This scoping review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses for Scoping Reviews (PRISMA-ScR) guidelines. No protocol was registered in advance (e.g., OSF), which represents a methodological limitation; however, all steps of the review process are transparently reported below to ensure reproducibility.

Given the heterogeneity of the literature, we explicitly defined the exposure of interest as ventilation and air-filtration strategies implemented in school environments (mechanical ventilation, natural ventilation, hybrid systems, HVAC optimization, portable air cleaners) for respiratory infections (upper and lower respiratory tract infections). We did not include studies focusing primarily on chemical indoor pollutants unrelated to ventilation.

The outcomes of interest were:

Incidence of respiratory infections (COVID-19, influenza, RSV, acute upper/lower respiratory infections),

Relevant publications were searched for through PubMed, Embase and Web of Science databases (Core Collection + Emerging Sources Citation Index + Science Citation Index Expanded). The inclusion of multiple WoS sub-databases explains the higher number of retrieved records compared with PubMed, particularly due to engineering and physics journals publishing modelling studies on ventilation. The systematic search was conducted on 19.03.2025 by the main author. The search combined controlled vocabulary and free-text terms related to ventilation strategies and respiratory infections. The full database-specific search strings are provided in Supplementary Material. A combination of the following words was used to find relevant studies in the existing literature:

“Indoor air,” “indoor air quality,” “air circulation” “air recirculation” “air filtration,” “ventilation,” “natural ventilation,” “mechanical ventilation,” “COVID-19,” “grippe,” “human flu,” “influenza*,” “RSV infection,” “school,” “school*,” “school building,” “university,” “educational building”.

The included publications had to be written in English, French and Italian and published within the study period of 2020 and 2025. This period was selected because the COVID-19 pandemic generated a substantial increase in empirical and modelling studies on ventilation in schools, making it the most relevant timeframe for current policy discussions. Language restrictions were applied based on the research team’s linguistic proficiency to ensure accurate interpretation of methodological details. Although ventilation research predates COVID-19, pre-2020 studies rarely focused specifically on school settings or respiratory infection outcomes. The period 2020–2025 was therefore selected because it corresponds to the highest volume of relevant, school-based empirical and modelling evidence, making it the most appropriate timeframe for this review. The scientific papers had to be original research articles, reviews, modelling studies, interventional and observational studies, or clinical studies and trials, RCT and guidelines. The settings included schools, universities or educational buildings. The outcomes included were respiratory infections or proxy indicators (CO₂, aerosol concentration, absenteeism).

The exclusion criteria were studies not conducted in educational settings, studies focusing solely on chemical pollutants unrelated to ventilation, and commentaries, editorials, and non-scientific reports.

The original research was conducted on 19.03.2025 and it resulted in: 28 results for PubMed, 39 results for Embase and 266 results for Web of Science. In total there were 333 articles, of which seven pertinent reviews and 59 duplicates.

The identified dataset was extracted using Rayyan web-tool and the duplicates were removed performed using Rayyan’s automated detection tool, followed by manual verification.

First, the reviews were searched by screening titles and abstracts. Seven reviews were found on the relationship between ventilation strategies-indoor air quality improvement and COVID-19.

The remaining publications were imported into Rayyan web-tool where the screening based on titles and/or abstracts was completed by the main author; to mitigate this, ambiguous cases were discussed with a second reviewer. 198 publications were excluded.

In the second stage, the full text of selected articles was read, and 55 articles and seven reviews were considered for this scoping review. A total of 62 studies met the inclusion criteria. To ensure completeness, we used “LiteRev” ¹ , a machine-learning–based tool developed at the University of Geneva, which identifies publications similar to a predefined set of included articles. “LiteRev” did not identify additional eligible studies but confirmed the consistency of the final selection.

A flowchart describing the process of study selection is shown below in Figure 1.

Relevant data were later extracted into a structured Word table, which included: title, author(s) names, date of publication, methods, aim, study period of investigation, type of indoor setting, city, main results, type of ventilation used and main limitations of the study (“Supplementary Appendix 3” in “Supplementary Material”). Extraction was performed by one reviewer (EE) and cross-checked by a second reviewer (AF).

