Ivana Turekova
Michal Cabaj1
Iveta Markova- 1Faculty of Education, Constantine the Philosopher University in Nitra, Nitra, Slovakia
- 2Faculty of Security Engineering, University of Žilina, Žilina, Slovakia
Schools operate in existing buildings owned by cities and municipalities, encompassing various building types. This paper presents results of microclimatic measurements in selected classrooms located in two distinct building types: a 19th century manor house and a school building from the 1960s. The measured parameters were temperature, relative humidity (RH), and CO2 concentration. The position of the measuring device within the room did not appreciably influence the recorded values. Simultaneously, a strong effect of natural ventilation on CO2 concentration was demonstrated. Building type did not materially affect the observed parameters. Notably, CO2 concentrations during lessons exceeded the recommended limit of 1,000 ppm and did not decrease to ambient outdoor levels.
1 Introduction
Indoor Environmental Quality (IEQ) is a complex concept that, according to various authors (Korsavi et al., 2020; Marino et al., 2012; Tureková and Marková, 2021; Tureková et al., 2022), can be divided into four main components: thermal comfort (TC), indoor air quality (IAQ), visual comfort (VC), and acoustic comfort (AC).
Ensuring high indoor environmental quality is especially important in school buildings (Papadopoulos and Avgelis, 2003), where students spend approximately 30% of their time (Catalina and Iordache, 2012), while, in general, people spend more than 90% of their time indoors.
In Slovakia, school buildings are currently influenced by demographic trends, notably a declining population and limited construction of new schools, which has led to an increased emphasis on renovating existing buildings (Statistical office of Slovak Republic, 2024). Key renovation measures include thermal insulation and replacement, which improve energy efficiency and thermal comfort (Makaveckas et al., 2023a). However, these modifications may also negatively affect indoor air quality, particularly with respect to carbon dioxide (CO2) concentration (Wargocki and Wyon, 2013). Replacing old windows with newer, more airtight ones and insulating the building envelope may minimize heat loss; however, these measures also reduce natural air exchange, leading to increased CO2 concentrations in classrooms. Elevated CO2 levels negatively affect students’ health and cognitive abilities (Mocová and Mohelníková, 2021). The primary factors influencing indoor CO2 concentration include building type, ventilation method, occupancy, and the nature of ongoing activities.
Several studies confirm the impact of the indoor environment on occupant comfort, health, and productivity (Haverinen-Shaughnessy et al., 2015; Oldham and Kim, 2019; Issa et al., 2011; Canha et al., 2016; Shendell et al., 2004a; 2004b; Toftum et al., 2015; Mendell et al., 2016; Kielb et al., 2015; Yang et al., 2015; Stranger et al., 2008). Measurements conducted in Slovak schools have revealed average CO2 concentrations during lessons exceeding 2,000 ppm, indicating an unfavourable condition for health (Mocová and Mohelníková, 2021).
The topic of determining acceptable indoor CO2 concentrations remains unresolved (Tureková and Marková, 2021). Von Pettenkofer (1858) proposed a threshold value of 1,000 ppm, above which air is considered contaminated.
Requirements for indoor air quality are specified in the United States and several other countries by the American Standard ASHRAE 62.1-2016, while in European countries they are set by the standard EN 13779:2007. The European standard defines four performance levels—IDA1, IDA2, IDA3, and IDA4—based on CO2 concentration, as presented in Table 1.
Table 1. Classification of indoor air quality in rooms with low pollutant emission levels and a smoking ban (Source: EN 13779:2007).
ASHRAE 62-2001 (United States) recommends a maximum indoor CO2 concentration of 700 ppm above outdoor levels (approximately 1,000 ppm) (Jones, 2004). Concentrations exceeding 2,000 ppm have been shown to reduce occupants’ comfort (Cetin, 2016), while levels above 3,500 ppm may pose long-term health risks (Wyrwoll et al., 2022).
An increasing number of studies focus on monitoring air quality in schools, its impact on pupils’ health, and their subjective perception of comfort (Mumovic et al., 2009; Ter Mors et al., 2011; Wargocki and Wyon, 2017; Turunen et al., 2014). In a systematic review, Lowther et al. (2021) identified a current consensus on the classification of CO2 concentration levels as follows:
• ≤ 1,000 ppm–good air quality,
• 1,000–1,500 ppm–moderate air quality,
• ≥ 1,500 ppm–poor air quality.
