Efficacy of Ventilation Air Purifiers in Improving Classroom Air Quality: A Case Study in South Korea

Abstract

Indoor air quality (IAQ) in schools significantly affects health and academic performance; however, effective interventions for poor air quality remain limited, particularly in settings with restricted natural ventilation. This study evaluated the effectiveness of ventilation-type air purifiers in improving classroom IAQ in a South Korean elementary school. PM₁₀, PM_2.5, and CO₂ concentrations were monitored over 18 days (14–31 May 2021) in two classrooms—one equipped with a ventilation-type air purifier and the other serving as a control. In the classroom with the air purifier, daily average concentrations of PM₁₀, PM_2.5, and CO₂ decreased by 23.7%, 22.8%, and 21.1%, respectively, from baseline levels. The air purifier effectively reduced pollutant infiltration during periods of severe outdoor air pollution and stabilized pollutant levels during active class hours. Its efficacy was particularly prominent under conditions of restricted natural ventilation, high indoor activity, and fluctuating outdoor pollution levels. IAQ varied significantly between weekdays and weekends; pollutant levels were higher on weekdays due to occupancy and classroom activities, whereas weekends exhibited reduced concentrations. These findings suggest that ventilation-type air purifiers provide a viable strategy for improving IAQ in schools with limited ventilation. Future research should examine their long-term performance across different seasons and architectural settings.

1. Introduction

Indoor air quality (IAQ) is a critical public health concern, particularly for children, who spend a substantial amount of time in enclosed environments such as classrooms. Children are more vulnerable to indoor pollutants due to their higher breathing rates, greater air intake relative to body weight, and developing physiological systems [1,2]. They inhale approximately twice as much air per unit of body weight as adults, increasing their susceptibility to pollutant exposure [3,4]. Consequently, IAQ plays a crucial role in safeguarding their healthy development and optimizing their learning capabilities.

The importance of IAQ is particularly evident in classrooms, where children spend an average of 6–8 h per day [5]. However, classrooms are especially prone to poor IAQ due to insufficient ventilation, indoor activities, and the infiltration of outdoor pollutants.

Furthermore, recent research [6] highlights the challenges of relying solely on natural ventilation to maintain adequate IAQ, pointing out that it often fails to effectively regulate CO₂ levels, which are crucial for maintaining cognitive function and overall health in classroom settings.

According to the World Health Organization (WHO), over 90% of children worldwide are exposed to harmful air daily, which affects them more severely than adults due to their underdeveloped respiratory systems [7,8].

Classroom IAQ directly affects children’s health and academic performance, making its effective management critically important. In addition, a systematic review has shown that CO₂ levels in naturally ventilated classrooms can impair cognitive function significantly [9], highlighting the need for integrated solutions that combine natural and mechanical ventilation systems to maintain a healthy learning environment [10].

IAQ influences concentration levels and learning abilities in classroom environments. For instance, studies have shown that elevated CO₂ levels are associated with reduced cognitive function, thereby negatively impacting academic outcomes [11]. Additionally, fine particulate matter (PM), such as PM₁₀ and PM_2.5, can lead to respiratory illnesses, allergies, and weakened immune responses, further affecting the overall learning environment and student attendance [12]. Inadequate ventilation in classrooms increases the concentrations of PM₁₀, PM_2.5, CO₂, and airborne bacteria. Activities such as physical exercise can resuspend particles, while increased breathing during these activities further elevates CO₂ levels.

Furthermore, poor building sealing or ventilation without consideration of external air pollution levels allows outdoor pollutants to infiltrate indoor spaces, worsening IAQ. Such conditions are associated with health issues, including headaches, fatigue, and dizziness, as reported in [12,13]. Classrooms with insufficient ventilation contain elevated concentrations of indoor pollutants, such as PM₁₀, PM_2.5, CO₂, and airborne bacteria. The levels of these pollutants can be increased by children’s activities.

The COVID-19 pandemic heightened awareness of air quality and ventilation, emphasizing the need for systematic and effective measures to manage IAQ. Improving and properly managing classroom IAQ is essential for both protecting children’s health and enhancing their academic performance [14,15].

Previous studies have primarily focused on measuring IAQ, specifically pollutants such as CO₂, particulate matter, and volatile organic compounds, as well as their significant impacts on student health and learning outcomes. For example, a systematic review [10] examined the effects of CO₂ levels in naturally ventilated classrooms on cognitive function and respiratory health. Similarly, a study [9] compared teachers’ perceptions of air quality with actual IAQ conditions, highlighting discrepancies in pollutant concentrations. Additionally, another study [16] emphasized the inadequacy of natural ventilation in reducing pollutant levels in schools and advocated for necessary improvements in IAQ management within educational environments.

However, research evaluating the combined effects of ventilation and air purification technologies in real classroom settings remains limited. Despite advancements in air purification technology, classroom environments present unique challenges. These environments are significantly influenced by children’s activities and varying seasonal conditions, which traditional ventilation methods often fail to address comprehensively. Recent studies have highlighted the necessity of integrating both mechanical and natural ventilation systems to effectively manage a broader range of indoor pollutants, including CO₂, which traditional air purifiers often fail to mitigate.

