Impact of Air Velocity on Mold Growth in High Temperature and Humidity Conditions: An Experimental Approach

Abstract

To address the challenges of indoor mold in southern China, this study designed and constructed an innovative experimental system to investigate mold growth in buildings under the combined influence of multiple factors. Using Fluent simulation (Ansys Fluent 19.0), we designed a suitably sized experimental chamber to realistically replicate the effects of factors such as temperature, humidity, and air velocity on mold growth. After establishing and fine-tuning the experimental system, we conducted two preliminary experiments, successfully validating the feasibility of our setup. Additionally, we observed that in a high-temperature, high-humidity environment of 28 °C and 80% relative humidity, the mold growth rate in the experimental chamber increased with the rise in inlet air velocity. This experimental system will serve as the foundation for future studies on indoor mold growth in building spaces in southern China.

1. Introduction

Research on the mechanisms of indoor mold growth in buildings has been emerging globally. These research studies aim to uncover the pathways through which mold infiltrates living environments, understand its impacts, and develop effective prevention and control strategies. A national survey in the United Kingdom revealed that 22% of respondents reported mold or dampness on the walls or surfaces in their homes [1]. In China, researchers are also investigating the key environmental factors contributing to indoor mold growth and the potential health risks while proposing various solutions [2,3]. Further studies have emphasized the importance of controlling indoor humidity and continuous monitoring in buildings [4,5,6]. Additionally, building design and construction are crucial for preventing indoor mold contamination. This includes selecting appropriate materials and proper wall configurations to inhibit mold growth [7]. Similarly, studies have suggested that controlling building infiltration issues in hot and humid climates through positive-pressure ventilation systems can prevent mold growth [8]. Appropriate mechanical and natural ventilation, along with effective humidity management strategies, can also effectively inhibit mold contamination in buildings [9]. Given the specific regional climate and growing health concerns, conducting localized research on indoor mold contamination is particularly critical [10,11].

In southern China, indoor mold growth has become a significant public health and residential environment issue [12]. The hot and humid climate in this region provides ideal conditions for mold proliferation, leading to frequent exposure of residents to indoor mold [4,13]. These molds can not only trigger allergic reactions such as rhinitis and asthma but may also lead to more severe respiratory infections and immune-related disorders [14,15,16,17,18]. Especially during the wet and rainy seasons in the southern region, humidity management and mold control in buildings are crucial to ensuring the health of residents [5]. Children and pregnant women, as high-risk groups for indoor mold exposure, exhibit heightened sensitivity to molds, which can lead to respiratory symptoms, increased infections, skin issues, and potential allergic risks [19,20]. In a study conducted in Foshan, a city in southern China, indoor humidity emerged as a significant risk factor for recurrent childhood pneumonia [21]. Research conducted in six cities in China indicated that prolonged prenatal exposure to indoor mold/damp stains and musty odor is a key factor contributing to low birth weight (LBW) [22]. The health impacts of indoor mold contamination should not be underestimated; compared to outdoor molds, indoor molds pose a more direct health risk due to their continuous exposure and inhalation [10,23]. Due to the continuous exposure to molds, the risk of respiratory diseases increases, especially in poorly ventilated living environments [24,25].

Under the collaborative research between Kitakyushu University and the Guangdong Provincial Academy of Building Research Group Co., Ltd. (Guangzhou, China), we conducted a preliminary questionnaire survey in Guangzhou to investigate and address indoor mold contamination issues in southern China. Among the 729 valid responses received, 5% of residential occupants reported significant mold problems in their homes. The survey revealed a strong correlation between indoor mold contamination and the health of building occupants. Additionally, preliminary on-site investigations were conducted with willing respondents, yielding valuable feedback. Severe cases of mold contamination were identified in several typical examples, and key points of temperature and humidity were measured, along with mold sampling and cultivation. Through these investigations and cultivations, we gained a sufficient understanding of indoor mold, adequately preparing for further action. Consequently, urgent laboratory research is needed to address mold contamination and improve indoor health environments in Guangzhou.