Our literature search confirms previous findings that mechanical ventilation is crucial to reduce airborne contamination in schools [9, 10, 34]. In fact, the risk of COVID-19 and other respiratory infections outbreaks in school should not be underestimated due to the fact that many children are not vaccinated and lack other preventive measures [11]. Busato and Cavallini [31] showed that mechanical ventilation, with or without a mask, reduced the infection risk of COVID-19 by three times or more, in different Italian classrooms compared to opening windows, and could even outperform the effect of solely wearing a mask. The retrospective cohort study of Buonanno et al. [26] reveals how mechanical ventilation strategies protected students from airborne exposure leading to infection of COVID-19 in more than 1400 school in Italy. For classrooms equipped with mechanical ventilation systems, the relative risk of infection of students decreased at least by 74% compared with a classroom with only natural ventilation, reaching values of at least 80% for ventilation rates >10 L s^-1 student^-1. Moreover, only 31 students became ill in classes with mechanical ventilation compared to 3090 in classes without, during the period of September 2021 and January 2022 [26]. In addition, mechanically ventilated classrooms seemed to have lower CO2 levels [34] and increased ventilation rates, which correlates with reduced infection probability in schools; however, they may need to be balanced with energy use [28]. At this purpose, double flow ventilation systems are recommended since they allow energy savings through heat recovery systems [2]. Mechanically ventilated classrooms also had lower levels of PM2.5 and the observational study of Deng et al. [28] found that every 1L/s increase per person in ventilation rates through mechanical ventilation corresponds to a 5·59 decrease in absent days per year (which translates into a 0·15% increase in the annual daily attendance rate). This clearly supports the recommendation that schools should be equipped with mechanical ventilation.

Overall, mechanical ventilation was associated with: lower CO₂ concentrations [28, 34], reduced aerosol or particle levels [20, 22, 27], lower estimated infection probabilities [26, 31], and, in some studies, reduced absenteeism [28]. These findings align with long-standing evidence that adequate air exchange reduces airborne pathogen accumulation [9, 10]. Mechanical systems also provide stable performance across seasons, unlike natural ventilation, which is highly weather-dependent [45, 52]. However, feasibility varies. Mechanical systems require: installation and maintenance cost [2], trained personnel [34], energy consumption considerations [28], and infrastructure that many older school buildings lack [2]. In low-resource settings, these constraints may limit implementation, highlighting the need for low-cost ventilation alternatives. Low-cost strategies for LMICs could include structured window-opening protocols, low-cost CO₂ monitors, locally manufactured HEPA units, simple architectural modifications (roof vents, cross-ventilation corridors) and improved classroom layout to enhance airflow [23, 25].

A significant number of publications found air filtration-purification systems, particularly HEPA and MERV-13, often used in combination with mechanical ventilation filters, showed strong reductions in aerosol concentrations [16, 37, 63] and, in some cases, respiratory infection incidence or absenteeism [13, 39]. Filtration was especially useful when natural ventilation was insufficient (cold climates, polluted outdoor air, security concerns) [1, 36], to be effective in reducing the transmission risk of infectious diseases.

A recent pre-print publication of 2025 (RCT, not included in our selection of published papers in this review) showed that air purifiers reduced pollution in 95 Italian classrooms by 28% and students’ absenteeism by 11% [69]. These devices significantly reduce airborne particle concentration regardless of the season or ventilation behavior [1] with the aim of controlling respiratory symptoms and infection rates [13, 15, 16], but their efficiency also depends on the type of air purifier and filter class [34]. Three types of air filtration devices were examined in the 62 publications: HEPA, MERV and PECO filters.

