judging the oxygen content in the air according to the time for which the red phosphorus in the gas bottle burns. The experimental equipment for this method is cumbersome, and white smoke may be inhaled after the manual ignition of the red phosphorus, causing discomfort [26]. At the same time, the phosphorus pentoxide generated by the combustion of red phosphorus pollutes the air, which is dangerous in a nonlaboratory environment. On the basis of this method, Wang Bing [27] used the thermal effect of a current to heat a resistance wire, igniting the red phosphorus; in doing so, they improved the operation of artificially igniting red phosphorus. Liu Yu [28] used a laser flashlight to instantly ignite red phosphorus, which reduced the amount of time required for this approach. Yu Bineng [26] placed a small amount of red phosphorus on a small circle at the end of a copper wire, inserted the plunger of a syringe, pulled the copper wire to make the rubber stopper tight, and heated the wire with an alcohol lamp to ignite the red phosphorus. Sha Qibo [29] used iron powder instead of red phosphorus to solve the problem of phosphorus pentoxide pollution generated by the combustion of red phosphorus; however, it was extremely inconvenient to operate indoors, and the cost was high. Therefore, designing a green, environmentally friendly, and energy-saving method for measuring indoor oxygen content will provide an effective solution to the issues posed by measuring and evaluating indoor oxygen content in high-rise buildings.

In this study, by designing an experiment where candles were burned in gas bottles, oxygen content was calculated according to the length of time between the lighting and the extinguishing of candles in a specific volume of the gas bottle. Here, this is proposed as a method for measuring the indoor oxygen content of high-rise buildings. This approach can be used in different functional areas, before and after greening, and under different natural ventilation conditions. The use of the proposed method enables the study of the relationship between vertical greening and the oxygen content of different functional areas in high-rise buildings; accordingly, it will provide support for the scientific design of green curtain walls and vertical greening approaches for architects and will aid architects in realizing energy-saving approaches for the maintenance of high-rise buildings, providing quantitative indicators for reference. At the same time, it will also provide a convenient, green, and environmentally friendly method for the measurement of oxygen content in indoor spaces and ecological environments.

2. Materials and Methods

2.1. Test Object and Test Conditions

Harbin City, Heilongjiang Province, China, is located in a high-latitude zone, between 125°42′~130°10′ east longitude and 44°04′~46°40′ north latitude. The temperature is generally cool, although it rises and falls rapidly in spring and autumn. The average annual temperature is 4.32 °C, the average temperature of the coldest month is −18.49 °C, and the average temperature of the hottest month is 22.93 °C; the average annual precipitation is about 520 mm, where summer precipitation is abundant, accounting for 60–80% of the whole year. Winter is dry and rainy, and precipitation is only about 5% of the annual precipitation. The southeasterly wind in summer and northwesterly wind in winter show little annual change in wind speed, with a monthly average between 3.2 and 5.4 m/s [30,31].

The test object for this study was located in Room 1, Floor 27, Unit 1, Building 5, New Faculty Apartment, Northeast Agricultural University, Xiangfang District, Harbin, and included the change in oxygen content in 5 indoor spaces including living room, master bedroom, second bedroom, study, kitchen, before and after placing vertical greening and under different window-opening states. The living room and master bedroom face south, while the second bedroom, study, and kitchen face north. The test time was from 2 May 2022 to 4 May 2022, 8:00~16:00. We completed the oxygen content measurement tasks before and after the green plants were placed in five rooms and in different window-opening states in one day, and we measured continuously for three days. The indoor and outdoor environmental conditions during the test and the window-opening states of each room are shown in

Table 1. Test times with different window-opening states and environmental conditions from 2 May to 4 May, 2022. (— represents a closed window, ● represents one open window, and ●● represents two open windows).