While numerous European studies have already highlighted the problem of elevated CO2 concentrations in school buildings (e.g., Wargocki, 2016; Toftum et al., 2015; Mendell et al., 2016), evidence from the Slovak context remains very limited. The contribution of our study lies in providing up-to-date data from Slovak schools, reflecting the specific circumstances of demographic development, the stagnation of new school construction, and the prevailing trend of renovations primarily focused on energy efficiency. In this respect, our results add to the European discourse by showing that renovation measures targeting energy savings alone may have unintended consequences for indoor environmental quality and the health of pupils. Our findings therefore provide an important perspective on how national approaches to school building renovation shape the need to balance energy objectives with requirements for a healthy indoor environment–an issue that is highly relevant in the context of European initiatives such as the ‘Renovation Wave’ (European Commission, 2020).
This article aims to analyse a series of measurements of microclimatic conditions within the school environment, focusing on temperature, relative humidity, and indoor air quality in two primary school buildings in Slovakia. Carbon dioxide concentration was used as the primary indicator of air quality. Since Slovakia does not have a specific limit set - the upper limit of the concentration of CO2 in the indoor environment, this is only paid marginal attention to. The currently implementing project considers it an obligation to pay key attention to CO2 concentrations. Based on measurements taken in various classrooms, the study examined the influence of building type and classroom location (first floors versus second floors) on CO2 concentration, the temporal progression of CO2 build-up during lessons, and the compliance of the measured values with current standards and recommended limits.
In Slovakia, no binding national limit has been established for CO2 concentration in school indoor environments. Therefore, the comparison of measured values is carried out with respect to Pettenkofer’s reference value of 1,000 ppm, which is widely accepted in the scientific literature as the upper threshold for maintaining adequate indoor air quality. This value is consistent with the recommendations of EN 16798-1 for the assessment of indoor environmental quality and is also frequently employed as a reference limit in WHO indoor guidelines. These results therefore do not meet the recommended reference of 1,000 ppm.
Another aim was to monitor and compare the occurrence of critical indoor air quality values and to identify the parameters that most pronounced contribute to poor IAQ in classrooms.
2 Materials and methods
The research was conducted in two different school buildings during March 2024, each representing a distinct stage in the building life cycle. The first building (Figure 1) is a nursery and primary school located in a classicist manor dating from the 19th century. The second building, constructed in 1960, has undergone renovations including thermal insulation of the building envelope and the replacement of windows (Figure 2).
Figure 1. (a) Nursery and primary school housed in a Classicist manor from the 19th century (labelled C–castle); (b) Primary school classroom on the second floor (labelled 2C).
Figure 2. (a) Primary school building from the 1960s (labelled M–modern school); (b) Classroom on the second floor (labelled 2M).
Table 2 outlines the experimental conditions, while Table 3 presents the corresponding outdoor conditions during the measurement period. Detailed characteristics of the individual buildings are provided in Table 5 for building Castle and Table 6 for modern building.
Table 2. Experimental procedure (Cabaj, 2024).
Table 3. Outdoor conditions during the experiment conducted in March 2024.
The experimental setup was preceded by a series of preliminary measurements aimed at identifying the optimal positions for the measuring points - specifically, at a height of 1 m (representing the typical head height of a child when seated at a desk) and 1.5 m (representing the average standing height of a child) - as well as at defining the operational procedure (see Table 2).
The weather was sunny during the measurement period. Each series commenced at 7:30 a.m., when outdoor temperatures were low (see Table 3) and relative humidity was relatively high. Although these parameters fluctuated throughout the day, they did not mark affect the indoor measurement results, which remained consistent across all series.
Measurements were carried out using the handheld measuring device (Fluke Corporation, 2006). Table 4 shows the device along with its technical specifications.
Table 4. Image and technical specifications of the measurement device (AirMeter test tool combines five instruments in one, 2024).