The observational and mathematical modeling study of Banholzer et al. [16] examined the effect of HEPA filters on school absenteeism in two Swiss secondary school classes during 7 weeks in winter: there were 22 absences related to respiratory infections without air cleaners and 13 with air cleaners. The researchers also estimated the number of respiratory infections in schools without air cleaners (36) as well as in cases where air cleaners had been installed during the study period (19) [16]. Granzin et al. [37] further showed that air purifiers with HEPA filters reduced the number of inhaled viral particles by a factor of 2·65, from November 2020 to May 2021, and Villiers et al. [63] found that the use of HEPA filters combined with students using face masks could reduce the mean aerosol concentration by 90%. In Gettings et al. [39], which evaluated the impact of preventive strategies against COVID-19 in 169 schools in Georgia (USA), the incidence of the disease was 39% lower in schools with improved ventilation, in particular 48% lower with HEPA (with and without purification UVGI) [39] versus 27% and 29% reduction respectively in Shen et al. [23] and Azimi et al. [25]. However, Falkenberg et al. [15] indicate that the use of HEPA filters might lead to a reduction in preventive behaviors because of the sense of security that they procure. Nevertheless, other studies conducted to examine the effectiveness of air filters to reduce indoor aerosol transmission agree on the positive effect of HEPA filters. In fact, compared to HVAC only systems which are designed to recirculate the air from one place to another, HEPA filters are relatively affordable and improve aerosol clearance [70].

The conclusion from the publications related to air filtration systems was that the use of filters with higher MERV ratings ensures an infection risk below 1% [40]. Indeed, Zand et al. [13] show how classrooms designed with MERV-13 filters had a lower COVID-19 incidence and positive PCR tests compared to MERV-11 [13], while Azimi et al. [25] revealed a 45% transmission risk for MERV-8 and 32% for MERV-13 [25]. Finally, only one study evaluated the efficiency of PECO air purifiers, and it showed a 99·98% removal of COVID-19 in an air chamber [36].

Despite all of the above, it is important to consider that HEPA filters do not remove gaseous pollutants and it is difficult to measure the exact effect of air filtration in classrooms because of the existence of confounding factors, such as the presence of other particles not emitted by humans [34], and there is a limitation to maintaining good indoor air quality when the outdoor air is very polluted [69]. Also, filtration does not provide fresh air [34], it does not remove CO₂ [34], and its effectiveness depends on proper sizing, placement, and maintenance [16, 37]. Some studies also noted concerns about noise levels [15], which may affect classroom comfort, and the potential for risk compensation, where occupants reduce other preventive behaviours when filters are present [15].

Natural ventilation (having windows and doors open) has been used in different studies [5, 54–57], but its role is controversial. Some publications show that it is not the most effective way to reduce transmission risk of infectious diseases in schools [49, 50, 58]. In France, according to a study by Cerema, the common practice of windows ventilation is to open the window for 5 min every 2 h. The minimum aeration time for good indoor air quality is 5 min every hour and 5 min for every break. This would add an extra cost of between 2 and 4 euros per student per year [2]. Windows ventilation is therefore considered more expensive and less efficient, considering that ventilation with heat recovery (double flow systems) would bring an economic gain between 6 and 7 euros per student per year and a social gain of better health, less absenteeism and better productivity [2]. The effectiveness of natural ventilation is often related to climate-sensitive regions and windows use, where in cold climates or densely populated areas its role ends up being insufficient, and many real-world case studies underscore the need for more reliable or combined alternatives [45, 52, 53]. The role of natural ventilation can be improved with the implementation of other measures, such as students wearing face masks or reducing the number of students in a class. Zivelonghi et al. [52] showed that natural ventilation could be more efficient for preventing transmission when combined with microphone use (20% risk reduction due to the reduced need to project the voice, which causes more droplets to spread), mask use (almost 40% risk reduction) and reduced children’s density (50% risk reduction). Moreover, Park et al. [45] proposed the use of cross ventilation (two or more openings on opposite or adjacent walls) compared to single ventilation (openings on the same wall) [45].

Overall, natural ventilation showed high variability in effectiveness [5, 12, 23, 49, 50]. Its performance depended on: window opening ratio [45], classroom orientation [53], occupancy density [52], outdoor temperature [45], and air pollution levels [54].

In cold climates, windows are often kept closed, limiting air renewal [45]. In hot climates, windows may be open but outdoor pollution or noise may reduce feasibility [54]. Seasonal constraints were highlighted in several studies, confirming that natural ventilation alone is unlikely to maintain adequate air quality year-round [52, 53].

