Monitoring Indoor Air Quality in Educational Facilities: Summary of the Relevant Parameters

1 LEPABE – Laboratory for Process Engineering, Environment, Biotechnology and Energy, Faculty of Engineering, University of Porto, Portugal

2 ALiCE – Associate Laboratory in Chemical Engineering, Faculty of Engineering, University of Porto, Portugal

3 Faculty of Environmental Engineering and Energy, Lublin University of Technology, Poland

4 Polymer Materials Engineering, Bursa Technical University, Turkey

5 Vocational School of Yenisehir Ibrahim Orhan, Bursa Uludag University, Turkey

6 Department of Mechanical Engineering, University of Western Macedonia, Greece

7 NosmoTech Ltd., Cambridge UK

8 German Environment Agency, Germany

9 Saxion University of Applied Science, Chair of Sustainable Building Technology, The Netherlands

*Corresponding author: sofia.sousa@fe.up.pt

Introduction: Why Indoor Air Quality in Schools Matters

Indoor air quality (IAQ) in educational buildings is increasingly recognised as a critical factor for students’ health, cognitive performance, and school attendance. Children and adolescents are particularly vulnerable to indoor air pollutants due to their developing respiratory systems and higher breathing rates per body weight. Ensuring healthy IAQ in schools is essential because students spend up to one third of their weekday time in classrooms.

This article summarises the most relevant findings from a comprehensive literature review from Branco et al. (2024), helping to translate academic insights into practical recommendations for HVAC engineers, building designers and facility managers. The review synthesised the literature to identify which parameters most significantly affect IAQ in schools and to guide future monitoring campaigns.

Key parameters influencing IAQ in educational facilities

The literature review comprehensively examined seven key parameters influencing IAQ in educational facilities. Those parameters were grouped into two categories: building-related and occupancy-related. Understanding how these factors interact with the indoor environment is essential for designing targeted IAQ monitoring and mitigation strategies. Figure 1 summarises the reviewed parameters.

Figure 1. Summary of the reviewed parameters influencing IAQ in educational facilities.

Building-related parameters

Building location

The geographical setting of a school plays a key role in shaping its indoor air quality (IAQ) profile. Schools located in urban or industrial areas, particularly those near high-traffic roads, tend to record higher levels of outdoor pollutants infiltrating indoor spaces. These include fine particulate matter (PM2.5), ultrafine particles (UFPs), nitrogen dioxide (NO₂) and volatile organic compounds (VOCs), which can compromise the student health. Polycyclic aromatic hydrocarbons (PAHs) are also relevant near industrial sites, while rural areas often experience higher ozone (O3) levels and less traffic pollution. The location also determines exposure to other region-specific pollutants, depending on surrounding land use, vegetation, and nearby sources of combustion or industry.

Seasonal variations and local climate further influence how these pollutants behave indoors, particularly for pollutants such as O3, which tends to be higher indoors during summer months due to elevated outdoor levels and ventilation patterns.

Understanding the building's location is therefore essential for planning an effective IAQ monitoring campaigns, helping to identify which pollutants are most relevant, including location-specific pollutants, where infiltration is likely to occur, and how seasonality may affect results.

Building layout and construction materials

The internal layout and construction characteristics of a school building significantly influence IAQ. Several physical factors, such as the floor level, room size and volume, ceiling height, and window orientation, affect how pollutants accumulate, disperse, and infiltrate indoor spaces. For example, ground-floor classrooms were found to be more exposed to dust resuspension and infiltration of outdoor UFPs due to proximity to the ground and external sources like playgrounds or roads. Larger or more open classrooms can increase particle resuspension, especially when occupied by students engaged in physical activities.

Window orientation and type also play an important role. Classrooms with windows facing busy roads or parking areas may exhibit elevated NO₂ and PM levels due to their close proximity to the outdoor source. Additionally, differences in glazing and sealing characteristics between window types can affect pollutant infiltration and indoor air exchange rates. These variations must be considered when locating the IAQ monitoring equipment.

Construction materials are another critical determinant of IAQ. Some materials can emit pollutants directly; for instance, gypsum board may release formaldehyde, and granite or other natural stones can emit radon gas. The choice of materials also influences airflow and the rate of infiltration or retention of contaminants. Formaldehyde and VOC levels also tend to be higher in newly constructed or recently renovated buildings, with emissions decreasing gradually over time.

Together, layout and materials define how air moves through a space and how long pollutants remain indoors. These factors must be carefully considered in IAQ monitoring plans. Understanding which areas are structurally more prone to pollutant build-up leads to better sensor placement.