Given that the study consisted solely of the objectivization of a physical factor of the indoor environment (CO2) in the classroom, without collecting personal or health data and without intervention on individuals, according to Slovak legislation (Act No. 355/2007 Coll. and Act No. 124/2006 Coll.), it does not constitute human research subject to ethical committee approval. The execution of the measurements was approved by the statutory representative of the school (the director), who is responsible for the hygiene of the educational premises according to the legislation. Teachers and parents were informed for transparency, beyond the scope of legal obligation. CO2 measurements were conducted in occupied classrooms with authorization from the school’s statutory representative. No personal data were collected, and the activity did not constitute human subjects research under Slovak law.
3 Results
3.1 Microclimatic conditions in a nursery and primary school housed in the 19th -century classicist manor
Results show one measurement run from chossen classes the same day by Table 2. The parameters of the nursery and primary school housed in the 19th century Classicist manor (hereinafter referred to as “Castle C”) are detailed in Table 5. The measurement results are presented in Figures 3–5.
Table 5. Description of classrooms in the nursery and primary school housed in the 19th-century classicist manor (Castle C).
Figure 3. Results from the first series of measurements without ventilation in classroom 2C at two measuring points: 1 m and 1.5 m (a) CO2 concentrations in ppm; (b) Temperature T in °C; (c) Relative humidity RH in %.
Figure 4. Comparison of microclimatic parameters in classroom 2C during the first measurement series (without ventilation) and second measurement series (with ventilation). (a) CO2 concentrations in ppm; (b) Temperature T in °C; (c) Relative humidity in %.
Figure 5. Comparison of classrooms 1C (ground floor) and 2C (second floor) during the ventilated series. (a) CO2 concentrations in ppm; (b) Temperature T in °C; (c) Relative humidity RH in %.
The first series of measurements was conducted exclusively in classroom 2C, located in the primary school section. Figure 3 presents the results of CO2, temperature T, and Relative Humidity RH measurements taken at both measuring points. It is evident that the position of the measuring point does not pronounced affect the measurement results.
CO2 levels rapidly increased during school hours, exceeding 1,000 ppm (European Committee for Standardization, 2007; World Health Organization, 2010), while temperature remained acceptable and relative humidity declined. This confirms the importance of ventilation for maintaining comfort and children’s performance (Dorizas et al., 2015; Madureira et al., 2016).
Based on the findings, which showed comparable values at the 1 m and 1.5 m measuring heights (with deviations observed only in CO2 concentrations), the data were averaged. Figure 4 presents a comparison of microclimatic conditions (CO2, T, and RH) between the first series of measurements (without ventilation) and the second series (with ventilation) in classroom 2C (the same classroom measured twice). A notable difference in measured CO2 concentrations for the first measurement series (without ventilation) and second measurement series (with ventilation) was obtained. The highest increase in CO2 occurred during the first series without ventilation, with the maximum value recorded in the sixth hour reaching 2.5 times the recommended limit of 1,000 ppm. In other cases, CO2 levels did not exceed 1,500 ppm but surpassed 1,000 ppm after the third hour of measurement.
Regular ventilation clearly reduced CO2 concentrations towards recommended thresholds, with only minor temperature differences and improved relative humidity. Ventilation thus proved effective for ensuring healthier classroom conditions (European Committee for Standardization, 2007; World Health Organization, 2010; Dorizas et al., 2015).
The second series of measurements enables a comparison of microclimatic parameters between classrooms 1C and 2C (Figure 5). Classroom 1C is situated on the first floor, while classroom 2C is on the second floor. The results from classrooms 1C and 2C are comparable. Minor differences were observed in relative humidity measurements, with classroom 1C exhibiting slightly lower values.
The lowest values were recorded in classroom 1C, which may be attributed to its location on the first floor (ground level).
CO2 and temperature were similar in both classrooms, while relative humidity was lower in 1C, likely due to ground-floor conditions. Floor level can subtly influence microclimate, although ventilation kept parameters within acceptable ranges (Seppänen et al., 1999; Wargocki and Wyon, 2013).
3.2 Microclimatic conditions in a primary school from the 1960s
The parameters of the primary school in the building from the 1960s (hereinafter referred to as “Modern M”) are detailed in Table 6. The measurement results are presented in Figures 6, 7.