Date	Time	Drawing Room	Master Bedroom	Second Bedroom	Study	Kitchen	Outdoor Temperature (°C)	Indoor Temperature (°C)	Outdoor Humidity (%)	Indoor Humidity (%)	Outdoor Wind Direction	Outdoor Wind Speed (m/s)	Illuminance
2 May	8:00–8:40	—	—	—	—	—	13–16	19–21	38	40	West wind	1.6–3.3	Natural lighting
	8:50–9:30	—	—	—	—	●
	9:40–10:20	●	—	—	—	●
	10:30–11:10	●●	—	—	—	●
	11:20–12:00	●●	●	—	—	●
	14:30–15:10	●●	●	●	—	●
	15:20–16:00	●●	●	●	●	●
3 May	8:00–9:40	—	—	—	—	—	15–20	19–21	36	40	West wind	1.6–3.3	Natural lighting
	8:50–9:30	—	—	—	—	●
	9:40–10:20	●	—	—	—	●
	10:30–11:10	●●	—	—	—	●
	11:20–12:00	●●	●	—	—	●
	14:30–15:10	●●	●	●	—	●
	15:20–16:00	●●	●	●	●	●
4 May	8:00–9:40	—	—	—	—	—	20–26	19–21	38	40	Southwest wind	1.6–3.3	Natural lighting
	8:50–9:30	—	—	—	—	●
	9:40–10:20	●	—	—	—	●
	10:30–11:10	●●	—	—	—	●
	11:20–12:00	●●	●	—	—	●
	14:30–15:10	●●	●	●	—	●
	15:20–16:00	●●	●	●	●	●

. On 2 May, the outdoor weather was 13–16 degrees Celsius, the indoor temperature was 19–21 degrees Celsius, the outdoor humidity was 38%, the indoor humidity was 40%, and the wind direction was a west wind. The outdoor weather on 3 May was 15–20 degrees Celsius, the indoor temperature was 19–21 degrees Celsius, the outdoor humidity was 36%, the indoor humidity was 40%, and the wind direction was a west wind. The outdoor weather on 3 May was 20–26 degrees Celsius, the indoor temperature was 19–21 degrees Celsius, the outdoor humidity was 38%, the indoor humidity was 40%, the wind direction was southwest, the wind speed was 1.6–3.3 m/s for three days during the test period, and the illumination was all-natural light. During the test, the states of different window opening in each room were divided into seven situations. From 8:00 to 8:40, the windows of the five rooms were all closed, and the measurement of the oxygen before and after placing the green plants in each room was carried out at the established test points. For the measurement of the content, the measurement was repeated six times at each point. At 8.50, the oxygen content was measured in another window-opening state, in intervals of 10 min. By 16:00, the test of the seven window-opening states was completed. Because oxygen and carbon dioxide are heavier than air, it can affect the distribution of gas after opening the window. In this experiment, the size of each openable window and the distance between its lowermost edge and the ground were determined. The sizes of the openable windows in the living room and kitchen were 98 cm in height and 47 cm in width, and the height from the bottom edge to the ground was 137 cm. The sizes of the windows that could be opened in the master bedroom, second bedroom, and study were 134 cm in height and 47 cm in width, and the height from the bottom edge to the ground was 102 cm. The specific test points A–E are shown in Figure 1. The vertical greening soft partition tested in this experiment is shown in Figure 2. A movable vertical greening soft partition wall was placed in the living room by the window. According to the number of commonly cultivated plants in the general living space, the wall was composed of 12 pots of green dill (Epipremnum aureum), which is a large evergreen vine in the Araceae family [32]. Green dill can release a very stable concentration of negative oxygen ions [33] and is the most common indoor potted leaf plant in northern China [34]. The total area of the vertical green wall was 0.98 square meters, accounting for 0.7% of the total indoor space area. In order to ensure the stability of the measurement environment during the experiment, a time when the residents were out was selected for testing. During the experiment, no residents conducted cooking or other activities.