Laboratory research plays a crucial role in understanding the growth of indoor mold in buildings. It offers excellent controllability, allowing for focused investigation into specific environmental variables such as humidity and temperature. When indoor relative humidity (RH) exceeds 60%, the risk of mold growth significantly increases [25,26]. Studies have also shown that placing test slides containing mold spores in buildings reveals that mold growth begins when humidity exceeds 71% [27]. Investigations into temperature effects indicate that while temperature’s influence is not as pronounced as humidity, higher temperatures, especially in conjunction with high humidity, accelerate mold growth. Controlled laboratory environments are crucial for quantifying this effect, providing predictive models for mold growth under various temperature and humidity conditions [28,29]. Previous research suggests that adequate ventilation, which brings airflow, aids in the removal of spore accumulation and helps manage indoor moisture levels [8,9]. However, some studies indicate that even with a significant increase in natural ventilation rates, the risk of mold growth may not be sufficiently mitigated in outdoor environments with high humidity levels [30]. Improving temperature and humidity conditions, certain building materials can also resist mold growth [31]. Experimental studies also indicate a significant correlation between the organic content of building materials and the rate of mold growth [32]. In Japan, Keiko Abe and colleagues established the Mold Index as an assessment standard. They determined the risk of mold contamination at various locations within buildings by placing slides containing encapsulated mold spores [33]. Existing research has not fully considered the diversity and dynamics of indoor microenvironments, nor has it addressed the combined effects of multiple influencing factors or focused solely on individual buildings. A common challenge faced is the difficulty in replicating the conditions of indoor mold growth accurately within laboratory settings to mimic real-life residential environments [32,34].

Current research provides us with a certain understanding and foundation for controlling indoor mold growth in buildings, primarily focusing on the impact of individual factors such as temperature and humidity. This study aims to more meticulously consider and attempt to replicate the complex indoor environments of the real world to better explore the combined effects of temperature, humidity, ventilation, lighting, and materials on indoor mold growth in the future. Only by fully understanding the growth mechanisms of indoor mold under multi-factorial influences can we develop more effective and widely applicable prevention and control methods.

2. Design and Method

2.1. Laboratory System Design

To achieve the complexity of real-world indoor environments, we conducted initial market research. Our findings revealed that existing laboratory equipment on the market fails to simultaneously simulate the multitude of factors influencing mold growth, including temperature, humidity, airflow velocity, lighting, materials, and installation positions. Consequently, we undertook a redesign of the test box and testing system to meet these requirements and conducted simulated experiments accordingly. Moreover, this test box can be utilized for mold growth simulation experiments in other regions once relevant data from those areas has been collected in the future.

The experimental system for this study should be capable of independently controlling various influencing factors affecting mold growth to facilitate control and simulation of different conditional variables. To achieve the objectives of this experiment, the design of the experimental system should meet the following requirements:

A.

Ease of assembly, with relatively low cost and the use of appropriate materials.

B.

Ability to accurately control airflow velocity, temperature, and humidity within the test box.

C.

Placement surfaces for building materials on the bottom, sides, and top of the chamber.

D.

Transparent front panel with controllable transparency to regulate the amount of light entering the chamber.

E.

Simple and convenient opening and closing mechanism for easy inoculation of mold and observation of mold growth.

F.

Reusability for multiple experiments.

According to the experimental requirements, the experimental system designed by us is illustrated in Figure 1.

The design aims to construct a laboratory capable of precisely controlling internal temperature and relative humidity. Such a laboratory can replicate various real climatic conditions in the Guangzhou region while conducting experiments on the test box groups under controlled temperature and humidity conditions. Adjustable valves are installed at the air inlet of the test box to regulate the inlet airflow velocity. At the air outlet of the test box, pipes and switch valves are set up, connecting to the negative pressure chamber and exhaust fan. Ultraviolet lamps are installed in the negative pressure chamber and the laboratory return air section to eliminate mold spores and mold present in the exhaust air from the test box.

The system comprises a total of 12 test boxes primarily made of stainless steel. The front of the test box is equipped with an acrylic (transparent) observation window. The test box dimensions are 0.4 × 0.3 × 0.3 m (M). An adjustable acrylic plate is set outside the observation window on the front of the test box as a baffle to facilitate adjustment of the light transmittance area of the test box. Inlets and outlets are placed on both sides of the test box, and different building materials are installed on the bottom, top, and back inside the box.

As the test boxes are scaled-down models of actual residential building rooms, the positions and sizes of the air inlet and outlet of the test box need to be determined according to the ventilation conditions inside actual residential building rooms. This ensures that the surface airflow velocity of each material placement area inside the trial box roughly matches the actual situation.

2.2. Test Box Design

In the Chinese national standard “Code for design of residential buildings” [35], it is stipulated that bedrooms, living rooms (halls), and kitchens should have natural ventilation. Furthermore, it specifies that the direct natural ventilation opening area for bedrooms, living rooms (halls), and en-suite bathrooms should not be less than 1/20 of the floor area of the room. In this study, Fluent simulation(Ansys Fluent 19.0) was first employed to simulate natural ventilation under various wind speeds in a simulated room with dimensions of 4 × 3 × 3 (m). This simulation aimed to determine the airflow velocity on each wall under natural ventilation conditions.