Building systems for ventilation, air cleaning and infiltration

Ventilation systems are among the most influential factors affecting IAQ levels in schools. Naturally ventilated buildings, which are common across Europe, often do not provide adequate air exchange – particularly during cold seasons when windows remain closed. In contrast, mechanical ventilation systems can deliver more consistent fresh air supply but require more energy and must be operated and maintained correctly to be effective.

The use of air cleaning technologies, whether portable or in-duct, has shown potential for reducing PM and VOCs indoors. However, the effectiveness of these systems depends on proper sizing, positioning, and maintenance. In some cases, highly airtight buildings, designed to conserve energy, can trap indoor pollutants if ventilation and air cleaning are not well integrated. Some air cleaning technologies may produce by-products, such as O3, that may degrade IAQ.

Infiltration, or the unintended flow of outdoor air through gaps and cracks in the building envelope, also affects IAQ. Depending on outdoor conditions, it can introduce pollutants including O₃, NO₂, or PM into classrooms.

Overall, IAQ monitoring in educational facilities requires a careful understanding of ventilation systems, air cleaning devices, and building envelope characteristics to ensure its effectiveness and representativeness.

Building finishing materials and class equipment

Finishing materials and classroom equipment are important indoor sources of air pollutants in educational buildings. New furniture, paints, floor coverings, adhesives, and coatings often emit VOCs, including formaldehyde and phthalates. These emissions tend to be highest shortly after installation or renovation and gradually decrease over time. Therefore, recently refurbished classrooms typically have higher concentrations of these indoor pollutants.

Electronic equipment and classroom tools can also contribute to indoor pollution. For example, laser printers emit O₃ and fine particles, while chalkboards may contribute to PM by generating dust. These sources are often overlooked but can significantly impact air quality, especially in poorly ventilated or densely occupied spaces.

The combination of these materials and devices, particularly when new or used intensively, creates a complex mix of indoor emissions. Their impact on IAQ is further influenced by room layout, ventilation efficiency, and cleaning practices.

To effectively monitor IAQ, it is essential to account for emissions from finishing materials and equipment, especially in newly renovated or furnished classrooms.

Occupancy-related parameters

Occupant demographics

The demographic profile of occupants, particularly age, plays an important role in shaping IAQ in educational facilities. The age group of the students impacts both the nature of classroom activities and the physiological characteristics affecting air quality. Younger children, such as pre-schoolers, often engage in a broader range of activities (including play, rest, and creative tasks) within the same room, leading to greater variability in IAQ. Their classrooms also tend to be more crowded and are often less ventilated to maintain thermal comfort, which can result in elevated CO2 concentrations and greater exposure to VOCs.

Differences in classroom layout, ventilation strategies, and sensitivity to temperature further contribute to varying exposure levels across age groups. These demographic-related patterns influence the distribution and intensity of indoor air pollutants.

To ensure effective IAQ monitoring, it is important to consider the specific age group using each space. Monitoring campaigns should be designed to reflect the distinct occupancy patterns and environmental conditions of classrooms for different age groups.

Occupancy and classroom activities

Occupant density and classroom activities have a major influence on IAQ in schools. High occupancy increases CO2 levels, due to human respiration, reflecting the lower amount of fresh air per occupant, and crowded classrooms also contribute to elevated VOCs and bioeffluents. Activities such as walking, playing, or using chalkboards can lead to significant particle resuspension, increasing PM concentrations in the indoor air.

Specific learning activities, such as art or science experiments, have also been linked to temporary spikes in VOC levels, particularly when materials such as solvents or markers are used. Additionally, emissions from human skin and personal care products can contribute to indoor VOC concentrations, particularly during warmer seasons.

Indoor air pollution preventive strategies already in use, such as shoe removal and adaptive behaviours such as intermittent window opening, may impact IAQ and may have implications when identifying polluting sources.

IAQ monitoring campaigns in educational settings should therefore include high-activity periods and reflect the real occupancy patterns. IAQ campaigns must also record during the different types of classroom activities, as they can often generate short-term peak pollutant levels and unique indoor air chemistry.

Cleaning activities

Regular cleaning is vital for hygiene but can also contribute to indoor air pollution. Cleaning activities in educational buildings can significantly affect IAQ by introducing or redistributing airborne pollutants. Common cleaning practices, such as sweeping, vacuuming, or surface disinfection, may lead to increased levels of PM, VOCs, and in some cases, PAHs. The type of products used (e.g., scented cleaners or disinfectants) and the cleaning schedule (during or after occupancy) strongly influence these emissions. Studies have highlighted that vacuuming during occupancy increases PM exposure, whereas wet cleaning after hours has less impact.