2.2. Testing Equipment and Materials

As shown in Figure 3, the test device consisted of a sealed water tank, a candle, a candle-fixing table, and a gas bottle, which was 180 mm high and 110 mm wide. The sealed water tank was used to hold water. The candle-fixing table was placed in the middle of the water tank and was divided into two parts: the upper part was the candle holder, which was cylindrical and could adjust the position of the candle; the lower part was a cube, a glass or metal, fixed object. The candles were fixed on the candle holder and could be placed in or taken out freely. The gas bottle was a glass bottle with a large diameter and round mouth, which could cover the entire candle and the candle-fixing table. The test materials in this experiment included water, candles, lighters, rulers, scissors, and stopwatches.

2.3. Testing Method

In this experiment, the oxygen content in the indoor environment was judged by the length of candle-burning time in the fixed volume of the gas bottle. The longer the burning time, the greater the oxygen content in the air; the shorter the burning time, the lower the oxygen content in the air. Figure 4 shows the procedure for the one-time candle-burning experiment. During the experiment, after holding the gas bottle and waving it in the air 2~3 times, the air in the bottle was considered the same as the air in the space where it was located; then, we used a lighter to light the candle on the candle-fixing table in the center of the sealed water tank. The depth of the water in the water tank was 2 cm. When the candle was burning, the flame was kept at least 1–2 cm away from the bottom of the bottle. The length of the candle wick was controlled at 1 mm using scissors. After lighting the candle, it was quickly covered with the gas bottle, and the bottle mouth contacted the bottom of the sealed water tank. At this time, the water in the water tank could seal the bottle mouth and isolate the air inside the bottle from the air outside the bottle. The candle burned for a period of time until it exhausted the oxygen in the bottle, at which point it went out. The burning time of a candle is positively correlated with the oxygen content in a bottle, which is positively correlated with the oxygen content of the space in which it is located. By measuring the burning time of the candle, the relative oxygen content in different spaces can be compared. Thus, the changes of oxygen content in the air before and after the indoor greening arrangement of high-rise buildings and before and after opening the windows can be obtained. This research method is suitable for the measurement of relative oxygen content in an indoor space, but it cannot give direct physical indicators for the volume of oxygen or the concentration value of the oxygen content.

2.4. Testing Process

First, place the sealed water tank filled with water on the ground in the center of the test site and add the candle holder with candles. Get the stopwatch ready and set to 00:00.00. Then, light the candle, hold the bottle until the candle flame is stable for 1–2 s, press the stopwatch when the bottle mouth touches the water surface, and observe any changes in the flame of the candle, while at the same time paying attention to whether the stopwatch has started to count. As soon as the candle flame is extinguished, press Start on the stopwatch and record the burn time. Next, tilt the gas bottle to let the water—which enters the bottle as a result of atmospheric pressure due to the consumption of oxygen during combustion—flow out, take it out of the water tank, and wave it in the air 2~3 times to fill the air-collecting bottle with the air to be tested. Observe the length of the wick of the candle after burning and use scissors to cut it to 1 mm. Repeat the above steps for the next test, and repeat for each test point at least 6 times; when switching test points or when testing two adjacent test points, allow for an interval of 10 min to wait for the air to be evenly distributed.