Figure 2 shows the geometric model of the simulated room. This geometric model was drawn using ANSYS Geometry 19.0.

The simulated room measures 4 m in length, 3 m in width, and 3 m in height. Air inlets and outlets are positioned on the left and right sides of the room, respectively. The air inlet is located 0.3 m from the top and 1 m from the front of the room. It has a width of 1 m and a height of 1.3 m. The air outlet is positioned 0.5 m from the back of the room. It has a width of 1.2 m and a height of 2.1 m.

In ANSYS Meshing 19.0, the grid was generated using the Automatic method with Curvature selected for the Size Function and all other options set to default. After meshing, the simulation of the room was performed using ANSYS Fluent 19.0.

In the simulation, gravity effects were considered, the Energy Equation was enabled, and the Standard k-epsilon (2 eqn) Model [36] in ANSYS Fluent 19.0 was used as the simulation model. Standard Wall Functions were applied in the Near-Wall Treatment, and Full Buoyancy Effects were considered. Solar loading was incorporated into the Radiation module, utilizing the latitude, longitude, and time zone of Guangzhou and simulating solar radiation at 13:00 on June 21st. To ensure simulation accuracy, a three-species transport model (water vapor, oxygen, nitrogen) was employed to simulate humidity conditions within the room.

In China, for buildings without high-power heat sources indoors, the ventilation conditions for “pressure-driven ventilation” should ideally maintain temperature within the range of 20 °C to 28 °C, wind speed between 0.1 m/s and 3.0 m/s, and humidity between 40% and 90% [37]. Additionally, due to the difficulty in utilizing outdoor airflow below 12 °C directly, the ventilation conditions for “thermal pressure ventilation” should be set between 12 °C and 20 °C, with wind speeds ranging from 0 to 3.0 m/s and humidity without restriction.

Hence, in the boundary condition setup for the simulation of this room, the inlet boundary condition was set as a velocity-inlet type, segmented into wind speeds of 0.5, 1, 1.5, 2, 2.5, and 3 m/s, with a temperature of 293.15 K (20 °C) and a relative humidity of 75%. The outlet boundary condition was set as a pressure-outlet type, with a temperature of 293.15 K (20 °C) and a relative humidity of 75%. The remaining walls were oriented with the room’s back facing north, considering solar radiation. All other settings were left at their default values.

During the simulation, a steady-state approach was adopted, with a simulation run of 250 iterations. Figure 3 depicts the vector plot of wall velocities at the inlet wind speed of 0.5 m/s.

Keeping all other conditions constant, only the inlet wind speed of the simulated room was varied. After several simulations, we obtained the wind speeds at the center points of each wall surface of the simulated room, ranging from an inlet wind speed of 0.5 m/s to 3 m/s. The wind speeds at the top, back, and bottom surfaces of the three experimental materials are shown in

Table 1. Wind speeds at key wall surface center under various inlet wind speed conditions (simulated room).

Inlet Wind Speed	Bottom Wind Speed	Back Wind Speed	Top Wind Speed
0.5 m/s	0.31 m/s	0.27 m/s	0.17 m/s
1 m/s	0.55 m/s	0.54 m/s	0.28 m/s
1.5 m/s	0.91 m/s	0.85 m/s	0.39 m/s
2 m/s	1.19 m/s	1.12 m/s	0.65 m/s
2.5 m/s	1.52 m/s	1.40 m/s	0.80 m/s
3 m/s	1.65 m/s	1.68 m/s	0.92 m/s

.

After the aforementioned simulations, the wind speeds at key wall surfaces under various inlet wind speed conditions in the simulated room have been determined. To ensure that the wind speeds at the key wall surfaces inside the test box match those in the simulated room, the position and size of the ventilation openings of the test box need to be determined through simulation. Additionally, the size of the ventilation openings in the test box should match standard pipe diameters available in the market. After multiple simulations and referencing the streamlined simulation results obtained from Fluent simulations of the simulated room, the geometric model of the test box is now determined, as shown in Figure 4.

The test box measures 0.4 m in length, 0.3 m in width, and 0.3 m in height. Circular inlet and outlet vents are positioned on the left and right sides of the test box, respectively. The center of the inlet vent is located 0.1 m below the top surface and 0.15 m from the front surface, with a radius of 0.055 m. The center of the outlet vent is positioned 0.15 m above the bottom surface and 0.1 m from the back surface, with a radius of 0.055 m.

