# Expanding the application of the TAIL rating scheme to schools: schoolTAIL

Minh Tien Tran, Wenjuan Wei, Claire Dassonville, Christophe Martinsons, Pascal Ducruet, Mickaël Derbez, Corinne Mandin, Valérie Héquet, Pawel Wargocki

► To cite this version:

Minh Tien Tran, Wenjuan Wei, Claire Dassonville, Christophe Martinsons, Pascal Ducruet, et al. Expanding the application of the TAIL rating scheme to schools: schoolTAIL. Energy and Buildings, 2026, pp.116942. 10.1016/j.enbuild.2026.116942. hal-05438571v2

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# Expanding the application of the TAIL rating scheme to schools: schoolTAIL

Minh Tien Tran<sup>a,b</sup> <sup>●</sup>, Wenjuan Wei<sup>a</sup> <sup>●</sup>, Claire Dassonville<sup>a</sup>, Christophe Martinsons<sup>a</sup> <sup>●</sup>, Pascal Ducruet<sup>a</sup>, Mickaël Derbez<sup>a</sup>, Corinne Mandin<sup>a</sup> <sup>●</sup>, Valérie Héquet<sup>b</sup>, Pawel Wargocki<sup>c,∗</sup> <sup>●</sup>

## ARTICLE INFO

### Keywords:

- Indoor air quality

- Noise

- Temperature

- Light

- Healthy school buildings

## ABSTRACT

Schools have no universally adopted scheme for rating indoor environmental quality (IEQ). In this work, we adapted the TAIL rating scheme that was developed for assessing IEQ in offices and hotels so that it can be used in schools. The original TAIL scheme assesses thermal, acoustic, and luminous conditions, along with indoor air quality, using ten measured parameters and one evaluated through observation and one through simulation. We supplemented it with two other parameters, reverberation time and the concentration of nitrogen dioxide, to adequately characterize a classroom's IEQ. We have also updated the ranges for PM2.5 in accordance with the latest updates to the WHO air quality guidelines. Otherwise, we kept the principles used to develop the original TAIL rating scheme unchanged. We used measurements from schools reported in the published literature to identify the relevant parameters. We refer to the newly developed scheme as TAIL for schools, in short, schoolTAIL. We examined its feasibility using an existing database, with measurements made in 308 randomly selected schools as part of a nationwide survey in France. We showed that the new scheme turned out to be a valuable tool for identifying potential problems and prioritizing IEQ remediations, providing a comprehensive yet easy-to-understand overview of the current situation in school buildings.

## 1. Introduction

Sixty million children are receiving primary and secondary education in the EU. With five million teachers, this means that about 15 % of the EU population spends on average 1000–1400 h per year (about 10 %) in school buildings [1]. The quality of the environment in school buildings, and especially classrooms, is crucial for learning and academic performance [2]; it can affect absenteeism and cause health problems [3], not to mention the overall well-being and comfort of children and adults occupying school buildings. Despite the importance of the quality of the indoor environment in school classrooms, there is no standard, agreed-upon method for characterizing it. Such a method could provide useful information and benchmarking, which is necessary to make informed decisions regarding school renovations and retrofits. It is especially needed considering that studies in Europe and around the world show that the quality of the environment in school classrooms is inadequate and frequently much worse than in offices [4,5,6]. Monitoring the indoor environmental quality (IEQ) in school buildings is therefore of paramount importance.

IEQ depends on the quality of the environment in four major domains: the thermal, acoustic, and luminous environments, and indoor air [7]. Each of them can be characterized by numerous parameters whose levels are then associated with different quality categories, defined most often using the criteria for achieving optimal comfort for building occupants [8]. As shown by Wei et al. in their review, a total of 90 parameters can be used to characterize the quality of indoor environments, whereby thermal environment has been characterized by 19, acoustic environment by 20, visual (luminous) environment by 12, and indoor air by 39 parameters [9]. Although the individual aspects of IEQ are all well described and represented by many parameters allowing their detailed characterization, there is generally no agreed-upon method for the overall characterization of IEQ, and no set of standard parameters in each of its components has been agreed on for use in monitoring the indoor environment and ensuring its high quality.