3. Results and Discussion

3.1. Indoor Oxygen Content with Vertical Greening under Different Window-Opening States

In this experiment, by calculating the burning time of a candle in a gas bottle, the changes in oxygen content in the living room, master bedroom, second bedroom, study, and kitchen of the study space was tested when zero to six windows were opened. The changes in oxygen content with different window-opening states are shown in Figure 5: 0 indicates no windows open; 1 represents one window open in the kitchen; 2 indicates one window open in the kitchen and one in the living room; 3 denotes one window open in the kitchen and two windows open in the living room; 4 represents one window open in the kitchen, two in the living room, and one in the master bedroom; 5 represents an additional open window in the second bedroom on the basis of 4; and 6 denotes an additional open study window on the basis of 5. At the stage denoted 6, all of the windows were opened. The results of the experiment showed that when only one window was open in the kitchen, the outside air entered the kitchen through the window due to the fact that the gas channel of the whole building was installed in the kitchen where the range hood was installed. Due to the temperature difference between indoors and outdoors, the indoor air was circulated. When two to six windows were opened, it was clearly felt that the airflow always flowed from the south window to the north window. It can be seen from the statistics of the burning time of the candles that, when the window was not open, the burning time of the candle in the master bedroom was evidently the longest, while the burning time of the candle in the second bedroom was the shortest. This is because the southern exposure of the master bedroom was more conducive to photosynthesis in the green plants and thus more oxygen was released; in the second bedroom, with northern exposure, there was less sunlight relative to the other rooms. Therefore, the oxygen content in the room was related to photosynthesis in the green plants.

When a window was opened in the kitchen for ventilation for 10 min, the oxygen content in the room changed. The burning time of the candles did not change much between the living room and the master bedroom, showing a slight decrease in average value, while the burning time of candles in the secondary bedroom, study, and kitchen increased significantly, with the average value increasing by 3 to 6 s when compared to the unopened state. When two windows were opened in the kitchen and the living room, the burning time of the candles in all rooms increased; the average burning time of the candles increased by 2 to 4 s when compared to the scenario where only one window was opened in the kitchen. The candles in the living room had the longest burning time, while the candles in the study had the shortest burning time. This was a result of air convection, formed by opening windows in the kitchen and living room for ventilation. After opening three windows in the kitchen and living room, the burning time of the candles in all rooms, except the kitchen, decreased by 1~3 s, while the burning time of the candles in the kitchen increased by 1.96 s. At this time, the candle-burning time was negatively correlated with the number of open windows. The reason for this is that during air convection, the two open windows in the living room and one open window in the kitchen formed a relative air pressure difference, which made the airflow in the direction of the kitchen faster and resulted in a remarkable ventilation effect, causing an increase in oxygen content; in addition, the airflow in the other rooms was relatively static, causing the candle to burn for a shorter time. When the four windows in the kitchen, living room, and master bedroom were opened for 10 min, the burning time of the candles in the living room was the longest, with an average of 23.01 s, and a maximum of 27.34 s was recorded in the master bedroom. However, the average burning time in the master bedroom, secondary bedroom, study, and kitchen decreased by 1 to 3 s compared to the average obtained when three windows were opened. The reason for this may be that the experiment was carried out in the afternoon, when sunlight exposure and thus the photosynthesis of green plants were weakened, and the oxygen content released by the plants was reduced. The second bedroom and study were found to be in a state of slow air circulation, and the oxygen content was reduced relative to the other rooms. When the five windows of the kitchen, living room, master bedroom, and second bedroom were opened, the burning time of candles in the second bedroom increased significantly, with an average increase of 3.68 s, while the burning time of candles in the master bedroom and study increased by 3 to 5 s. At this stage, the burning time of the candles was reduced by about 1 s; this is because the convection of air between the living room and the kitchen tended to be stable, and the volume of air exchange was decreased. Generally, as oxygen consumption increases, the oxygen content decreases. Therefore, along with oxygen consumption, the relatively stable indoor airflow caused the oxygen content value to decrease. When six windows were opened for ventilation for 10 min, the average burning time of candles in the study and kitchen was longer than when five windows were opened, while the burning time of candles in the living room, master bedroom, and second bedroom decreased; however, the overall burning time of candles in the room tended to be consistent. This is likely related to the fact that when all the indoor windows were opened, airflow increased and was relatively stable.