In ANSYS Meshing 19.0, the grid was generated using the Automatic method with Curvature selected for the Size Function, while all other options were set to default. After meshing, the simulation of the experimental chamber’s geometric model was performed using ANSYS Fluent 19.0. After partitioning the domain into grids, the simulation of the room was conducted using the same simulation model and boundary conditions as the simulated room. For the inlet vent, wind speeds were segmented into 0.5, 1, 1.5, 2, 2.5, and 3 m/s for simulation calculation. The vector plot of wall velocities when the inlet vent wind speed is 0.5 m/s is illustrated in Figure 5.

After maintaining all other conditions constant and conducting multiple simulations by varying the inlet wind speed of the test box, we obtained the wind speeds at the center points of each wall surface of the test box, ranging from an inlet wind speed of 0.5 m/s to 3 m/s. The comparison between the wind speeds at the center points of key wall surfaces in the test box and those in the simulated room is presented in

Table 2. Comparison of the wind speeds between the test box and the simulated room.

Object	Inlet Wind Speed (m/s)	Bottom Wind Speed (m/s)	Back Wind Speed (m/s)	Top Wind Speed (m/s)
Simulate room	0.5	0.31	0.27	0.17
Test box		0.26	0.20	0.16
Simulate room	1	0.55	0.54	0.28
Test box		0.53	0.47	0.27
Simulate room	1.5	0.91	0.85	0.39
Test box		0.81	0.82	0.38
Simulate room	2	1.19	1.12	0.65
Test box		1.10	1.05	0.55
Simulate room	2.5	1.52	1.40	0.80
Test box		1.42	1.45	0.71
Simulate room	3	1.65	1.68	0.92
Test box		1.51	1.65	0.85

.

From the table, it can be observed that the wind speeds at the key wall surfaces of the test box are generally consistent with those in the simulated room. We consider this geometric design of the test box acceptable. The assembly and experimentation of the test system are based on this design.

2.3. Methodology for Mold Growth Experiments

After the assembly of the laboratory, mold growth experiments were conducted in accordance with the Chinese national standards [38,39]. Initially, a comprehensive formal experimental procedure was employed to validate the experimental system’s capability to facilitate the normal growth of mold inoculated in the test box. Subsequently, based on the outcomes of this initial experiment, further experiments were conducted. The specific experimental flowchart is shown in Figure 6.

The following outlines the complete experimental procedure:

A.

Preparation for demonstration experiment:

Instruments: refrigerator, biochemical incubator, autoclave, electronic balance, high-temperature furnace, thermal comfort instrument (manufacturer: METREL; model: MI6401; airflow measurement accuracy: ±0.01 m/s within the range of 0.1 m/s–9.99 m/s).

Consumables and reagents: Sabouraud glucose liquid (SDB, 021091), Sabouraud glucose agar contact dish (CP0301J), analytical pure sodium chloride 500 g, 84-disinfectant solution, 75% concentration alcohol, Aspergillus niger strain, glass triangular flask 500 mL, 250 mL, sealing film, rubber band, inoculation ring, inoculation needle, coating rod, pipette, weighing paper, medicine spoon.

B.

Disinfection of test box:

Firstly, use 75% concentration alcohol to evenly spray inside each test box to be used in the experiment. Then, use a wet alcohol cloth to wipe off the dust and dirt inside the test box. Finally, use 84-disinfectant to evenly spray inside the box. Let it stand for 3 h to completely evaporate the 84-disinfectant inside the test box.

C.

Test board disinfection:

The various building materials required for the experiment must first be rinsed with running water. After rinsing, soak them in 84-disinfectant for 10–15 min before being taken out and rinsed again with running water. Leave it for 3 h to allow the 84-disinfectant to completely evaporate.

D.

Preparation of Aspergillus niger and SDB mixture:

Weigh 1.7 g of NaCl and prepare 200 mL of physiological saline. Weigh 10.02 g of SDB powder and prepare 200 mL of liquid Sabouraud medium (SDB). After configuration, sterilize with high-temperature steam.

Take 10 mL of commercial strain Aspergillus niger and physiological saline solution with a concentration of 15–100 cfu/0.1 mL from a −18 ℃ refrigerator. Take 10 mL of prepared bacterial solution into 200 mL of physiological saline, then add 200 mL of SDB and prepare a mixture of Aspergillus niger and SDB.

After the mixture preparation is completed, immediately start the experiment and carry out mold inoculation.

E.

Building board mold inoculation:

Take 10 mL of the mixture and apply it evenly to the entire surface of each building material board using a coating rod. After the mixture on the board has dried, place the board into each test box. Close the top cover of the test box tightly to ensure its airtightness.

F.