Our research aims to fill this gap by exploring the effectiveness of ventilation-type air purifiers, which are more affordable and adaptable to various educational settings than full-scale mechanical systems. Additionally, studies analyzing the practical effectiveness of air purifiers in enclosed environments, such as classrooms, and developing optimal strategies to improve IAQ remain scarce.

Recent studies have reported that air purifiers effectively reduce indoor pollutant concentrations [17,18,19]. For example, research conducted in residential and office settings has demonstrated their efficacy in removing PM, such as PM₁₀ and PM_2.5. However, such studies have rarely examined dynamic environments like classrooms, where children’s activities and outdoor pollutant infiltration significantly influence IAQ.

The U.S. Environmental Protection Agency (EPA) has reported that ventilation reduces pollutant concentrations, including viruses, by introducing outdoor air into indoor spaces [20]. Inadequate ventilation in densely populated environments, such as schools, increases CO₂ levels, which are associated with higher pathogen concentrations and elevated infection risks [21].

According to a 2021 survey by the Ministry of Education of Korea, 82.4% of schools use air purifiers designed specifically for dust removal, whereas only 2.8% are equipped with mechanical ventilation systems. South Korean schools often rely on manual ventilation through open windows due to the absence of centralized air control systems. This dependence poses challenges for maintaining IAQ, particularly under varying seasonal conditions. For example, natural ventilation is limited during winter and spring due to high levels of outdoor PM and during summer to preserve cooling efficiency. As a result, indoor pollutants, including PM₁₀, PM_2.5, and CO₂, accumulate [22].

Ventilation-type air purifiers offer a unique solution to these challenges by introducing fresh outdoor air while filtering particulates, thereby improving IAQ. Their dual functionality is particularly suitable for South Korean schools, where natural ventilation is constrained [23]. However, their adoption involves economic considerations, such as installation costs, energy consumption, and maintenance expenses. These factors highlight the importance of evaluating their practical effectiveness and balancing IAQ benefits with operational costs.

For example, the initial purchase and installation cost of these ventilation-type air purifiers is approximately USD 1376, with an annual maintenance cost of approximately USD 69 for consumable replacements. These economic considerations are pivotal in the decision-making process for schools regarding the adoption of such technology, as they directly influence budgeting and long-term financial planning.

Therefore, in this study, we assessed the performance of ventilation-type air purifiers in real classroom environments and investigated their potential to address the limitations of conventional ventilation and air purification methods. A previous study noted that ventilation-type air purifiers effectively reduce CO₂ concentrations, making them superior to conventional air purifiers [24].

High CO₂ levels, primarily generated by breathing, can cause reduced concentration, fatigue, and headaches. Although conventional air purifiers effectively remove PM, their ability to lower CO₂ levels is limited. Ventilation-type air purifiers mitigate this limitation by introducing fresh outdoor air.

Furthermore, beyond merely improving IAQ, ventilation-type air purifiers play a key role in fostering a healthier learning environment. Unlike previous studies that primarily focused on reducing PM concentrations, we aimed to examine CO₂ reduction, a critical factor in maintaining a healthy classroom setting. Given the high occupancy and limited natural ventilation in school classrooms, this study provides valuable insights into air purification strategies for educational environments.

Our investigation was conducted in a South Korean elementary school, representative of typical conditions where traditional ventilation methods (primarily manual window ventilation) are often insufficient, particularly during extreme seasonal conditions such as winter and summer [25]. Pollutant concentrations, including PM₁₀, PM_2.5, CO₂, and airborne bacteria, were measured in classrooms with and without air purifiers during active school hours.

We examined the correlation between IAQ and outdoor air conditions within classrooms, analyzed time-dependent fluctuations in pollutant concentrations across different child activity periods (including class hours, breaks, and lunchtime), and compared pollutant levels on weekdays and weekends to evaluate the impact of children’s presence on IAQ.

Our analyses aimed to validate the performance of ventilation-type air purifiers, highlight the importance of improving classroom environments, and address gaps in both research and policy. Our findings provide evidence-based insights into IAQ improvement strategies, supporting informed decision-making for enhancing IAQ in educational settings worldwide. By effectively reducing CO₂ levels, ventilation-type air purifiers can help prevent cognitive decline and enhance academic performance [26,27]. Evaluating their practical effectiveness in real classroom settings is essential for developing evidence-based strategies to optimize IAQ management in schools. The development and validation of these strategies are detailed in

Table 1. Technology roadmap for indoor air quality improvement in schools.

Stage	Activities
Problem recognition	Recognition of indoor air quality in schools and limitations of conventional air purifiers Recognition of the limitations of natural ventilation through windows
Technology requirements	Identification of technical requirements for managing indoor pollutants (PM10, PM2.5, and CO2)
Current technology assessment	Evaluation of existing ventilation air purifiers on the market
Technology development goals	Setting goals for enhanced indoor air quality improvement performance under various conditions and stable air quality management
Development stages	Development and testing of the initial ventilation air purifier prototype Performance improvement and testing under various environmental conditions Final product development and validation
Results and plans	Evaluation of the effectiveness and market applicability of the developed technology Directions for future research and development

This table provides an overview of the development stages of air purification technology for improving indoor air quality in schools. Each stage outlines the process from problem recognition to the development of related technology and future directions for technological advancement, detailing the necessary activities and goals.