As shown in Figure 6, by comparing the average values for candle-burning times in each room, it can be seen that the master bedroom and living room located in the south had higher oxygen content when the windows were not opened; in contrast, the study, kitchen, and secondary bedroom located in the north had relatively low oxygen content. It can be seen that photosynthesis by plants brought about by sufficient care directly affected the oxygen content in the room. When one window was opened in the kitchen, the oxygen content in the three northern rooms increased significantly. When two windows were opened in the kitchen and living room, the oxygen content in each room continued to increase steadily, and the oxygen content in the second bedroom reached its peak. When the three windows in the kitchen and living room were opened, the oxygen content in the second bedroom and the study room was close to the same, while the oxygen content in the kitchen, living room, and master bedroom was almost the same. When the three windows in the kitchen and living room were opened, the oxygen content in the secondary bedroom and the study room was almost the same; the oxygen content in the kitchen, living room, and master bedroom was the same; and the oxygen content in the kitchen reached its peak. At this time, air convection had a significant impact on oxygen content. When the four windows in the kitchen, living room, and master bedroom were opened, the oxygen content in the living room reached its peak value, while the oxygen content in the remaining rooms showed a downward trend. When there were five open windows, the oxygen content in the master bedroom, second bedroom, and study increased by varying degrees; the oxygen content in the living room and kitchen decreased; and the oxygen content in the master bedroom reached its peak. When the six windows in the kitchen, living room, master bedroom, second bedroom, and study were all opened, the oxygen content in the living room reached its lowest level, the oxygen content in the master bedroom and the second bedroom was decreased relative to the other rooms, and the oxygen content in the kitchen and study room was increased relative to the other rooms. From this, it can be concluded that, when greening is placed indoors, photosynthesis in the plants and the flow of indoor air generated by opening windows directly affect the indoor oxygen content.

3.2. Indoor Oxygen Content without Vertical Greening under Different Window-Opening States

The same experimental method was used to test the oxygen content in different functional spaces without greening when zero to six windows were opened by analyzing the candle-burning time in each space. The results are shown in Figure 7. When the windows were not opened, the burning time of the candle in the living room was the longest, while the burning time of the candle in the master bedroom, secondary bedroom, study, and kitchen tended to be stable, with an average between 11 and 13 s. After a window was opened for 10 min, the oxygen content in the room changed. Except for the living room, the burning time of the candles in all rooms increased by an average of 1~2 s. When the two windows in the kitchen and living room were opened to generate convection currents, the burning time of the candles in the living room and the master bedroom increased; the average time increased by an additional 2~3 s compared to that achieved when one window was open. The burning time of the candles in the other rooms was reduced by 1~2 s. The secondary bedroom and study were found to be in a relatively static or slow circulation state, which reduced the oxygen content in these rooms. When the three windows in the kitchen and living room were opened for ventilation for 10 min, the candle-burning time in the secondary bedroom reached its lowest value of 8.86 s. The average burning time of the candles in the living room and second bedroom decreased; in the living room, the average burning time decreased by 5.27 s compared to the previous group, and this was the largest observed reduction. The average burning time of the candles in the master bedroom, study, and kitchen increased by about 1 to 3 s. One of the windows in the living room faced the door to the study; after the window was opened, the air flowed into the study, increasing the airflow and oxygen content in the room. When the four windows in the kitchen, living room, and master bedroom were opened for 10 min, the candles in the living room reached the maximum burning time observed in this experiment, 22.38 s. The burning time of the candles in the living room and second bedroom increased, while the burning time of candles in the master bedroom, study, and kitchen was reduced by 1~4 s. Because the experiment was carried out in the afternoon, the sun was strong, the indoor temperature was high, and there were no green plants for photosynthesis; thus, the oxygen consumption in the master bedroom was considerable. Although the window in the master bedroom was opened, the candle-burning time was reduced; after 10 min of ventilation through the five windows of the living room, master bedroom, and second bedroom, convection occurred between the living room and the kitchen and the master bedroom and the second bedroom. The average burning time of the candles in the other rooms, except the living room, increased by about 1 s. The convection of air between the living room and the kitchen tended to be stable; the air exchange in the kitchen decreased, and with the increase in oxygen consumption, the oxygen content also decreased. After all six windows in the kitchen, living room, master bedroom, second bedroom, and study were opened for ventilation for 10 min, the convection of air between the living room and the kitchen, the master bedroom and the second bedroom, and the living room and the study room resulted in increased airflow. Except for the study, the average burning time of the candles in the other rooms increased by 1–2 s. The reason for this was that the study window was opened last, and the air pressure in the remaining rooms was stable due to long-term ventilation. When the window of the study was opened, the air flowed to other spaces; however, the air in the study was in an unstable state, which caused the overall indoor air pressure to fluctuate, resulting in a decrease in the burning time of the candles in the study and an increase in the burning time of the candles in the other rooms.