Green building certification schemes have identified specific parameters for assessing indoor environmental quality (IEQ); however, none have been established as a standardized or widely adopted reference for monitoring the indoor environment [10,11,12]. In many cases,

<footnote><sup>a</sup> Scientific and Technical Centre for Building (CSTB), Health and Comfort Department, French Indoor Air Quality Observatory (OQAI), Marne-la-Vallée, France</footnote>

<footnote><sup>b</sup> IMT Atlantique, CNRS, GEPEA, UMR 6144, CS 20722, 44307 Nantes Cedex 3, France</footnote>

<footnote><sup>c</sup> Department of Environmental and Resource Engineering (DTU Sustain), Technical University of Denmark, 2800 Kongens Lyngby, Denmark</footnote>

<footnote>* Corresponding author.
E-mail address: pawar@dtu.dk (P. Wargocki).</footnote>
the evaluation in the green certification schemes is made using pre-defined arbitrary credits decided by experts based on the levels of parameters impacting IEQ and parameters selected for inclusion in the scheme [10,11]. This approach rates the building assets that make it possible to achieve high IEQ. The same approach was adopted in other proposed methods for describing IEQ, and was included in the IEQ-Compass [13] and Danish Standard DS3033 [14], among others.

A more systematic approach to rating IEQ was proposed by the WELL Standard, which, besides assessing the usual IEQ components by using their measured levels in a building, also characterizes other aspects of well-being, including the water and food quality [15]. There have been other attempts seeking to assess the quality of the indoor environment in buildings, including OsmoZ [16] and RESET [17], but none of them have been widely adopted or used as a widely accepted measure of IEQ. Recently, the European Commission published Level(s), an assessment and reporting method providing a common language for the sustainability performance of buildings, where thermal environment and indoor air quality (IAQ) levels are the major components characterizing the IEQ, while the parameters for describing acoustic and luminous domains are only supplementary [8].

An alternative approach for rating IEQ in buildings was proposed in 2020; it was an outcome of the EU Aldren project [18], whose purpose was to develop incentives for increasing the rate at which office buildings and hotels undergo deep energy renovation. As a part of this challenge, the project proposed a scheme that would, on the one hand, ensure that IEQ in buildings is characterized before the renovation process so that after renovation, it can be documented that it was not degraded, and on the other hand, be used to monetize the potential co-benefits of an energy retrofit. The rating scheme for IEQ named TAIL [18] was proposed. It uses twelve parameters to characterize the quality of the four domains of IEQ: thermal (T), acoustic (A), luminous (L), and indoor air (I); the worst quality is then used to characterize the overall IEQ in a building to create a strong incentive for improvement and to avoid trade-offs. The twelve parameters are air temperature, sound pressure level, benzene, formaldehyde, radon (in radon-prone areas only), particulate matter (PM<sub>2.5</sub>), and carbon dioxide concentrations (as a proxy-indicator for ventilation), relative humidity, ventilation rate (if technical systems are present), and illuminance level, which are to be measured; the presence of mould, which is to be observed (if relevant), and the daylight factor, which is simulated. They were selected, on purpose, by reference to the existing standards and guidelines describing parameters and their levels characterizing IEQ in buildings including EN Standard 16798-1 [7] and the World Health Organization (WHO) air quality guidelines [19], as well as the Level(s) [8] to avoid making too strong arbitrary decisions on which parameters with which levels should be selected and included in such a scheme; as indicated above, the documents from which the parameters were selected also prescribe their acceptable levels and/or limit values. Consequently, the proposed TAIL rating scheme simply integrates existing methods for characterizing the components of IEQ to create a new scheme. This approach was also motivated by the need to ensure that the proposed scheme is credible and refers to parameters which are prescribed by existing standards and guidelines, have already been used in practical applications, and are all well-documented in terms of their impact on IEQ. It was expected that this approach would support the adoption of the scheme.

Four quality categories were proposed in the scheme for the four domains (T, A, I, and L) and the overall IEQ, aligning with the categories proposed in EN Standard 16798-1 [7]. The feasibility of the TAIL scheme was examined in eleven office and hotel buildings [18]. Moreover, the method was supplemented by the supporting tool to be used during design – the predicTAIL scheme, which is based on simulations and allows selection and comparison of design solutions that can influence IEQ when the building is in operation [20], but it is not used for the final rating of IEQ in a building.