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2. Methods

2.1. Study Site and Participants

This study was conducted at a South Korean elementary school located at 37.8344° N, 127.1451° E (Figure 1). Figure 1 and Figure 2 illustrate the study site, classroom layout, air purifier placement, and school environment. The daily schedule of students, including class hours and breaks, is presented in

Table 2. Daily schedule of classes, breaks, and lunch for children at the observed elementary school. The schedule consists of four 40 min class periods, with 10 min breaks between each class. Lunchtime begins immediately after the third period. Meals in the cafeteria were prepared from 10:00 to 11:00.

Class and Lunch	Class Time	Break Time	Cooking Time
1st class	09:00–09:40	09:40–09:50
2nd class	09:50–10:30	10:30–10:40	10:00–11:00
3rd class	10:40–11:20
Lunch break	11:20–12:10
4th class	12:10–12:50	12:50–13:00
5th class	13:00–13:40

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The four-story building housed administrative offices and a cafeteria on the first floor, while classrooms occupied the second to fourth floors. It was representative of a typical South Korean elementary school in terms of building design, classroom size, and student density. Two classrooms on the third floor, adjacent to the left staircase, were selected for this study. One classroom was equipped with a ventilation-type air purifier (Classroom 1), while the other lacked an air purifier and served as the control (Classroom 2). Each classroom measured 58.5 m², and both were similar in terms of location, student capacity (25 students in the air-purified classroom and 26 in the control classroom), and the students’ ages.

The classrooms were selected based on their comparable ventilation characteristics, building location, and exposure to outdoor air. Their adjacency minimized differences in airflow patterns and external air exposure. The layout of Classroom 1, including the placement of the air purifier, is shown in Figure 2, where the purifier is represented by a red box near the rear window on the left side of the classroom.

Classes ran from 09:00 to 13:40, with each class lasting 40 min, followed by a 10 min break and a lunch break after the third class. In line with typical Korean practices, cafeteria meal preparation began two hours before lunch, and these activities were considered in the analysis due to their potential influence on IAQ. The ventilation-type air purifier operated from 09:00 to 14:00, covering the full school activity period.

The study was conducted over 18 days, from 14 to 31 May 2021, ensuring stable seasonal climatic conditions and avoiding abrupt changes in outdoor air quality or temperature that could affect the results. Additionally, the study period was selected to align with the school’s academic schedule during the COVID-19 pandemic and to avoid major external environmental disturbances, such as nearby construction activities, ensuring that the results were not skewed.

To minimize external confounding variables, outdoor air quality data—including PM₁₀, PM_2.5, and CO₂ concentrations—were obtained from publicly available datasets and continuously monitored. Weather conditions, such as temperature and precipitation, were tracked to assess their potential impact on IAQ. Additionally, student activities and behaviors were observed and documented to ensure consistency between classrooms during data collection.

2.2. Ventilation-Type Air Purifier

The ventilation-type air purifier is a device that simultaneously performs both air purification and ventilation functions. Unlike conventional air purifiers, which recirculate indoor air to remove pollutants, the ventilation-type air purifier introduces fresh outdoor air while purifying indoor air. This dual functionality effectively improves IAQ and reduces pollutant concentrations. In Korea, a development project for ventilation-type air purifiers was initiated to enhance IAQ in enclosed spaces, such as schools.

The ventilation-type air purifier used in this study was designed to reduce PM_2.5, PM₁₀, and CO₂ concentrations at an airflow rate of 150 m³ h⁻¹. Related studies have indicated that air purifiers are more effective in enclosed buildings and that IAQ improves more rapidly when outdoor PM_2.5 concentrations are low [26]. The ventilation-type air purifier employed in this study enhances IAQ by combining mechanical filtration with forced ventilation. These devices utilize high-efficiency particulate air (HEPA) filters, effectively capturing up to 99.97% of airborne particles measuring 0.3 microns or larger from both indoor and outdoor sources. The purifiers actively introduce and filter fresh outdoor air, which is crucial during high pollution events to reduce indoor contaminant levels. This system recirculates air while simultaneously introducing fresh, filtered air, maintaining energy-efficient ventilation at a rate of 150 m³ h⁻¹ to optimize air exchange without excessive energy use.

The air purifier was installed on the classroom window, filtering both indoor and outdoor air through a built-in system to ventilate the space. It was equipped with a high-efficiency particulate air filter and an ultraviolet-C (UV-C) LED lamp for virus removal. The filter had an efficiency of 99.95% with a pressure drop of 3.4 mm H₂O.

2.3. Measurement Parameters and Equipment

DustTrak II Aerosol Monitors (TSI Incorporated, Shoreview, MN, USA) were used to measure PM (PM₁₀ and PM_2.5), and CO₂ concentrations were measured using an IQ-610Xtra (Graywolf Sensing Solutions, Shelton, CT, USA). Before the study, the instruments were calibrated according to the manufacturer’s standard protocols to ensure the accuracy and reliability of the measurements. Specific details regarding the accuracy of the equipment used are provided in

Table 3. Specifications of monitoring equipment used in study.