Adjustment of wind speed at the inlet of the test box:

Turn on the fan of the test box system, use a thermal comfort meter to measure the wind speed at the center point of the test box inlet, and adjust the wind speed at the test box inlet through the valve at the inlet. If the wind speed needs to be adjusted to 0 m/s, fully close the valve at the outlet of the test box and fully open the valve at the inlet of the test box.

G.

Laboratory temperature and humidity settings:

After the test box system is set up, close the laboratory door tightly and go to the laboratory control room to set the laboratory temperature and humidity. Laboratory temperature and humidity are controlled by adjusting the dry-bulb and wet-bulb temperatures. During the experiment, the laboratory door remained closed except for sampling.

H.

Sampling and cultivation of mold on building panels:

After the experiment begins, enter the laboratory every 3 days and use SDA contact dishes to conduct contact sampling on each building material. Each sampling should be in contact with different positions of each building material, and the contact time should not be less than 10 s. When sampling, it is necessary to indicate the sampling date, sampling box number, sampling material, and sampler of the contact vessel on the cover. After sampling, incubate in a 26 ℃ incubator for 3 days and count the colony count in the contact dish.

After counting, cover all contact dishes with a sealed bag and indicate the cultivation period before storing them in a 3 ℃ refrigerator for inspection.

I.

End day sampling:

On the end of the experiment, conduct the final sampling and complete the corresponding counting work 3 days later.

J.

Close the laboratory:

Turn off the temperature and humidity control equipment in the laboratory and check for any malfunctions. Turn off the fan of the test box system. Keep the laboratory open for better ventilation.

K.

Cleaning of test boxes and test plates:

After the experiment, use 75% concentration alcohol to spray evenly inside each test box that will be used in the experiment. Use a wet alcohol cloth to wipe off the dust and dirt inside the test box, and then use 84-disinfectant to spray evenly inside the test box. Keep all test boxes open.

The various building materials used in the experiment must first be rinsed and wiped with running water. After rinsing, they soak them in 84 disinfectant for 10–15 min before taking them out and rinsing again with running water. Then, placed them on a shelf for future use in the experiment.

L.

Discard contact dishes after use:

After confirming that the bacterial count on all contact dishes in the experiment is correct, all contact dishes should be uniformly sterilized with high-temperature steam and discarded.

M.

Cultivate colony count analysis and draw experimental result charts.

3. Experiment and Result

3.1. Experimental Platform Construction

According to the experimental system design diagram in Figure 1, we first needed a laboratory that could precisely control internal temperature and humidity. This laboratory provides a controlled and stable temperature and humidity environment for the experimental chamber groups. We built such a laboratory based on the enthalpy differential laboratory from Guangdong Provincial Academy of Building Research Group Co., Ltd.

The enthalpy differential laboratory, also known as the air enthalpy differential method laboratory, was constructed to determine the cooling and heating capacity of air conditioning units based on the principle of the air enthalpy differential method. The components of the enthalpy differential laboratory include the peripheral insulation structure, air handling units, temperature and humidity sampling system, airflow measurement device, laboratory measurement and control system, and data acquisition system.

The peripheral insulation structure serves to obstruct the heat transfer between the internal space of the laboratory and the external environment, as well as between the internal and external sides, reducing heat loss and lowering energy consumption for environmental temperature regulation, thereby demonstrating significant insulation and energy-saving effects. The air handling unit mainly consists of air conditioning cabinets, fans, heaters, humidifiers, refrigeration systems, etc. Its function is to regulate the indoor air state of the enthalpy differential laboratory to meet the required operating conditions during testing.

The temperature and humidity sampling system mainly includes temperature samplers, platinum resistors, sampling fans, temperature transmitters, temperature control instruments, and computer measurement systems. Its function is to collect indoor dry and wet bulb temperatures, outdoor dry and wet bulb temperatures, and outlet dry and wet bulb temperatures, which are fundamental parameters for enthalpy differential method testing. The airflow measurement device comprises an inlet chamber, nozzles, exhaust chamber, exhaust fan, pressure transmitter, frequency converter, static pressure control instrument, connecting hoses, and computer measurement system. Its function is to measure the airflow of the test unit, which is also a fundamental parameter for enthalpy differential method testing.

The control system provides users with a convenient measurement and control operation platform consisting of various measurement and control instruments, transmitters, computers, switches, indicator lights, etc. Its main function is to act as the control center for the operation of the enthalpy differential laboratory, ensuring its normal operation.

The laboratory equipment includes an air volume testing device, direct evaporation unit, compressor condenser unit, humidifier, air sampler, air-cooled chiller (heater), water tank, cooling tower, test unit water pump, evaporator water pump, and condenser water pump. The photograph of each system of the enthalpy differential laboratory is shown in Figure 7.