The purpose of the present work was to extend the use of TAIL rating scheme so that it can serve as a method for rating IEQ in school

classrooms, because such a method does not currently exist. The intention was to develop a method that researchers, school authorities, and other interested parties and stakeholders can all use. We examined whether the parameters included in the TAIL scheme provide sufficient information for an adequate characterization of IEQ in classrooms, and whether additional parameters should be included. However, we did not change the underlying principles based on which the original TAIL was developed. We also examined whether the new method is feasible for practical applications.

## 2. Methods

### 2.1. Developing a method for rating IEQ in schools

The proposed rating scheme for IEQ in schools is based on the existing scheme called TAIL that was described in the Introduction; the full rationale, parameters, quality categories, and application, as well as measuring protocol, have already been comprehensively described, and the method has been successfully used in practical applications [18]. We do not repeat these discussions and considerations in the present paper and refer the reader to the original open-access publication [18]. The principles underlying the development of TAIL were thoroughly scrutinized during the development stage. Basing the rating scheme for schools on the TAIL scheme allows the use of the same parameters and protocols in different types of buildings, making IEQ measurements more systematic, repeatable, and comparable across the entire building stock, as is the case for energy rating. If adopted in practical applications, it is expected that a family of schemes with similar principles and backgrounds would be created, which would facilitate comparability and strengthen the overall monitoring methodology needed for quantifying and assessing IEQ in the entire building stock.

To proceed with the adaptation of the TAIL rating scheme, it was necessary to review the parameters included in the original TAIL rating scheme and their ranges in different quality categories to determine whether they were pertinent for school buildings and classrooms, and to identify any missing parameters, while keeping the overall approach for creating TAIL unchanged to ensure comparability. As a first step in pursuing this task, we utilized the published review by Tran et al. [21], which was specifically prepared for the purpose of identifying IEQ parameters relevant in classrooms, and summarized the parameters that had been measured in school classrooms, discussed which were relevant to the health and comfort of the occupants of school buildings and to the learning process. The parameters identified by Tran et al. [21] were compared to those included in the original TAIL scheme [18]. Besides examining the parameters to be included in the rating scheme for schools, we also verified whether the ranges of the quality categories for the parameters included in the scheme would require updates and revisions, while referring to the same standards and guidelines used during the development of the original TAIL rating scheme. Finally, we examined the measurement protocol and the method used to determine the quality categories in the original TAIL scheme.

### 2.2. Applying the new method for rating IEQ in schools

The newly developed rating scheme for IEQ in school buildings was examined using the published results from the 2013–2017 nationwide French survey, which included measurements in 602 randomly selected classrooms in 308 schools representing the school building stock in mainland France [22]. The school locations are indicated in Fig. S1 in the Supplementary Data. Overall, 238 schools were investigated during the heating season (with the heating system in operation), and 70 schools were investigated outside the heating season. This data set was used due to its comprehensiveness; it also created opportunities to examine the pros and cons of the newly developed scheme and its dissemination capabilities. To date, it is one of the largest surveys ever conducted in schools. As the data were obtained through measurements
<figure><img src="image_2.png" /><figcaption>Fig. 1. The graphical presentation of the schoolTAIL rating scheme. Green indicates the desired quality (Category I), yellow the refined quality (Category II), orange for the ordinary quality (Category III), and red for the undesirable quality (Category IV) according to EN Standard 16798-1 [7].T stands for thermal environment, A for acoustic environment, I for indoor air, and L for luminous (visual) environment. The Roman numerals describe the quality of the overall environment, where I is the highest quality and IV the lowest.</figcaption></figure>

performed only in France, they were compared with those from the European SINPHONIE project that investigated 114 schools from 23 countries in 2010–2012 [23] to examine the extent to which the data from France represented the IEQ data obtained in other countries. The SINPHONIE project is one of the largest monitoring campaigns in schools that has been conducted in Europe.

### 3. Developing a new rating scheme for IEQ in schools (schoolTAIL)

The subsequent subsections describe the development of the rating scheme for the indoor environment in schools. The scheme has been named schoolTAIL because it is based on the original TAIL rating scheme developed for offices and hotel rooms. This is also why the graphical design is the same (Fig. 1). The new rating scheme extends the TAIL family of rating schemes by introducing additional methods that should be applied in school buildings. The parameters included in the TAIL and schoolTAIL are compared in Fig. 6.

#### 3.1. Rating of thermal environment

Air temperature was selected to characterize the thermal environment in the original TAIL rating scheme for offices and hotels [18]. The selection of air temperature was also motivated by studies showing its effects on occupants’ well-being and cognitive performance [25,26,27].