During the COVID-19 pandemic, more frequent and intensive cleaning raised concerns about secondary pollutant formation from commercial disinfectants, with potential impacts comparable to outdoor traffic emissions. Additionally, surface hygiene plays a crucial role in IAQ in schools, especially in classrooms with young children.

IAQ monitoring should be planned with awareness of cleaning routines. Measurements taken before and after cleaning can help distinguish pollutant sources and assess the impact of different products and methods on air quality.

Practical guidance for IAQ monitoring in educational facilities

One of the main objectives of the literature review by Branco et al. (2024) was to identify which parameters should be considered when designing and implementing IAQ monitoring campaigns in educational buildings. While the academic paper was designed to support researchers in designing future monitoring campaigns, the findings are equally relevant for practitioners, particularly IAQ and HVAC professionals, building managers and designers, and public health officials working with schools. This section presents actionable guidance based on the most consistent and significant results from the literature, with a focus on supporting effective and context-specific monitoring.

Design a representative and context-sensitive monitoring campaign

IAQ in schools is shaped by a combination of structural, environmental, and behavioural factors. Therefore, monitoring strategies should consider both the diversity of indoor microenvironments and how these spaces are used. Monitoring locations should include not only conventional classrooms but also canteens, computer rooms and gymnasiums, as each presents distinct pollutant profiles due to different activities, ventilation regimes, and occupancy patterns. Multi-space monitoring may be required to capture overall exposure, especially considering how students move between areas during the day. Different age groups also experience varying pollutant exposure due to differences in activity levels, classroom density, and ventilation practices. Including classrooms for different age groups (preschoolers, primary, and secondary students) ensures more representative data.

In practical terms, IAQ monitoring campaigns should be tailored to understand how layout and materials interact with airflow and pollutant sources. this will affect planning, including placing sensors at different heights and locations within a room, targeting classrooms with different window orientations, and ensuring coverage across multiple floors – especially in radon-prone areas. Renovated or new buildings may require more frequent monitoring to capture emissions from new materials and their decay over longer times. Special attention should also be given to older buildings where materials such as asbestos may be present.

Select pollutants based on likely sources

Monitoring campaigns should prioritise pollutants that align with the expected sources of pollution. For instance, CO2 is a key marker of occupancy and ventilation adequacy and should be routinely measured in all educational settings. Where outdoor traffic or industrial emissions are expected, NO2, PM2.5 and UFPs become more relevant. Buildings with new or recently installed materials, furnishings, or electronics should be assessed for VOCs, formaldehyde, and O3. The same guidelines apply to kindergartens and preschools, where materials such as toys and flooring may contribute to higher phthalate exposure. Understanding the building’s context and recent history (e.g., renovation, furniture replacement) is essential when selecting which pollutants to monitor.

Account for temporal variability and seasonality

IAQ in schools varies significantly with the season, occupancy schedules, and even the time of day. The review highlights that ventilation effectiveness, infiltration patterns and indoor activities vary between cold and warm seasons, influencing pollutant levels such as PM, O3, and VOCs, and CO2. This underlines the need for extended monitoring campaigns that capture both daily and seasonal variations rather than relying on one-time measurements.

Record contextual data for accurate interpretation

Raw pollutant data are only meaningful when linked to a relevant context. To support accurate interpretation of monitoring results, it is important to record metadata during measurements. Metadata includes occupancy numbers and schedules, surface cleaning frequency, methods and products, ventilation mode and operation, use of air cleaning technologies, temperature and humidity, and weather conditions and outdoor air pollution levels. For example, analysing PM peaks depends on timed records of activities including cleaning, blackboard use, window opening, or other actions. This information is critical to avoid misinterpretation and to support correct source identification.

Consider the effects of mitigation strategies

A small but growing number of studies in the review addressed the effectiveness of mitigation solutions such as source removal, improved ventilation and the use of portable air cleaners. These studies suggest that even low-cost interventions can produce measurable improvements in IAQ. However, their effects vary depending on pollutant type and building context. Monitoring campaigns should be used to both assess baseline conditions and the effectiveness of mitigation measures, ideally comparing data before and after mitigation. Professionals are encouraged to validate mitigation strategies with IAQ monitoring data, ensuring that improvements in pollutant concentrations are sustained and that unintended consequences (e.g., O3 generation from some air cleaners) are avoided.

Consider how building design and use shape exposure

IAQ exposure is not only about pollutant concentrations but also about how people use the building. Therefore, practitioners are encouraged to move beyond pollutant concentration measurements and consider how building layout and usage patterns influence exposure. Factors such as floor level, room orientation, and proximity to outdoor pollution sources (e.g., busy roads, industrial settings, parking lots, playgrounds) can modify exposure risks. For instance, lower-floor classrooms facing busy roads are more exposed to traffic-related PM and NO2, and possibly radon. Thus, assigning younger children to such rooms may increase their risk. Monitoring should then be tailored to both building use and exposure risks.