Nevertheless, natural ventilation can be enhanced through cross-ventilation [45], scheduled airing [2], reduced occupancy [11], and mask use [45, 52].

Finally, a growing body of evidence promotes a multi-layered strategy to prevent the rise of infections, combining ventilation and filtration measures, CO2 monitoring, mask use and occupancy control. For example, Pollozhani et al. [4] considered adding mask use to mechanical and hybrid ventilation strategy, and Yen at al [11]. studied the reduction of the number of occupants in the room and the duration of their stay, finding both a strong reduction in transmission risk of COVID-19. Yet, even if some proposals such as using a microphone or implementing social distancing measures have been tested, the use of a mask, in combination with ventilation strategies, has been recognized as the most efficient supplementary strategy. In Harrington et al. [64], the probability of infection transmission was reduced from 75%–100% to 18%–43% over an 8-h school day with mask use. According to Gettings et al. [39] there was also a difference in incidence of COVID-19 if the teacher or students wore the masks: a 37% lower incidence rate when the teachers used face masks compared to a 21% rate if students wore a mask. Overall, hybrid systems combining natural and mechanical ventilation provided: improved air renewal [21], lower CO₂ levels [21, 45], and reduced infection risk across seasons [4, 46, 47, 52].

These systems may offer a cost-effective compromise, particularly in temperate climates where natural ventilation can be used part of the year [21]. They also allow schools to reduce energy consumption by alternating between modes [28].

However, while significant progress has been made to understand the role of improving indoor air quality in infectious disease prevention in schools, disparities persist in implementation. In fact, even though in the 62 publications selected for this scoping review, mechanical ventilation with and without filters was found to be a very efficient strategy [26, 30, 40], it is important to consider that there are many lower-resourced schools around the world which often lack mechanical systems and real-time monitoring capacities. So, it is necessary to take these differences into consideration and, as far as possible, opt for low-cost, effective alternatives [33, 37, 44, 52, 53, 57, 60, 62]. This is the reason why some researchers have proposed hybrid ventilation as a balance between the efficiency and feasibility of reducing transmission risk [4, 35, 46].

Across modelling and empirical studies, mask use consistently produced large reductions in infection probability [4, 11, 45, 52, 64], often exceeding the effect of ventilation alone. Occupancy reduction [11], microphone use (reducing voice projection) [52], and CO₂ monitoring [4] also improved outcomes.

These findings support a multi-layered approach, where ventilation is combined with behavioural and organizational measures [4, 11, 39].

Few studies explicitly addressed safety concerns, such as: noise from portable air cleaners [15], thermal comfort when windows are opened [2], or the carbon footprint of mechanical systems [28].

These factors are important for policymakers and may influence acceptability among teachers and students [15]. Future research should evaluate: long-term energy-health trade-offs [28], noise mitigation strategies [15], and the environmental impact of different ventilation technologies [2].

Nonetheless, despite the fact that schools are well known to be settings for the spread of airborne infectious diseases, there are different limitations to the comparison between ventilation strategies in this scoping review.

First, it is difficult to compare the results of interventional, observational and mathematical modeling studies because of their different natures. Moreover, in the 62 publications analyzed, there was no general scheme for analyzing ventilation methods and risk transmission reduction, and the results were only qualitatively analyzed. For instance, for the interventional studies, mechanical ventilation was compared to natural ventilation strategies [26], HVAC systems were compared to fan coil units [9], and HEPA filters to natural ventilation [15, 36] and a combination of measures [44]. The 17 observational studies conducted indoor air quality measurements for different time periods, one study collected molecular data (bioaerosol and saliva samples) [16], only four studies examined COVID-19 cases [13, 15, 16, 61], two studies collected students attendance records (absenteeism) [16, 28], and one conducted a survey on preventive measures [39], and another on students’ activity levels [68]. Mathematical models mainly used the Wells-Riley equation, but others used the Monte Carlo [46] and Eulerian-Lagrange methods [51]. The Wells–Riley model is based on a different assumption that represents a limitation for the studies: indoor air and aerosols are well mixed and the steady state (which represents a constant concentration of quanta overt time, which may not be held in larger rooms or those with irregular shapes and varies with ventilation rates) [20, 45]. It also requires measurement of outdoor air supply rates [64].