[24]. With the development of sensors (e.g., https://www.instructables.com/How-to-Build-an-Indoor-Comfort-Monitoring-Sensor-S/) and the extension of the TAIL scheme to all buildings, not just energy-retrofitted ones, it may be advisable to include operative temperature in any future revisions of the rating scheme. Other parameters, such as mean radiant temperature and air speed, can also be used to characterize the indoor thermal environment, because their inclusion leads to a better prediction of an occupant's thermal comfort [21]. However, measuring these parameters in buildings, especially on a large scale, is now relatively expensive and complicated.

The review of Tran et al. [21] performed specifically to identify parameters measured in schools, reported that the air temperature was the predominantly monitored parameter to characterize the thermal environment. Other parameters like mean radiant temperature, operative temperature, and air speed were measured only sporadically (Fig. 2), not least because of the complicated protocol required, as discussed above. For example, Simonic et al. argued that using globe thermometers to measure mean radiant temperature caused economic problems as the equipment had to be installed in 145 classrooms, and opted for measuring only air temperature [24]. Concerning the measuring protocols, Tran et al. observed that there was no consistency regarding measurements of air temperature: Studies employed different measurement positions, duration, and even the number of sensors per classroom, depending on the study’s objectives [21].

Considering the conclusions drawn by Tran et al. [21] and additionally considering that studies in schools have documented a strong relationship between classroom temperatures and learning [25,26,27], we decided to retain air temperature as the sole parameter used to characterize the thermal environment in the new method for rating IEQ in schools; however, we did not use learning as the outcome to define the quality levels because it is not currently used in any standards regarding IEQ in classrooms. We also retained the values defining the four quality categories of the thermal environment (Table 1) because the thermal responses of children in schools have been assumed to be the same as those of adults for the same operative temperature, as stipulated in the EN 16798-1 standard. [7]. Recent studies by Teli et al. [28] and War-gocki and Wyon [27] suggest that the thermal responses of children may be shifted toward lower temperatures by approximately 2–3 °C; however, these results are not yet reflected in existing standards for school buildings, and therefore not considered for defining the quality levels in schoolTAIL. Once these data, which are specific to children, become available and are included in standards, the ranges used for rating thermal environments in classrooms in the schoolTAIL rating scheme would need to be revised.

Table 1 shows two different sets of values used during and outside the heating season. For the measurements made outside the heating season in buildings without a mechanical cooling system, the upper and lower limit values are defined based on the calculation of the outdoor running mean temperature ($\theta_{rmas}$) as follows [7]; the same methodology is stipulated in the original TAIL rating scheme [18]:

$$ \theta_{rm} = (1 - \alpha)(\theta_{d-1} + \alpha\theta_{d-2} + \alpha^2\theta_{d-3}) $$

The following approximate formula can be used as an alternative [7]:

$$ \theta_{rm} = \frac{(\theta_{d-1} + 0.8 \times \theta_{d-2} + 0.6 \times \theta_{d-3} + 0.5 \times \theta_{d-4} + 0.4 \times \theta_{d-5} + 0.3 \times \theta_{d-6} + 0.2 \times \theta_{d-7})}{3.8} $$

The use of operative temperature was considered when the original TAIL rating scheme was developed [18]. However, air temperature was primarily chosen because it is more challenging to measure operative temperature and because, in modern well-insulated buildings, the differences between air temperatures and operative temperatures are small

where:

$\theta_{rm}$ = the outdoor running mean temperature for the measured day (°C).

$\theta_{d-i}$ = the daily mean outdoor air temperature for the i^th^ previous day
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Typically measured parameters characterizing the thermal environment in classrooms and the number of studies in schools where they were measured as reported in the literature review by Tran et al. [21].</figcaption></figure>

<caption>**Table 1**</caption>

The ranges of air temperature defining the quality categories of the thermal environment in the rating scheme for schools (schoolTAIL); the quality categories are represented by colours, green being the highest.