Model	Dust Trak II 8530	IQ-610Xtra
Measured particles	0.1–10 μm	Gas
Range	0.001–400 mg/m3	0.1–10,000 ppm
Accuracy	±0.1% or 0.001 mg/m3	±3% of reading, ±50 ppm
Response time	Not specified (Measures aerosol mass in real-time)	90% response < 1 min

. Data collection was conducted under consistent conditions to enhance accuracy. Specifically, particulate matter sensors were calibrated using zero calibration with HEPA filters and span calibration with polydisperse aerosol standards simulating ambient air conditions, ensuring high measurement accuracy. Calibration and sensor adjustments were performed rigorously, and the sensors underwent daily stability checks. Any deviation exceeding the specified tolerance (±3%) by the manufacturer triggered recalibration, ensuring consistent data accuracy throughout the 18-day monitoring period. The precision of the DustTrak II and IQ-610Xtra instruments was ±0.1% and ±3% of the measured values, respectively, confirming their suitability for IAQ monitoring. External air quality data from the Korea Environment Corporation and Korea Meteorological Administration were analyzed in real-time, and a weighted averaging method was applied to ensure time synchronization.

Airborne bacteria were measured using a KAS-120 (KEMIK Corporation, Seongnam, Republic of Korea) at a flow rate of 1 CFM. The measurement instruments were installed on the rear wall of the classroom, 2 m away from the ventilation-type air purifier. This location was selected to capture the average concentration of purified air diffused throughout the classroom. The instruments were carefully positioned away from windows, doors, and high-activity areas to minimize bias caused by localized airflows, ensuring that the collected data accurately reflected overall classroom air quality.

2.4. Indoor and Outdoor Air Quality Data Collection

Indoor air pollutant concentrations fluctuate in response to children’s activities and increased respiration rates [27,28]. In the present study, we primarily focused on variations in PM₁₀, PM_2.5, and CO₂ concentrations to assess the impact of ventilation-type air purifiers on IAQ. The pollutants were measured every 5 min from 09:00 to 14:00 throughout the study period. This approach facilitated a quantitative analysis of the effects of child activities, cooking processes, and fluctuations in outdoor air quality on indoor pollutant levels, as well as differences between weekdays and weekends.

Contrastingly, airborne bacteria were measured on a single day as a supplementary analysis to observe general trends rather than as a primary focus of the study. This aligned with our objective of evaluating the effectiveness of ventilation-type air purifiers in reducing particulate matter and CO₂ levels while also examining the impact of child activities and cooking on airborne bacterial concentrations. Although airborne bacteria measurements were conducted for only one day, nine repeated samples ensured data reliability.

However, IAQ, particularly PM₁₀, PM_2.5, and CO₂ levels, is influenced both by indoor activities and outdoor air quality [28]. Outdoor air enters the classroom through doors and windows, affecting indoor air conditions. In this study, outdoor PM_2.5 and PM₁₀ concentration data were obtained for the specific dates corresponding to IAQ measurements from the Korea Environmental Corporation (https://www.airkorea.or.kr/, accessed on 9 October 2023). Similarly, outdoor CO₂ concentration data for the same dates were sourced from the Korea Meteorological Administration (https://data.kma.go.kr/, accessed on 9 October 2023). Outdoor air quality data were provided at 1 h intervals. To synchronize these data with the 5 min interval indoor measurements, a time-weighted averaging method was applied.

To enhance the validity of the analysis, major external environmental factors, including temperature, humidity, and precipitation, were collected from the Korea Meteorological Administration and incorporated into the analysis. A preliminary investigation was also conducted to identify potential external influences, such as nearby construction activities and variations in traffic volume. The investigation confirmed that no significant external events occurred during the study period that could have influenced the results.

2.5. Data Analysis Methods

This study aimed to evaluate the effectiveness of ventilation-type air purifiers in reducing indoor concentrations of particulate matter (PM₁₀, PM_2.5) and CO₂ in classrooms. To achieve this, the following analytical procedures were employed:

• Daily Average Comparisons: Daily average concentrations of PM₁₀, PM_2.5, and CO₂ were compared to assess the influence of outdoor air on IAQ.
• Time-Dependent Changes: Air quality was analyzed during different child activity periods, such as class hours, breaks, and lunchtime, to evaluate the impact of child activities on pollutant concentrations.
• Weekday vs. Weekend Analysis: Changes in IAQ were compared between weekdays and weekends to assess the influence of child presence.

3. Results and Discussion

3.1. Analysis of Impact of Indoor and Outdoor Air Quality

Figure 3 shows the daily average PM₁₀, PM_2.5, and CO₂ concentrations recorded during the study period, illustrating the differences between outdoor levels and those in classrooms with and without ventilation-type air purifiers. Over the 18-day observation period from 14 to 31 May 2021, Classroom 1 exhibited reductions of 23.7%, 22.8%, and 21.1% in PM₁₀, PM_2.5, and CO₂ concentrations, respectively. These results were consistent with the observed temporal variations in IAQ, which revealed more pronounced pollutant reductions on weekends due to lower occupancy rates. The PM₁₀ concentration in Classroom 1 was 14.5 µg/m³, whereas in Classroom 2, it was 19.0 µg/m³ (Figure 3A). During high yellow dust periods (24–26 May), PM₁₀ levels in Classroom 1 remained significantly lower (see Figure 3A).