Gaps and priorities for future research

Although the reviewed literature offers a comprehensive understanding of the parameters that influence IAQ in educational settings, several knowledge gaps remain that may limit the effectiveness of interventions and monitoring strategies. These gaps point to key priorities for future research and development that are directly relevant for HVAC and building professionals, researchers, policymakers, and school administrators.

Underrepresentation of certain school levels and spaces

Most reviewed studies focused on nursery and primary schools, with limited attention to middle schools, high schools, or universities. Furthermore, research has primarily concentrated on classrooms, while other indoor microenvironments, such as canteens, libraries, gymnasiums, laboratories, or corridors, remain understudied. These areas may have different pollutant sources and activity patterns that influence indoor exposures, so future studies should address this imbalance by including a broader range of educational spaces.

Short-term studies dominate

The majority of existing studies rely on short-term IAQ measurements. While these snapshots can provide useful data, they do not capture long-term trends, seasonal effects, or the durability of interventions. There is a pressing need for longer monitoring periods to better understand pollutant accumulation, seasonal ventilation changes, and degradation or emission decay of building materials.

Limited evaluation of material emissions and chemical reactions

While several studies confirmed emissions from new materials (e.g., flooring, furnishings, coatings), few explored how these emissions evolve over time or how secondary reactions, such as those between O3 and VOCs, affect indoor air chemistry. In renovation or new construction projects, this information is vital for selecting safe, low-emission materials and planning proper ventilation.

Few studies address occupant movement and real exposure

Traditional IAQ studies often focus on a single classroom, yet children and staff move through different zones throughout the school day, and this dynamic use of space affects their cumulative exposure. Future research should consider personalised exposure assessment and time-location patterns to better reflect real-world scenarios.

Cleaning practices and their by-products need further study

Cleaning methods and frequencies vary widely, and their impact on IAQ is still insufficiently studied. Some cleaning practices increase VOCs and PM levels, and the use of disinfectants can generate harmful by-products, particularly when used during occupancy. These effects deserve greater attention, especially in the post-COVID-19 context where cleaning protocols have increased.

Limited evaluation of mitigation effectiveness

Although several low-cost mitigation solutions (e.g., portable air cleaners and increased ventilation) were found to be effective in lowering indoor concentrations of CO₂, PM2.5, PM10, and VOCs, the number of controlled intervention studies remains low. Moreover, some solutions may produce unintended side effects (e.g., ozone generation). Future studies should focus on quantifying the benefits and limitations of these interventions under real-world conditions.

Lack of pollutant-specific monitoring standards

IAQ monitoring campaigns often face challenges in selecting pollutant thresholds or interpreting results due to inconsistent or outdated IAQ standards. The review encourages alignment with the most recent guidelines and standards, and calls for the development of pollutant-specific reference levels (i.e. limit values) for compounds not yet adequately covered.

Conclusions

IAQ in educational buildings is a complex, multifactorial issue influenced by building characteristics, occupancy dynamics, ventilation and human activities. The review summarised in this article provides actionable insights that support evidence-based decisions in the design of IAQ monitoring campaigns in schools. These monitoring campaigns should be planned to reflect the specific characteristics and uses of the building. Decisions about which pollutants to monitor, when, and where, should be based on an understanding of building layout, occupancy patterns and potential sources. This ensures that the collected data are meaningful and actionable. Aligning IAQ practices with the unique features of each educational facility is critical for protecting students’ health, well-being, and academic performance.

Acknowledgements

This publication is based upon work from COST Action INDAIRPOLLNET (CA17136), supported by COST (European Cooperation in Science and Technology) (www.cost.eu). The support and original concept designed by Nicola Carslaw (U. York) is hereby acknowledged.

Two authors (PTBS Branco and SIV Sousa) are integrated members of LEPABE, financially supported by national funds through FCT/MCTES (PIDDAC): LEPABE, UIDB/00511/2020 (DOI: 10.54499/UIDB/00511/2020) and UIDP/00511/2020 (DOI: 10.54499/UIDP/00511/2020) and ALiCE, LA/P/0045/2020 (DOI: 10.54499/LA/P/0045/2020); project 2023.15742.PEX (DOI: 10.54499/2023.15742.PEX). PTBS Branco thanks FCT for the financial support of his work contract with the reference 2022.05461.CEECIND/CP1733/CT0011 (DOI: 10.54499/2022.05461.CEECIND/CP1733/CT0011).

References

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