<table><thead><tr><th>Green (Category I)</th><th>Yellow (Cat. II)</th><th>Orange (Cat. III)</th><th>Red (Cat. IV)</th></tr></thead><tbody><tr><td colspan="4">Building with mechanical cooling*</td></tr><tr><td>Heating season:<br>22 ± 1 °C</td><td>Heating season:<br>22 ± 2 °C</td><td>Heating season:<br>22 ± 3 °C</td><td rowspan="2">If other quality levels cannot be achieved</td></tr><tr><td>Outside heating season:<br>24.5 ± 1 °C</td><td>Outside heating season:<br>24.5 ± 1.5 °C</td><td>Outside heating season:<br>24.5 ± 2.5 °C</td></tr><tr><td colspan="4">Building without mechanical cooling*</td></tr><tr><td>Heating season:<br>22 ± 1 °C</td><td>Heating season:<br>22 ± 2 °C</td><td>Heating season:<br>22 ± 3 °C</td><td rowspan="2">If other quality levels cannot be achieved</td></tr><tr><td>Outside heating season:<br>Upper limit:<br>0.33 θ<sub>rm</sub> + 18.8<br>+ 2 °C</td><td>Outside heating season:<br>Upper limit:<br>0.33 θ<sub>rm</sub> + 18.8<br>+ 3 °C</td><td>Outside heating season:<br>Upper limit:<br>0.33 θ<sub>rm</sub> + 18.8<br>+ 4 °C</td></tr><tr><td>Lower limit:<br>0.33 θ<sub>rm</sub> + 18.8 – 3 °C</td><td>Lower limit:<br>0.33 θ<sub>rm</sub> + 18.8 – 4 °C</td><td>Lower limit:<br>0.33 θ<sub>rm</sub> + 18.8 – 5 °C</td><td></td></tr></tbody></table>

$θ_{rm}$ = outdoor running mean temperature on the day of measurement (°C).

*Standard EN 16798-1, 2019[7].

(°C).

$α$ = constant between 0 and 1 (recommended value is 0.8).

## 3.2. Rating of the acoustic environment

The background noise level (LAeq) was used to characterize the acoustic environment in the original TAIL rating scheme for offices and hotels [18] in accordance with the EN 16798-1 standard [7]; LAeq is defined as the background sound pressure level at a given location. The review of Tran et al. [21], performed specifically to identify parameters measured in schools, showed that the background noise level was the most frequently measured acoustic parameter in schools (Fig. 3). In some studies, the quality of sound transmission in the classroom was additionally measured by monitoring the reverberation time (RT) and/or speech intelligibility.

A key difference between the acoustic environments in offices and

hotels compared to classrooms is the need for conditions supporting effective verbal communication. Effective communication between teachers and students is crucial for learning, especially in lower grades, where children are less familiar with the spoken language than adults due to their limited vocabulary and grammar skills [29]. Additionally, children with concentration deficits and those in two-language primary schools may have difficulty understanding spoken language [34]. Consequently, it would be ideal to monitor speech intelligibility in school classrooms to measure how much the classroom environment supports verbal communication; however, these measurements are complex and require specialized technical skills [21]. Even if alternative methods are used, such as word articulation tests [30], there are generally no accepted reference guideline values for these parameters in international standards or national building regulations. It would thus be difficult to justify the ranges defining different quality classes if included in the schoolTAIL. Consequently, the parameters describing speech intelligibility were not selected.

Instead of measuring speech intelligibility, the reverberation time (RT) can be determined in classrooms to assess how much the acoustic environment facilitates verbal communication. RT is defined as the time it takes for the sound to decay to a certain level after the sound ends abruptly; it is used to determine whether sounds could be a nuisance in a closed room. Studies have documented improved phonological processing in classrooms where RT was short, although there was no significant effect on children's spelling ability [4,31]. A recent simulation study of the acoustic environment in a classroom showed that both LAeq and RT influenced speech intelligibility, and classrooms with a low LAeq and a short RT showed high-quality speech intelligibility [32,33]. Bis-tafa et al. [31] suggested that in a classroom with low background noise, RT in the 0.4 to 0.5 s range would ensure 100 % speech intelligibility. Moreover, several studies in schools have documented that the background noise level (LAeq) affects children's well-being and academic performance [4,34]. For example, Shield et al. showed that children can read faster in a quiet classroom, and the background noise level in classrooms should usually be below 35–40 dB(A) [34]. Some studies have shown that anthropogenic noise from outside sources can also reduce children's performance of schoolwork, and the younger the children, the more negatively affected they are by noise [34,35,36]. For these reasons, it is clear that measurements of the background noise level together with RT are essential in assessing the acoustic
<figure><img src="image_4.png" /><figcaption>Fig. 3. Typically measured parameters characterizing the acoustic environment in classrooms and the number of studies in schools where they were measured as reported in the literature review by Tran et al. [21].</figcaption></figure>

<caption>**Table 2**</caption>

The ranges of LAeq and RT defining the quality categories of acoustic environment in the rating scheme for schools (schoolTAIL); the quality categories are represented by colours, green being the highest.