Classroom 1 exhibited a 38% reduction in PM₁₀ concentration compared to the outdoor concentration (23.2 µg/m³), whereas Classroom 2 showed an 18% reduction. This indicated that the ventilation-type air purifier provided an additional 20% reduction in indoor PM₁₀ concentrations compared to relying solely on natural ventilation.

This reduction rate remained consistent throughout the study, with standard deviations of 1.2 µg/m³ and 1.5 µg/m³ for Classrooms 1 and 2, respectively. The 95% confidence intervals for the mean reductions were as follows:

• Classroom 1: 14.5 µg/m³ ± 0.8 µg/m³.
• Classroom 2: 19.0 µg/m³ ± 1.0 µg/m³.

These results underscore the significant role of the ventilation-type air purifier in reducing indoor PM₁₀ concentrations, providing an approximately 20% additional reduction compared to classrooms relying solely on natural ventilation.

Building on the reductions in PM₁₀, PM_2.5, and CO₂ concentrations documented here, a comparative analysis was conducted using settings similar to those outlined in previous studies [6,16]. A previous study [6] highlighted the role of natural ventilation strategies in enhancing IAQ across various classroom activities, providing an analysis of different ventilation approaches. Similarly, another study [16] investigated the effectiveness of various air quality management strategies in different classroom settings and environmental conditions. However, the specialized dual functionality of the ventilation-type air purifiers used in our experiments (combining efficient particulate removal with the introduction of fresh air) facilitated superior management of CO₂ and particulate matter concentrations compared to the simpler natural ventilation methods discussed in these studies. The integration of advanced air purification technologies is particularly crucial in classrooms with limited natural ventilation options, especially in high-density settings.

We also compared the temperature and humidity levels in the two classrooms. Classroom 1 maintained an average temperature of 25.2 °C (range: 22.3–26.2 °C) and 33.4% RH (range: 23.0–44.3% RH). In contrast, Classroom 2 recorded an average temperature of 25.7 °C (range: 22.3–26.7 °C) and 33.2% RH (range: 26.0–43.6% RH). These results suggest that the impact of ventilation-type air purifiers on classroom temperature and humidity is minimal. Consequently, the ability of these devices to regulate temperature and humidity appears limited, indicating that additional interventions are required to improve the classroom environment.

During the period of high yellow dust (24–26 May), outdoor PM₁₀ concentrations increased to a maximum of 64.1 µg/m³. However, PM₁₀ levels in Classroom 1 remained low, ranging between 16.4 and 16.7 µg/m³, whereas those in Classroom 2 ranged between 20.4 and 22.9 µg/m³. These results suggest that closing windows to prevent the entry of heavily polluted outdoor air, combined with the use of an air purifier, effectively maintained low indoor PM₁₀ levels.

The PM_2.5 concentrations in Classrooms 1 and 2 were 14.2 and 18.4 µg/m³, respectively. In contrast, the outdoor PM_2.5 concentration was 13.5 µg/m³, which was lower than both indoor values. The higher indoor PM_2.5 levels can be attributed to the resuspension of dust caused by child activities and indoor sources, such as PM from desks and classroom materials.

The ventilation-type air purifier effectively mitigated elevated PM_2.5 levels, maintaining concentrations 30% lower in Classroom 1 than in Classroom 2. This finding highlights the purifier’s ability to address indoor PM_2.5 sources, including those generated by occupant activities.

These results emphasize that indoor PM_2.5 concentrations in classrooms are influenced both by outdoor air quality and indoor pollutant sources. The data further demonstrate that during periods of increased outdoor PM_2.5 concentrations (24–26 May), both classrooms maintained relatively low PM_2.5 levels due to closed windows. However, this passive ventilation strategy has limitations, particularly when indoor pollutant sources (e.g., child activities) contribute to elevated PM_2.5 levels. Notably, passive ventilation is less effective during the winter and spring months in Korea, as outdoor PM₁₀ and PM_2.5 concentrations frequently exceed air quality standards. Thus, ventilation-type air purifiers are essential for maintaining IAQ under such conditions [29,30].

The average CO₂ concentration in Classroom 1 was 21.1% lower than that in Classroom 2 (Figure 3C). This demonstrates that the purifier effectively mitigated CO₂ accumulation and ensured superior air quality by introducing fresh outdoor air, which cannot be achieved by conventional air purifiers. This result is particularly significant in classroom settings where natural ventilation is limited [29]. Classroom 1 consistently maintained CO₂ concentrations below the Korean IAQ standard of 1000 ppm, whereas Classroom 2 exceeded or approached this threshold on three occasions. These instances were likely caused by the increased respiration rates of children.

3.2. Temporal Variations in IAQ

Classroom IAQ fluctuates significantly depending on children’s activities [31,32]. To account for this variability, we analyzed changes in air quality during different periods (class hours, breaks, and lunchtime) to evaluate the effectiveness of the ventilation-type air purifier. Hourly variations were examined using data from 17 May and 21, as these dates had lower outdoor pollutant concentrations compared with those of other days in the study period, minimizing their impact on IAQ.

Figure 4 displays the hourly variations in PM₁₀, PM_2.5, and CO₂ concentrations on May 17, highlighting the impact of class activities and ventilation effectiveness.