The average candle-burning times from each room without greening were compared, and the results are shown in Figure 6. The oxygen content in the master bedroom, study, and kitchen did not change considerably, and the distribution was relatively uniform whether the windows were open or not. The oxygen content curves of the living room and the second bedroom showed relatively obvious changes; when the three windows in the kitchen and living room were opened, the oxygen content in the living room and the second bedroom was significantly reduced. The reason for this is that a window in the living room was opposite to the study, forming a convection current with the study. The air in the living room flowed to the study, reducing the oxygen content in the living room while the oxygen content in the study reached its highest value. The second bedroom was located in the northeast corner; it was only when the windows in the second bedroom and the master bedroom were open—that is, when six windows were open—that a convection current was formed and the oxygen content in the second bedroom reached its peak. When two windows were opened in the living room and one window was opened in the kitchen, the air in the living room mainly flowed to the study and kitchen, and the oxygen content in the second bedroom was the lowest. It can be concluded that in the absence of green plants, the oxygen content in the room was directly related to the air convection state formed by opening windows. The number and direction of open windows will have a direct impact on the flow of air, thereby affecting the change in indoor oxygen content.

3.3. Influence of Vertical Greening on Oxygen Content under the Same Window-Opening States

By comparing the average candle-burning times in the same functional space before and after greening when the windows were opened in the same state, i.e., with zero to six windows open, we obtained the change in oxygen content. The results are shown in Figure 8. With the windows closed, only the master bedroom and living room had increased oxygen content after the placement of green plants, among which the master bedroom showed the most obvious increase, while the rest of the rooms had higher oxygen content without green plants. The southern master bedroom and living room received sufficient sunlight in the morning, allowing for adequate photosynthesis in the green plants. The release of oxygen from the plants thus increased the oxygen content in the two south-facing rooms. When one window was opened in the kitchen for ventilation for 10 min, the average burning time of the candles in the study and kitchen without green plants was higher than that with green plants. The reason for this is that the green plants were placed near the window in the living room, far from the study and kitchen. In this window-opening state, the air in the kitchen flowed quickly, and the kitchen was closest to the study room; therefore, the oxygen content in the kitchen and study room was relatively high when there were no green plants. When the two windows in the kitchen and living room were opened to form convection currents, the indoor oxygen content in the state with green plants was higher than that in the state without green plants. When the three windows in the kitchen and living room were opened for ventilation for 10 min, only the study had a slightly lower oxygen content when there were green plants; it was lower than the oxygen content when there were no green plants. When the four windows in the kitchen, living room, and master bedroom were opened for 10 min, the oxygen content in each indoor space with green plants was higher than that without green plants. When the five windows in the kitchen, living room, master bedroom, and second bedroom were opened for ventilation for 10 min, the average burning time of the candles in the study and kitchen without green plants was higher than that with green plants. The reason for this is that with this ventilation, convection currents were formed between the air in the living room and the kitchen and the master bedroom and the second bedroom. At this stage, the green plants had little effect on the oxygen content in the kitchen and study. When all six windows in the kitchen, living room, master bedroom, second bedroom, and study were opened for ventilation for 10 min, convection currents were formed between the master bedroom and the second bedroom and the living room and the study, and airflow increased. The oxygen content in the living room and the second bedroom was slightly higher when there were no green plants, which is related to the fact that the windows were all open and airflow has a higher impact on oxygen content than green plants. From the above analysis and comprehensive comparison of experimental data, it can be concluded that indoor greening is positively correlated with changes in the oxygen content of air.