<table><thead><tr><th>Parameter</th><th>Green (Category I)</th><th>Yellow (Cat. II)</th><th>Orange (Cat. III)</th><th>Red (Cat. IV)</th></tr></thead><tbody><tr><td>Background sound pressure level (LA<sub>eq</sub>)*</td><td>≤ 30 dB(A)</td><td>≤ 34 dB(A)</td><td>≤ 38 dB (A)</td><td>If other quality levels cannot be achieved</td></tr><tr><td>Reverberation time*</td><td>Classroom with volume < 250 m<sup>3</sup>: 0.4 – 0.6 s<br>Classroom with volume ≥ 250 m<sup>3</sup>: 0.6 – 0.8 s</td><td>Classroom with volume < 250 m<sup>3</sup>: 0.6 – 0.8 s<br>Classroom with volume ≥ 250 m<sup>3</sup>: 0.8 – 1.2 s</td><td>No criteria</td><td>If other quality levels cannot be achieved</td></tr></tbody></table>

environment in classrooms.

To characterize the acoustic environment in the rating scheme for school (schoolTAIL), we retained the background sound level proposed in the original TAIL scheme and supplemented it with reverberation time. Table 2 presents the values that define the four quality categories of the acoustic environment in classrooms included in the schoolTAIL. The values of LAeq are in accordance with those defined in the EN 16798–1 standard [7]. The values of RT were taken from Article 5 of the order of April 25, 2003, related to the limitation of noise in educational buildings in France [37], and from the Danish Building Regulations [38]; they basically follow the general recommendations regarding RT in classrooms.

### 3.3. Rating of indoor air quality

Eight parameters were included in the original TAIL rating scheme for offices and hotels to assess indoor air quality (IAQ) [18]; currently, there is no agreed-upon metric for IAQ. They include the concentrations of benzene, formaldehyde, radon (in radon-prone areas only), carbon dioxide (CO₂) (as a proxy-indicator of ventilation), and airborne particles (PM₂.₅), the extent of visible mold, relative humidity, and ventilation rate (if there is mechanical ventilation); CO₂ and ventilation rate should be used interchangeably as CO₂ is a proxy-indicator of ventilation, and the latter is difficult to measure in many practical applications. These parameters are intended to characterize indoor air quality in terms of the outdoor pollutants entering the indoor spaces, indoor

sources of pollution, and the means available for controlling their levels. The review of Tran et al. [21] performed specifically to identify parameters measured in schools, showed that CO₂ concentration was measured most frequently in schools, followed by PM₂.₅, radon, and formaldehyde (Fig. 4).

Previous studies showed that increasing ventilation in school classrooms from 2 to 7.5 L/s per person would improve long-term academic achievement on standardized tests by 5% [39], while reducing CO₂ concentration from 2100 ppm to 900 ppm, indicating improved ventilation rate, was shown to improve the performance of children on psychological tests and academic tasks by up to 12% [39]. Concerning other pollutants, benzene, formaldehyde, and radon are classified as Group 1 carcinogens, i.e., carcinogens that are known to cause cancer in humans, by the International Agency for Research on Cancer (IARC) [40]. Lam et al. concluded from their meta-analysis that there was sufficient evidence of an association between exposure to formaldehyde and asthma diagnosis and asthma symptoms in children [41]. PM₂.₅ can penetrate deep into the respiratory system, causing inflammation and irritation of the airways, as well as respiratory issues such as asthma, bronchitis, and other infections in children [42]. Maintaining optimal relative humidity is essential for comfort and well-being. Low relative humidity can cause discomfort in the respiratory tract, throat, and eyes because of dryness, while high relative humidity can promote mold growth [43]. Because schools are special environments designed mainly for children whose bodies are still developing and who breathe a higher volume of air relative to their body weight compared to adults, it is essential to consider all relevant pollutants also those not included in the original TAIL. One of them is nitrogen dioxide (NO₂), whose main source is combustion, particularly from traffic exhaust [44]. It is particularly harmful for children, and the recent revision of the WHO Air Quality Guidelines halved its guideline value, mainly for this reason [42]. Despite the potential harm to health, this pollutant was not included in the original TAIL rating scheme [18], though it had been measured in schools (Fig. 3). We considered that this set of parameters would best characterize IAQ in classrooms, yet we did not change the outcomes based on which the reference values for these parameters were selected to maintain similarity with the original TAIL.