Second, school settings vary significantly from one article to another, which might create some measurements’ bias: not only settings of different age groups (kindergarten, primary school, secondary school, university), but also different settings around the world (Europe, USA, Latin America, Asia) with specific and unreproducible characteristics (climate, temperature, humidity, building size and features). In fact, children of different ages are exposed to significantly different environmental conditions (such as physical environments and activities) [12] and differences in height and body weight between children and adults modify the CO2 concentration [11]. Most included studies were conducted in high-income countries [1, 4, 9–13, 15–18]. Evidence from LMICs is scarce, despite the fact that many schools in these settings rely exclusively on natural ventilation and have limited resources for mechanical systems [23, 25].

Third, in many articles it was not revealed whether the school setting had preventive measures in addition to the ventilation strategies that were implemented, and in some schools mask use and vaccines were already applied. This could influence the accuracy of the results and could be a confounding factor. For instance, in [12, 13, 15, 26, 29, 33, 39, 56, 61, 64], there were a number of possible bias, such as the use of masks during lessons (but not for preschool children [12, 59] and only for 30% of students and all the staff [13]), infection control strategies (frequent hand hygiene and cleaning of surfaces [12]) and maximum physical distance (1·5 m distance at least) [12].

Finally, the lack of published experimental trials is a relevant limitation of this scoping review and further research should be conducted. Additionally, this scoping review does not contain randomized clinical trials (except for one 2025 pre-print study mentioned in the discussion section [69]), as they are unfortunately uncommon in this domain. Therefore, we need more experimental research to assess the relationship between improved indoor air quality through ventilation strategies and the reduction of infectious diseases in schools.

Taken together, the evidence suggests that ventilation and filtration strategies may reduce airborne exposure and respiratory infection risk in schools [9–13, 15–18]. Mechanical and hybrid systems offer the most reliable performance [21, 26, 28, 52], while natural ventilation remains highly context-dependent [45, 52, 53]. Multi-layered approaches combining ventilation, filtration, and behavioural measures appear most effective [4, 11, 39, 52].

However, conclusions must remain cautious due to heterogeneity in study designs, outcomes, and modelling assumptions [1, 4, 10, 23].

This scoping review highlights a growing body of evidence supporting the role of improved indoor air quality, ventilation and air-filtration interventions, in reducing respiratory infection incidence in school settings. Across diverse study designs, the findings suggest that improving air renewal through mechanical, hybrid, or optimized natural ventilation may contribute to reducing airborne exposure and respiratory illness risk among students and staff.

Mechanical ventilation, with and without filtration systems, or hybrid ventilation appears consistently more effective than natural ventilation alone in lowering infection risk, absenteeism and improving indoor air quality across empirical and modelling studies. Hybrid systems offered a balance between performance and feasibility, while natural ventilation demonstrated highly variable effectiveness depending on climatic, structural, and behavioural factors. Multi-layered approaches combining ventilation, filtration, masking, occupancy control, and CO₂ monitoring appeared to provide the greatest overall protection.

Despite the promising findings, the outcome measures and heterogeneity of the study designs with limited real-world interventions, scarce data from low- and middle-income countries, few evaluations of long-term sustainability, noise, safety, or energy-health trade-offs, as well as different school environments, limit the generalizability of the results. There is a strong need for clear goals, up-to-date guidelines for indoor air quality in schools, transdisciplinary coordination and further scientific data to increase prevention and investments by governments. The main goal is to reduce the burden of respiratory infectious diseases in schools by improving indoor air quality in schools with solutions that can balance energy resources and good indoor air quality. The evidence in the literature analyzed here supports prioritizing indoor air quality improvements as part of a comprehensive prevention strategy. Overall, the available evidence suggests that ventilation and filtration strategies represent essential components of healthier and more resilient school environments. Future work should also address safety, noise levels, and the carbon footprint of ventilation systems, as well as absenteeism, which remains an important but underreported outcome. Strengthening these measures—while ensuring feasibility, equity, and sustainability—may contribute to reducing the burden of respiratory infection and promoting healthier learning environments for students and staff.