Table 6. Description of classrooms in the primary school building from the 1960s (Modern M).
Figure 6. Results from the second series of measurements in classroom 2M at two measuring points: 1 m and 1.5 m (a) CO2 concentrations in ppm; (b) Temperature T in °C; (c) Relative humidity RH in %.
Figure 7. Comparison of classrooms 1M (ground floor) and 2M (second floor) during the ventilated series. (a) CO2 concentrations in ppm; (b) Temperature T in °C; (c) Relative humidity RH in %.
As in the previous case, the use of two measuring points was found to be unnecessary (Figure 6). Differences between heights were negligible, confirming that vertical gradients of CO2, Temperature and relative humidity are minor in naturally ventilated classrooms. A single measuring point is sufficient for monitoring (Seppänen et al., 1999; Santamouris et al., 2008).
The comparison between the first and second series of measurements in classrooms 1M and 2M mirrors the pattern observed in classrooms 1C and 2C (Figure 4). Important differences in CO2 concentrations were recorded, with notably lower values in classrooms where ventilation was applied. This reduction is evidently influenced by the method, timing, and frequency of ventilation (Diz-Mellado et al., 2020; Bain-Reguis et al., 2022). Meškauskienė and Juškevičienė (2019) highlight the important role of teachers in managing classroom microclimatic conditions (including ventilation practices).
Notable findings emerged from the comparison of measured parameters in classrooms 1M and 2M during the second series of measurements. Both rooms were ventilated in the same manner - by opening windows after 60 min (Figure 7). The most pronounced variation was observed in relative humidity levels (Figure 7C). Relative Humidity within classrooms in the same school differed by as much as 5%–10% (Krawczyk et al., 2017). Both classrooms showed comparable CO2 and temperature, with slightly lower relative humidity in 1M classroom. These results confirm the stabilizing effect of ventilation, with only minor differences due to floor level (Madureira et al., 2016; Dorizas et al., 2015).
4 Discussion
CO2 production is directly proportional to the level of physical activity, resulting in increased CO2 concentrations in exhaled air (Tureková et al., 2021). Exhaled air from an adult typically contains between 35,000 and 50,000 ppm of CO2, a concentration pronounced higher than that of outdoor air (Merisalu et al., 2018).
Indoor carbon dioxide concentration is therefore strongly influenced by the number of occupants, the size of the space, and the intensity of ventilation. Spaces with higher occupancy tend to exhibit elevated CO2 levels unless sufficient air exchange is ensured. This was confirmed in the present study, where classroom 2C, which had the lowest number of pupils, also recorded the lowest CO2 concentrations (Figure 8). Castle classrooms exceeded 1,000 ppm after the third hour, while 1M and 2M were above the recommended reference from the start. The results were not meet recommended reference in Slovakia and confirm insufficient ventilation, consistent with earlier findings (Brečka et al., 2018; Tureková et al., 2022). Elevated CO2 levels are linked to reduced comfort and learning performance (Wargocki and Wyon, 2013).
Figure 8. Comparison of CO2 concentrations across all classrooms during the ventilated series.
An acceptable indoor carbon dioxide concentration is 1,000 ppm; concentrations at or below this level indicate satisfactory indoor air quality (Tureková et al., 2022). In the “Castle” classrooms, CO2 levels exceeded 1,000 ppm after 3 hours, whereas in classrooms 1M and 2M, values already exceeded this limit at the beginning of measurements. These results do not comply with Slovak standards. A similar study by Brečka et al. (2018) reported comparable findings. Jovanović et al. (2014) obtained the similar results.
Makaveckas et al. (2023b) assessed the feasibility of meeting IAQ requirements in schools ventilated solely by opening windows. They monitored carbon dioxide (CO2) concentrations, temperature (T), and fluctuations in relative humidity (RH). Their study indicated that achieving the defined IAQ parameters in classrooms through window ventilation alone is not feasible. During lessons, measured CO2 concentrations frequently exceeded the 1,000 ppm limit and did not return to ambient levels even when windows were opened during breaks.