To characterize indoor air quality in the rating scheme for schools (schoolTAIL), we decided to retain all other parameters included in the original TAIL rating scheme and supplement them with NO₂. Table 3 presents the values that define the four quality categories characterizing indoor air quality in the school rating scheme (schoolTAIL). The most stringent limit for the benzene concentration was based on the WHO guidelines for indoor air quality [19] for an excess lifetime risk of 1 in 100,000, rounded to the nearest unit, while the least stringent limit was taken from the EU Directive 2008/50/EC on ambient air quality and

<footnote>* Standard EN 16798–1, 2019[7].</footnote>

<footnote>** French regulations [37] and Danish Building Regulations[38].</footnote>
<figure><img src="image_5.png" /><figcaption>Fig. 4. Typically measured parameters characterizing indoor air quality in classrooms and the number of studies in schools where they were measured, as reported in the literature review by Tran et al. [21]; BTEX: benzene, toluene, ethylbenzene, xylenes; SVOCs: semivolatile organic compounds; VOCs: volatile organic compounds.</figcaption></figure>

cleaner air for Europe. The values for the formaldehyde and CO₂ con-
centrations, relative humidity, and ventilation rate are from the EN
16798-1 standard [7]. The most stringent limit for the NO₂ concentra-
tion is based on the WHO Air Quality Guidelines, revised in 2021 [42],
and the least stringent limit is from the French Agency for Food, Envi-
ronmental, and Occupational Health & Safety [45]. The values for radon
concentration are based on the WHO guidelines for indoor air quality
[19] and the EU Directive 2013/59/EURATOM. The values for the PM₂.₅
concentration were adjusted compared with the original TAIL rating
scheme to align with the revision of the WHO Air Quality Guidelines in
2021 [42]. Finally, for mold, the values from the Nordic classification
system for mould assessment were used [8].

3.4. *Rating of the luminous environment*

The daylight factor and the total illuminance were selected to assess
the quality of the luminous environment in the original TAIL rating
scheme for offices and hotel rooms [18]. The daylight factor is impor-
tant, as daylight has been shown to be the most potent light source for
regulating the circadian rhythm [46,54]. The review by Tran et al. [21],
which was specifically conducted to identify parameters measured in
schools, showed that total illuminance measured at various locations in
the classroom was predominantly used to characterize the visual envi-
ronment in classrooms (Fig. 5). Artificial light plays a crucial role, as it
provides adequate illuminance levels when there is insufficient daylight
[46]. However, other factors also influence total illuminance. A

comprehensive study by Leccese et al. [47] reported that daylight glare
is the second most important factor contributing to students’ visual
comfort; however, assessing glare is currently rather complicated.

To characterize luminous environment in the rating scheme for
schools (schoolTAIL), we decided to retain the original parameters
included in the TAIL rating scheme, which are daylight factor and illu-
minance, together with their ranges defining different quality cate-
gories. However, we have extended the measuring protocol to include
assessments of total illuminance on students’ tables and teachers’
blackboards, both of which require adequate lighting levels.

Table 4 presents the values that define the four quality categories characterizing the luminous environment, as used in the method for rating IEQ in schools. The values for the quality categories of the daylight factor are based on the EN 17037 + A1 standard [48], for the latitude of Brussels. Schools in different geographic zones should consult the standard and change the guideline value to that of a nearby city. The values for the quality categories of artificial light are based on the EN 12464-1 standard [49]. When the original procedure specified in EN 12464-1:2021 [48] cannot be fully implemented to provide a set of representative records of measurement while reducing operational costs, spot measurements in at least six points should be made, one directly under artificial light, one directly situated between two overhead lights, and one point situated between these two points; another set of these three points should be measured in the same classroom.
<caption>**Table 3**</caption>

The ranges of the nine parameters defining the quality categories of indoor air in the rating scheme for schools (schoolTAIL); the quality categories are represented by colours, green being the highest.