In brief, when greenery was placed indoors and the four windows in the kitchen, living room, and master bedroom were opened for 10 min, the oxygen content in the living room was the highest compared to the other window-opening states. When the five windows in the kitchen, living room, master bedroom, and second bedroom were opened for 10 min for ventilation, the oxygen content in the master bedroom was the highest compared to the other window-opening states. When the two windows in the kitchen and living room were opened for 10 min for ventilation to form convection currents, the oxygen content in the second bedroom and study room was the highest compared to the other window-opening states. After 10 min of ventilation, achieved by opening the three windows in the kitchen and living room, the oxygen content in the kitchen was the highest compared to the other window-opening states. When there was no indoor greenery and the two windows in the kitchen and living room were opened for ventilation for 10 min, the oxygen content in the living room was the highest compared to the other ventilation states. When the six windows in the kitchen, living room, master bedroom, second bedroom, and study were opened for ventilation for 10 min, the oxygen content in the master bedroom and the second bedroom was the highest compared to the other ventilation states. After 10 min of ventilation, achieved by opening the three windows in the kitchen and living room, the oxygen content in the study and kitchen was the highest compared to the other window-opening states.

In this study, an external environment with sunny weather and a breeze was selected. During the whole experiment, the indoor airflow rate was relatively stable, and the flame of the candle burning was always in a vertical upward state. The indoor airflow was mainly due to the indoor space air convection formed by the indoor and outdoor temperature difference. Therefore, in this study, the wind volume and velocity were not used as an important reference index. Weather with a higher wind speed will be selected for further in-depth study in future research. In addition, the experimental method of candle burning used in this study measures the relative oxygen content of the indoor space, and it does not provide the absolute value of the oxygen content. In future research, the absolute value of oxygen content can be measured more comprehensively by a negative oxygen ion tester.

4. Conclusions

To test the oxygen content in this experiment, the method of burning candles in a fixed-volume gas cylinder instead of burning red phosphorus solved the problem of phosphorus pentoxide polluting the air when burning red phosphorus. The method provides a low-cost, easy-to-operate, and eco-friendly method for measuring the relative oxygen content in an indoor space and in the ecological environment.

Through the comparative analysis of the indoor oxygen content changes before and after placing green plants indoors, it can be concluded that the placement of indoor vertical greenery directly affects the indoor oxygen content. This was especially observed in the south room with sufficient photosynthesis; the released oxygen content was increased. Therefore, the indoor vertical design of high-rise buildings needs to consider the lighting conditions in the area, so as to ensure the highest rate of survival of vertical greenery to maximize its effectiveness. Under normal circumstances, more cold-resistant green plants that do not require much sunlight should be placed in the north room, and the greenery in the south room should be an appropriate amount, i.e., not too much.

In breezy weather conditions, the indoor oxygen content is related to the indoor airflow caused by the indoor and outdoor temperature changes after the windows are opened. A relatively stable air convection is formed and is conducive to the increase in indoor oxygen content. Therefore, daily window opening should not open one window alone. If only one window is opened, only the oxygen content of that room will increase, while the oxygen content of other rooms will not change much. Especially in the case of placing green plants indoors, the best state of window opening should enable the air in the entire room to form a smooth convection current. The stable airflow in the room is conducive to driving the oxygen produced by the green plants to be evenly distributed in each room, increasing the overall oxygen content in the room. At the same time, to determine the window-opening time, the window-opening state should consider the latitude, seasonal changes of the region, temperature, wind direction, and wind speed.