Utilizing the enthalpy differential laboratory as the foundation for our experimental system offers several advantages. Firstly, the laboratory employs multiple chiller units and humidifiers with linked control, enabling the setting of a wide range of room temperatures (0–40 °C) and humidities (0–100%RH). Secondly, the enthalpy differential laboratory features a highly precise control system, ensuring accurate temperature and humidity control (±0.1 °C, ±2%RH). Thirdly, the laboratory utilizes ceiling diffusers for uniform air distribution, ensuring even airflow organization and temperature–humidity distribution, effectively mitigating the influence of supply airflow on the test area. Fourthly, through the system platform, we can monitor indoor temperature and humidity in real time. Fifthly, the equipment system is comprehensive and fully functional, meeting all requirements without the need for additional configuration, thus reducing experimental costs. Finally, the laboratory is a fully enclosed room with a high-insulation enclosure structure, which prevents the test area from being affected by external environmental conditions.

However, it also has notable drawbacks. The laboratory lacks a safety warning control module, and in the event of excessively high temperatures, there is no alarm or automatic power-off function, posing a safety risk. To address this, we installed additional real-time monitoring devices with remote transmission capabilities for temperature and humidity, as well as cameras for visual real-time remote monitoring, allowing us to monitor the laboratory’s status in real time and take timely action.

Through simple modifications and successful trial operation of the enthalpy differential laboratory, we completed the initial establishment of the experimental platform.

3.2. Installation and Debugging of Test Box

Once the temperature and humidity-controlled laboratory was set up, we first needed to determine the parameters of the exhaust fan in the experimental system. The aim of this experimental system was to simulate indoor ventilation conditions in real-life building environments, thus requiring control over a low airflow velocity range of 0 to 1 m/s. With a fixed radius area of the outlet, the required flow adjustment range for the experimental system was calculated to be 0 to 34.19 m³/h.

Table 3. Calculation of air inlet flow rate.

Wind Speed	Radius of Outlet	Area of Outlet	Rate of Flow
m/s	m	m2	m³/s	m³/min	m³/h	L/min
0.1	0.055	0.0094985	0.00095	0.06	3.42	56.99
1			0.00950	0.57	34.19	569.91
1.5			0.01425	0.85	51.29	854.87
2			0.01900	1.14	68.39	1139.82
2.5			0.02375	1.42	85.49	1424.78
3			0.02850	1.71	102.58	1709.73

presents the calculated outlet flow rates under various inlet airflow velocity conditions.

To achieve adjustable airflow velocity, we installed damper adjustment valves at the inlet of each test box, enabling flexible and convenient airflow regulation. During adjustment, airflow velocity is measured using handheld airflow sensors at the inlet while simultaneously adjusting the valves. To ensure undisturbed airflow during testing, a section of ductwork is installed at the inlet.

When all 12 test boxes reach the required maximum airflow velocity for the project, considering the total airflow requirement at this point, including head resistance and allowing for a 10% reserve airflow. Based on the calculated airflow value, variable frequency exhaust fans were selected.

In the experimental system designed in Figure 1, we proposed to install ultraviolet lamps at the return air outlet of the temperature and humidity control system to sterilize the entire air environment. This is to prevent mold or microbial contamination in the experimental chambers from affecting mold growth. However, the return air outlet of the enthalpy differential laboratory consists of grille doors, and the return air section is the machine room, which has a large area and makes precise sterilization challenging. Therefore, in the later stage, small-capacity ultraviolet lamps were installed on six surfaces within the negative pressure chamber to serve as a disinfection section. This setup ensures that the air in the entire environment remains unaffected by mold in the test boxes, thus avoiding any impact on the experimental results.

The testing chamber of the enthalpy differential laboratory has a floor area of 15 square meters, which exceeds the allowable limit of less than 10 square meters for flat building areas. In this experimental system, it is necessary to arrange equipment such as exhaust fans and static pressure boxes within the limited space of the enthalpy differential laboratory. Additionally, considerations include the resistance of duct elbows, duct head resistance, the position of the outlet of the test box, the orientation of the transparent glass surface of the test box, the spacing between the air supply outlets of each test box, and the spacing between the transparent glass surfaces of each test box.

To address these concerns, we placed the test chambers on a platform, dividing them into upper and lower levels and arranging them on either side of the room. This layout ensures both convenience of operation and accessibility for maintenance of the laboratory equipment.

Figure 8 illustrates the installation and final arrangement of the experimental system.