He et al. (2021) conducted an extensive study on microclimatic conditions in elementary schools in Japan. They focused on the location of schools through screening temperature fields and highlighted important urban morphology parameters affecting the microclimate around the elementary schools, such as building density, floor area ratio, green plot ratio, impervious ground surface fraction, and sky view factor (Sun et al., 2022). A similar study was later conducted by Namazi et al. (2024) in the United Kingdom.
The role of teachers in managing classroom ventilation represents an important factor for indoor environmental quality, which was not empirically examined in this study. Effective control of CO2 concentrations requires systematic classroom monitoring and the education of teachers about strategies for regulation through appropriate ventilation. The lack of data on actual teacher practices in managing the indoor environment constitutes a limitation of the study, which is being addressed in the authors’ current research.
5 Conclusion
The measurement results indicate that the height of the measuring points (1 m and 1.5 m above the floor) did not affect the recorded microclimatic parameters, confirming a uniform distribution within the classroom spaces.
A comparison of classrooms with identical technical characteristics but situated in two different buildings revealed very similar microclimatic conditions, suggesting that technical design has a greater influence than the broader architectural context.
From the perspective of indoor environmental quality, CO2 concentrations during lessons were found to pronounced exceed the recommended hygienic limit of 1,000 ppm and did not return to typical outdoor levels even after classes ended.
Differences were also observed in the dynamics of relative humidity changes between ventilated and non-ventilated spaces. Despite these findings, the study acknowledges certain limitations, particularly concerning the scope and duration of the measurements. To ensure objectivity of the results and their wider generalisability, further research will continue, next season, with an emphasis on different ventilation methods (including mechanical ventilation) and on variations in classroom occupancy, both of which can influence indoor environmental quality.
It should be emphasized that the monitoring was carried out over the period of only 1 month (March 2024) and included four classrooms in two primary schools. The findings therefore have the character of a screening study and cannot be directly generalized to all school buildings in Slovakia. The primary aim was to compare conditions in an older historic building (a manor house) and in a more recent, insulated school in order to identify fundamental differences in indoor environmental quality. The results also highlight the need for more extensive monitoring covering longer periods and seasonal variability, as indoor environmental quality is strongly influenced by outdoor climatic conditions as well as by building operation throughout the year. Such a research design would enable more robust conclusions and provide a stronger basis for systematic recommendations regarding school building renovation.
Slovak schools are currently undergoing a phase of reconstruction of external facades in order to reduce the cost of heating. The purpose is to save money on heating. On the other hand, it is not monitored how the mentioned construction modification will affect the quality of the internal environment. This opens up the possibility of showing the real situation based on experimental data and pointing out the need to address the issue of the quality of the internal environment in classrooms.
Data availability statement
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.
Ethics statement
Ethical approval was not required for the study involving humans in accordance with the local legislation and institutional requirements. Written informed consent to participate in this study was not required from the participants or the participants’ legal guardians/next of kin in accordance with the national legislation and the institutional requirements.
Author contributions
IT: Conceptualization, Data curation, Investigation, Methodology, Writing – original draft, Writing – review and editing. MC: Conceptualization, Investigation, Supervision, Writing – original draft, Resources. IM: Formal Analysis, Writing – original draft, Writing – review and editing, Data curation.
Funding
The authors declare that financial support was received for the research and/or publication of this article. The authors declare financial support by Resear Grant Agenture from Slovakia: VEGA 1/0213/23 Developing the learning potential of preschool and younger school-age children with a focus on the STEM concept.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Generative AI statement
The authors declare that no Generative AI was used in the creation of this manuscript.
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Keywords: classroom, temperature, relative humidity, CO2 concentration, school
Citation: Turekova I, Cabaj M and Markova I (2026) Monitoring microclimatic conditions in classrooms of various school types. a Slovak case study. Front. Built Environ. 11:1672543. doi: 10.3389/fbuil.2025.1672543
Received: 12 August 2025; Accepted: 17 November 2025;
Published: 07 January 2026.
Edited by:
Justin Yang, Boston University, United StatesReviewed by:
Sahar Zahiri, Oxford Brookes University, United KingdomGiovanni Ciampi, Università degli Studi della Campania Luigi Vanvitelli, Italy
Copyright © 2026 Turekova, Cabaj and Markova. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Iveta Markova, aXZldGEubWFya292YUB1bml6YS5zaw==