Temporal variations in PM concentrations were observed in Classrooms 1 and 2 on 17 May 2021. From the beginning of C1 until C2, Classroom 1 consistently maintained lower PM concentrations than Classroom 2, with a peak difference of 11 µg/m³ in PM₁₀. However, during C3 and lunchtime, PM₁₀ and PM_2.5 concentrations in Classroom 1 increased, mirroring a similar rise in Classroom 2. This increase was likely caused by the opening of classroom doors, which allowed polluted air from hallways to enter, as well as the rise in outdoor PM₁₀ and PM_2.5 concentrations during this period.

Notably, during this time, PM concentrations in Classroom 1 briefly exceeded those in Classroom 2. This suggests that opening the doors allowed pollutants, including cooking fumes from the cafeteria, to enter the classroom, thereby reducing the effectiveness of the purifier. During C4, when doors were closed and classes resumed, PM concentrations in Classroom 1 decreased significantly, coinciding with lower outdoor PM levels. In contrast, concentrations in Classroom 2 remained elevated.

Toward the end of the school day, PM concentrations in both classrooms increased again due to dust resuspension and the opening of entrance doors as children departed, allowing the entry of outdoor air.

In summary, the ventilation-type air purifier effectively reduced indoor PM concentrations under controlled conditions. However, its efficiency was diminished by external factors, including door openings, hallway pollutants, and cooking fumes. This highlighted the importance of managing environmental factors, such as ventilation practices, cooking emissions, and door usage, to maintain optimal IAQ in classrooms.

Classroom 1 maintained an average CO₂ concentration of 790 ppm, consistently lower than the 965 ppm recorded in Classroom 2. However, during C2, CO₂ concentrations in Classroom 1 were temporarily higher than those in Classroom 2. This anomaly was likely due to the windows being closed in Classroom 1, which caused CO₂ to accumulate from respiration. In contrast, the windows were open in Classroom 2, enabling natural ventilation to lower CO₂ concentrations.

Additionally, many school cafeterias in Korea are located on the lower floors of buildings, where pollutants generated during food preparation are not fully ventilated by hood systems. These pollutants tend to flow into stairways and hallways, affecting nearby classrooms. Previous studies have demonstrated that pollutant emissions during cooking are influenced by factors such as temperature, ingredients, and cooking methods [29,33].

Except during C2, CO₂ concentrations in Classroom 1 were, on average, 18% lower than those in Classroom 2. In Classroom 2, CO₂ levels primarily increased during class hours, likely due to children’s higher breathing rates in a closed environment. This was particularly noticeable compared to periods such as lunch or break times, when doors were open, facilitating greater ventilation.

Figure 5 shows the temporal changes in air quality concentrations from 09:00 to 14:00 on 21 May 2021. During school hours, outdoor PM₁₀ and PM_2.5 levels exceeded indoor concentrations, with Classroom 1 consistently maintaining lower pollutant levels compared to Classroom 2.

During class hours, the average concentrations of PM₁₀ and PM_2.5 in Classroom 2 were 16.6 µg/m³ and 9.6 µg/m³, respectively. Classroom 1 showed reductions of 29.5% and 26% in PM₁₀ and PM_2.5 concentrations, respectively. During break times, Classroom 1 achieved reductions of 28%, 20.6%, and 39% for PM₁₀, PM_2.5, and CO₂, respectively.

Our findings on the efficacy of ventilation-type air purifiers in reducing PM₁₀, PM_2.5, and CO₂ concentrations demonstrate significant improvements in IAQ. However, similar studies have shown varied results, reflecting differences in environmental conditions and purifier technologies. For instance, a study [10] observed that mechanical filtration systems in classrooms did not significantly reduce PM levels compared to those using natural ventilation methods in Dutch schools. This discrepancy can be attributed to differences in ventilation methods, variations in outdoor air quality, and differences in air purifier performance. In contrast, another study [5] found that mechanical ventilation systems effectively reduced PM levels in classrooms near Amsterdam’s highways, suggesting their heightened efficacy in areas with high outdoor pollution.

Our results were particularly significant, as they demonstrated the additional benefit of reducing CO₂ levels, a feature often overlooked by traditional air purification systems. This capability is attributed to the incorporation of fresh outdoor air, contrasting with the findings in [29], where only PM levels were significantly affected.

These comparisons validate the effectiveness of the ventilation-type air purifiers used here and underscore the importance of considering various pollutants, including CO₂, when assessing air purifier performance in educational settings.

Notably, PM concentrations in Classroom 2 temporarily increased during C2 and C3. This rise was likely due to the infiltration of PM generated by cooking activities in the cafeteria, which entered the classroom through open doors. However, despite these external influences, Classroom 1 maintained superior IAQ compared to Classroom 2.

In summary, Classroom 1 consistently maintained lower PM concentrations than Classroom 2 throughout the observation period. These results highlight the critical role of air purifiers in improving IAQ under varying conditions.

The CO₂ concentration in Classroom 2 showed an initial increase during C1, followed by a sharp increase between C2 and C3, and subsequently decreased abruptly after C3. The initial increase during C1 was attributed to children’s respiration in an enclosed space. During C2 and C3, cooking activities in the cafeteria likely contributed to elevated CO₂ levels. The natural ventilation achieved by opening windows in the classroom without the purifier led to a rapid reduction in CO₂ concentrations.