After activating the exhaust fans of the experimental system, the ultraviolet lamps in the static pressure box were operating normally. With all the covers of the test boxes tightly closed and the inlet adjustment valves of each box opened, we conducted measurements using handheld airflow meters. We confirmed that the inlet airflow velocity of each test box could be adjusted within the required range for the experiment. With this, the installation and commissioning of the test boxes were successfully completed.

3.3. Test Box Mold Growth Experiment

From 26 June 2023, to 10 July 2023, we conducted Experiment 1, which aimed to assess the feasibility of the test box system. The feasibility assessment experiment of the test box system was designed to evaluate the viability of the experimental setup. Concurrently, we observed the impact of inlet airflow velocity on mold growth within the boxes.

During the experiment, the test box system was placed in the prearranged laboratory environment. To ensure the reliability of each experimental result, multiple test boxes with identical conditions should be set up for each trial. In boxes 1 to 3, the inlet airflow velocity was set to 0 m/s; in boxes 5 to 6, it was set to 0.3 m/s; in boxes 7 to 9, it was set to 0.6 m/s; and in boxes 10 to 12, it was set to 1 m/s. Wooden boards inoculated with mold were placed on the bottom surface of each box, while wooden boards coated with latex paint inoculated with mold were placed on the top surface. Each board was inoculated with 10 mL of a mixture of black mold strain and physiological saline or Sabouraud liquid culture medium, with a concentration of 30–100 colony-forming units per 5 mL. Sampling of mold was conducted every three days during the experiment, and the number of mold colonies on the sampling dishes was counted. The laboratory was set to a controlled temperature of 28 °C and humidity of 80%.

The number of mold colonies sampled from each group of test boxes and the sample cultivation of mold samples in the experiment are shown in Figure 9 and Figure 10.

From 8 August 2023 to 22 August 2023, we conducted Experiment 2, which focused on mold growth within the test boxes under high inlet airflow velocity conditions. The objective of this experiment was to observe the growth of mold on building materials inside the boxes under extreme inlet airflow velocity conditions.

During this experiment, the test box system was placed in the prepared laboratory environment. To ensure the reliability of each experimental result, multiple test boxes with identical conditions should be set up for each trial. In boxes 1 to 3, the inlet airflow velocity was set to 1 m/s; in boxes 4 to 6, it was set to 2 m/s; and in boxes 7 to 9, it was set to 0 m/s. Wooden boards inoculated with mold were placed on the bottom surface of each box, while latex boards inoculated with mold were placed on the top surface. Each board was inoculated with 10 mL of a mixture of black mold strain and physiological saline or Sabouraud liquid culture medium, with a concentration of 30–100 colony-forming units per 5 mL. Sampling of mold was conducted every three days during the experiment, and the number of mold colonies on the sampling dishes was counted. The laboratory’s temperature and humidity control were activated, with the temperature set to 28 °C and humidity set to 80%.

The number of mold colonies sampled from each group of test boxes and the sample cultivation of mold samples in the experiment are shown in Figure 11 and Figure 12.

Through two formal experiments, we demonstrated the feasibility of the experimental system. Our constructed experimental setup facilitates the normal growth of mold inoculated on building materials within the test boxes, yielding favorable experimental outcomes.

4. Discussion

To better analyze the experimental data from the two trials, we first processed the data obtained. Since the number of experimental chambers under identical conditions was at least two, we calculated the mean colony counts from samples in each group of Experiments 1 and 2 to ensure the reliability of the results. This provided us with the average colony growth for each group. Consequently, we summarized the colony counts on wooden boards and latex-painted wooden boards under different inlet air velocities. The summarized results are shown in Figure 13 and Figure 14. This method allows us to observe the differences in mold growth on wooden boards and latex-painted wooden boards under varying inlet air velocity conditions more clearly.

By comparing the area of the petri dish to the area of the experimental board, we can easily calculate the total colony count on the experimental board based on the colony count on the petri dish. Using Equation (1), we can determine the mold growth rate for each sampling stage. Here, ${Sp}_d$ represents the daily colony growth rate (cfu/day), $P_d$ is the colony count at each sampling stage, $s_b$ is the area of the experimental board, $s_c$ is the area of the petri dish, and $d$ is the total number of days of cultivation.

$${Sp}_d=\frac{P_d\times \frac{s_b}{s_c}}{d}$$

(1)

Following the calculation using Equation (1), we obtained the variation in colony growth rates on wooden boards and latex-coated wooden boards, as shown in Figure 15 and Figure 16.