In Classroom 1, CO₂ concentrations remained steady during C1, increased during C2 and C3, and then decreased sharply at lunchtime. This pattern suggests that the CO₂ increase during class hours was primarily caused by respiration, whereas the decrease during lunchtime occurred as outdoor air entered through open doors, facilitating ventilation.

These findings underscore the potential of ventilation-type air purifiers to improve IAQ and create a healthier learning environment. However, further investigation across diverse settings and varying environmental conditions is needed to validate these results. As such, this study provides a foundation for future research exploring the broader implications of ventilation-type air purifiers on child health and educational outcomes [34,35].

Suspended bacteria significantly influence IAQ and pose health risks to children [35]. Effective management of these pollutants is essential for improving educational environments and safeguarding children’s health. The hourly variations in indoor airborne bacterial concentrations, illustrating the effectiveness of the air purification system during school hours, are shown in Figure 6. On 16 May 2021, the concentration of suspended bacteria in Classroom 1 was 44.8% lower than that in Classroom 2. Higher concentrations during break times were likely due to increased child activity, leading to elevated respiration rates and subsequent bacterial release. The decline in suspended bacteria during lunchtime was attributed to the influx of outdoor air through open doors, which promoted natural ventilation.

The ventilation-type air purifier, equipped with a UV-LED sterilization system, effectively reduced suspended bacteria concentrations, keeping levels below Korea’s management standard of 800 CFU/m³. This result highlights the importance of using ventilation-type air purifiers to improve classroom air quality and protect children’s health.

3.3. Changes in IAQ Between Weekdays and Weekends

The concentration of indoor air pollutants is significantly influenced by occupant activities, with the presence and movement of children contributing to increased PM (PM₁₀ and PM_2.5) concentrations [35]. The present study assessed the effectiveness of ventilation-type air purifiers by comparing PM₁₀, PM_2.5, and CO₂ levels between weekdays and weekends over a study period of 11 weekdays and 6 weekends. Figure 7 presents a comparison of the average PM₁₀, PM_2.5, and CO₂ concentrations between weekdays and weekends, highlighting the significant reduction achieved with the ventilation-type air purifiers.

The results showed that the average PM₁₀ concentration in Classroom 2 during weekdays was 21 µg/m³, whereas in Classroom 1, it was reduced by 27%, reaching 15.3 µg/m³. PM₁₀ concentrations were notably lower on weekends than on weekdays due to the absence of children. The average PM₁₀ concentration in Classroom 2 was 13.6 µg/m³, whereas in Classroom 1, it was 12.2 µg/m³, representing a comparative reduction of 10.3%. Similarly, PM_2.5 concentrations in Classroom 1 showed a significant reduction of 37.1% on weekdays. On weekends, the PM_2.5 concentration was reduced by 8.9%, indicating a lower influence of non-child-related pollutant sources during periods of low occupancy. These findings align with the results presented in Section 3.2, which demonstrated an increase in PM concentrations during active school hours due to particle resuspension caused by children’s movement.

The smaller difference in PM concentrations between the classrooms on weekends reflects the absence of child-driven indoor pollution. However, the continued presence of pollutants highlights the influence of non-child-related sources, such as emissions from meal preparation or building ventilation systems. These findings emphasize the need for further investigation into these sources to develop comprehensive air quality management strategies.

Classroom 1 maintained an average CO₂ level of 577 ppm during weekdays, which was 26% lower than the 784 ppm recorded in Classroom 2. This reduction demonstrates that ventilation-type air purifiers effectively mitigate CO₂ accumulation due to respiration. The consistent decrease in CO₂ levels during active school hours highlights the long-term feasibility of using ventilation-type air purifiers for IAQ management in educational settings.

In contrast, CO₂ concentrations during weekends were similar in Classroom 1 (408 ppm) and Classroom 2 (411 ppm), indicating that CO₂ generation is primarily linked to children’s presence and activities. During weekends, when children were absent, the air purifier’s CO₂ reduction effect was less evident, as natural ventilation and the absence of occupants minimized CO₂ accumulation.

These findings emphasize the critical role of ventilation-type air purifiers in maintaining a healthy indoor environment, particularly during weekdays when children’s activities significantly influence pollutant levels. The results underscore the need for structural and operational improvements in schools, including optimizing the placement of ventilation-type air purifiers in areas with high pollutant generation, such as near classrooms or cafeterias. Furthermore, they highlight the need for policy interventions to enhance IAQ in educational settings. For example, operational guidelines should be established to regulate the opening and closing of windows and doors during class hours to minimize pollutant ingress. Additionally, infrastructure guidelines could support the systematic integration of air purifiers into school buildings to ensure optimal performance.

The introduction of ventilation-type air purifiers in classroom settings represents a significant advancement in managing indoor air quality (IAQ). Unlike traditional air purifiers that primarily target particulate matter, these devices also effectively reduce gaseous pollutants, which are often neglected in IAQ management. This dual functionality not only improves the healthfulness of the indoor environment but also offers a more cost-effective alternative to the high expenses associated with conventional mechanical ventilation systems.