Based on the data obtained from the test boxes in these two experiments, we observed that the inlet air velocity affects the growth rate of mold inside the experimental chambers. In a high-humidity environment at 28 °C and 80% relative humidity, boxes with higher inlet airflow velocities exhibited faster mold growth rates. The mold growth rate in boxes with higher inlet airflow velocities consistently remained at a higher level, which is markedly different from boxes with lower inlet airflow velocities or boxes with zero inlet airflow velocities.

From the comparison of the two images in Figure 14 and the two images in Figure 15, we can observe that different building materials and positions have an impact on the growth rate of mold. In both experiments, the wooden boards placed at the bottom of the test boxes exhibited higher mold growth rates compared to the latex-coated wooden boards placed at the top of the boxes. This elevated growth rate resulted in a significant presence of mold colonies on the wooden boards placed at the bottom of the boxes. Towards the later stages of both experiments, sampling personnel observed visible mold colonies on the wooden boards placed at the bottom of the boxes.

The observed influence of the placement position of building materials under laboratory conditions may be attributed to the combined effects of the humidity microenvironment within the test boxes and the surface characteristics of the materials. Under high humidity conditions, the wooden boards at the bottom of the test boxes possess rough and porous surfaces, and the boards themselves can serve as a nutrient source for mold. This may facilitate the attachment of mold to the boards and promote its growth. Conversely, latex paint has a smoother surface, which may limit the attachment and spread of mold. Additionally, there may be differences in humidity levels between the top and bottom surfaces of the boxes. These disparities suggest the importance of selecting suitable materials and coatings in architectural design, as well as accurately controlling the humidity in specific areas for the prevention and control of indoor mold.

Our findings regarding the influence of airflow velocity on mold growth raise important considerations for indoor ventilation design in the Guangzhou region. Traditional views suggest that ventilation helps reduce indoor mold levels [2,9,40,41]. However, current test box studies show that under high outdoor humidity conditions, increasing the inlet airflow velocity actually promotes mold growth. This finding aligns with the conclusions drawn from research conducted in Ireland [30]. This may be due to the increased velocity causing spores to spread more widely within the box or because enhanced airflow allows high-humidity air from outside to diffuse better into the box, providing favorable conditions for mold growth. This finding suggests that, in indoor environmental control, simply increasing ventilation may not necessarily effectively prevent mold contamination, and a more comprehensive consideration of the interactive effects of airflow and other environmental factors is warranted. In future research, we will conduct additional similar experiments to further validate this finding.

In summary, the results of this study provide new insights into the indoor environmental design of buildings in southern China, especially regarding the control of indoor ventilation airflow velocity. Further empirical research will provide scientific evidence for the formulation of best practices in ventilation and indoor air quality management.

5. Conclusions

This study presents the design and successful establishment of a laboratory capable of simulating the influences of various factors on indoor mold growth. Preliminary experiments were conducted to investigate the effects of building materials, material placement, and inlet air velocity on mold growth. The experimental system designed and constructed allows precise control of temperature and humidity within the laboratory, as well as accurate adjustment of the inlet air velocity for each test box. Different building materials can be placed on the bottom, sides, and top of the chambers for mold growth simulation experiments. Our validation confirms that mold can grow in our simulated experimental system, yielding reliable experimental results. This experimental setup will serve as the foundation for our future research and contribute significantly to the study of mold growth in different regions and environments.

Preliminary experimental results indicate that the type and placement of building materials on the floor and ceiling affect mold growth rates, with mold growth being faster on wooden boards placed on the bottom surface compared to those coated with latex paint on the ceiling. Additionally, mold growth rates increase with higher inlet air velocities, with significant differences observed between conditions of 1 m/s and 2 m/s compared to stagnant air environments. This finding contrasts with previous research findings. In our future research, this conclusion needs to be thoroughly examined.

Our study reveals the complex effects of indoor environmental factors on mold growth and suggests new considerations for building design and indoor environmental management. Reassessing ventilation strategies, particularly in airflow control, holds practical significance for reducing mold growth in southern China. Furthermore, the selection and treatment of building materials directly influence mold growth conditions; hence, while ensuring indoor air quality, careful consideration should also be given to the arrangement of indoor materials.

Although this study provides insights, further experiments under different temperature and humidity conditions and a broader range of environmental parameters are needed due to differences between laboratory conditions and actual building environments. Also, we have not yet monitored the air conditions inside the experimental chamber, nor have we assessed their potential impact on the health of occupants. In the future, we will expand our research scope to explore how ventilation and other environmental factors such as temperature, humidity, and light influence mold growth. Additionally, we will attempt to measure the air conditions inside the test box. This will provide a more solid scientific basis for optimizing building design and indoor air quality control strategies in southern China, laying the groundwork for creating healthier indoor spaces.