Our findings indicate that these systems effectively maintain lower concentrations of both PM₁₀ and CO₂, even in scenarios with high external pollution. This is particularly crucial in educational environments where the health and cognitive functions of children are a priority. By integrating these advanced air purification technologies with natural ventilation strategies, schools can achieve a comprehensive solution to indoor air quality challenges.

Moreover, our results underscore the necessity for a multifaceted approach to air quality management in schools. It is recommended that future policies consider the systematic implementation of these technologies to optimize IAQ. Guidelines should be developed to regulate the operation of windows and doors during school hours to minimize the ingress of pollutants. Such operational protocols, coupled with the strategic placement of ventilation-type air purifiers, can significantly enhance the effectiveness of air quality management in school settings.

In light of these results, further research is needed to explore the long-term effectiveness and operational sustainability of these systems across various climatic conditions and building types. Additionally, a more in-depth analysis of the economic aspects of installing and maintaining such systems will provide valuable insights into their practical implementation in schools.

In conclusion, this study not only fills a significant gap in the literature on ventilation-type air purifiers but also highlights the potential for considerable health benefits and cost savings. The integration of these systems into existing school infrastructures could profoundly impact students’ health and learning outcomes, advocating for a broader adoption of such technologies in educational settings globally.

4. Conclusions

4.1. Summary of Findings

This study evaluated the effectiveness of ventilation-type air purifiers in enhancing IAQ in elementary school classrooms. The analysis considered the influence of outdoor air pollution, variations in pollutant concentrations by hour, and differences between weekdays and weekends. The findings revealed that the use of the air purifier led to notable improvements in IAQ. Specifically, indoor PM₁₀ concentrations were reduced by 23.7%, PM_2.5 concentrations decreased by 22.8%, and CO₂ levels exhibited a decline of 21.1%. These results highlight the potential of ventilation-type air purifiers to substantially mitigate indoor air pollutant levels in educational settings.

These findings underscore the critical role of ventilation-type air purifiers as an effective and scalable solution for reducing indoor air pollutants in schools. The ventilation-type air purifiers processed 150 m³ h⁻¹ of air, contributing significantly to the reduction in key pollutants. This capability was particularly crucial during class hours when increased child activity necessitated effective IAQ management.

Furthermore, we observed dynamic changes in IAQ throughout the day and between weekdays and weekends. These temporal fluctuations were most pronounced during class hours, with significant reductions in PM₁₀, PM_2.5, and CO₂ concentrations observed on weekends. This suggests that managing child-related activities and optimizing classroom ventilation could further enhance air quality. Importantly, stable IAQ was maintained even during class hours, indicating that air purifiers can effectively support healthier learning environments, particularly in classrooms with limited natural ventilation.

4.2. Limitations

Although this study provides valuable insights into IAQ and air purifier effectiveness, it has several limitations that must be acknowledged. First, the research period was limited to just 18 days, which is relatively short and may not adequately capture long-term trends or the influence of seasonal variations on indoor air conditions. Second, since the study was conducted within a single educational facility, its findings may have limited generalizability to other environments with different building structures, ventilation systems, or usage patterns. Third, the study did not adequately consider potential confounding factors, such as cooking emissions from the school kitchen or the frequency of door and window openings. These factors can markedly influence indoor pollutant levels. Lastly, although ambient temperature and humidity are known to affect pollutant behavior and the performance of air purification systems, these environmental factors were not comprehensively analyzed within the scope of this study.

4.3. Future Research Directions and Policy Implications

Future studies should further build on the current findings by addressing these limitations and expanding the scope of the investigation. Research should explore the performance of ventilation-type air purifiers under various climatic conditions, especially during transitions between heating and cooling seasons, as these periods may significantly affect ventilation efficiency and indoor air dynamics. Additionally, long-term studies are needed to evaluate the durability of air purifiers, focusing on possible degradation in their performance over time in real-world educational settings. Further investigations should also compare air purifier effectiveness across diverse building designs and classroom configurations to ensure their applicability in a wide range of contexts. Furthermore, research should examine the interaction between ventilation-type air purifiers and existing HVAC systems, aiming to develop integrated strategies that optimize air quality management in schools and other public facilities.

Based on the results of this study, several strategic recommendations are proposed to enhance air quality management within educational environments. First, the mandatory installation of ventilation-type air purifiers in schools located in high pollution zones is crucial to mitigate the adverse effects of external air quality, providing a safer and healthier learning environment for students. Second, investment in real-time air quality monitoring systems integrated with AI technologies is recommended. These systems enable proactive management and automated adjustments, significantly enhancing the responsiveness and effectiveness of air quality control measures.

Additionally, it is imperative to address the pollutants emitted during cooking processes in school cafeteria facilities, as they may infiltrate classrooms. Improving ventilation systems in these areas and developing standardized management guidelines are essential to ensure clean and safe air throughout all school environments. Third, it is crucial to establish and implement standardized operational and maintenance guidelines across all educational institutions. Such guidelines will ensure consistent performance of air purification systems and promote uniform best practices. Furthermore, it is recommended to prioritize the development of AI-driven systems that can predict pollutant levels and automatically adjust operational settings based on real-time data. This approach addresses immediate air quality challenges as well as enhances the long-term health and educational environment for students by maintaining optimal indoor air conditions.