<table><thead><tr><th>Parameters</th><th>Green (Category I)</th><th>Yellow (Cat. II)</th><th>Orange (Cat. III)</th><th>Red (Cat. IV)</th></tr></thead><tbody><tr><td>Benzene<sup>a,b</sup></td><td>&lt; 2 µg/m³</td><td>≥ 2 µg/m³</td><td>No criteria</td><td>≥ 5 µg/m³</td></tr><tr><td>Formaldehyde<sup>a,c</sup></td><td>&lt; 30 µg/m³</td><td>≥ 30 µg/m³</td><td>No criteria</td><td>≥ 100 µg/m³</td></tr><tr><td>NO<sub>2</sub><sup>d,e</sup></td><td>&lt; 10 µg/m³</td><td>&lt; 20 µg/m³</td><td>No criteria</td><td>≥ 20 µg/m³</td></tr><tr><td>Radon<sup>b,f</sup></td><td>&lt; 100 Bq/m³</td><td>≥ 100 Bq/m³</td><td>No criteria</td><td>≥ 300 Bq/m³</td></tr><tr><td>PM<sub>2.5</sub><sup>d</sup></td><td>&lt; 5 µg/m³</td><td>≥ 5 µg/m³</td><td>No criteria</td><td>≥ 15 µg/m³</td></tr><tr><td>Relative humidity<sup>c</sup></td><td>30 – 50 %</td><td>25 – 60 %</td><td>20 – 70 %</td><td>If other quality levels cannot be achieved</td></tr><tr><td>Visible mould<sup>g</sup></td><td>No visible sign</td><td>&lt; 400 cm²</td><td>&lt; 2500 cm²</td><td>≥ 2500 cm²</td></tr><tr><td>CO<sub>2</sub><sup>c</sup></td><td>550 ppm above the outdoor concentration</td><td>800 ppm above the outdoor concentration</td><td>1350 ppm above the outdoor concentration</td><td>If other quality levels cannot be achieved</td></tr><tr><td>Ventilation rate<sup>c</sup></td><td>≥ (10 L/s/p + 2.0 L/s/m² floor)</td><td>≥ 7 L/s/p + 1.4 L/s/m² floor and &lt; 10 L/s/p + 2.0 L/s/m² floor</td><td>≥ 4 L/s/p + 0.8 L/s/m² floor and &lt; 7 L/s/p + 1.4 L/s/m² floor</td><td>If other quality levels cannot be achieved</td></tr></tbody></table>

### 3.5. Rating of the overall quality of the indoor environment in a school building

The proposed method for rating the overall quality of the indoor environment is aligned with the methodology developed for the original TAIL rating scheme [18].

In every classroom where IEQ measurements are conducted, each parameter is evaluated and classified into one of the four TAIL quality categories, following the guidelines described in Tables 1 to 4. Then, for the school building, the quality category of each parameter is obtained by calculating an interim rating using the respective categories obtained in the investigated classrooms. The interim rating is the average of the ranks (R) of the quality categories, where R = 1 for Category I (green level), R = 2 for Category II (yellow level), R = 3 for Category III (orange level), and R = 4 for Category IV (red level).

The quality category of T, A, I, and L, is determined based on the parameter that has the lowest category within each component defining TAIL. Once the quality category of each component has been determined, the overall indoor environmental quality (IEQ) for the building is defined by the lowest quality category among the four components. The reason for this conservative approach is explained in detail in the publication describing the original TAIL [18]. Briefly, this approach was proposed to ensure that deficiencies in any single component are not overlooked and to avoid trade-offs between different components defining TAIL. The Discussion section lists other arguments supporting this approach.

### 3.6. Assessment protocol for IEQ rating in schools

The proposed protocol for assessing the parameters included in the schoolTAIL rating scheme is aligned with the protocol of the original TAIL scheme [18]. Each parameter must be assessed in several classrooms. The number of classrooms should be balanced, representing the diversity in the entire building while ensuring technical and economic feasibility. In the original TAIL rating scheme for offices and hotels, a suggestion was made to cover at least 10 % of the total building area or make measurements at a minimum of two and at most ten locations [18]. The same approach is retained in the schoolTAIL scheme.

In each classroom, the assessments should ideally be performed in the centre, not closer than 1 m from the wall, about 0.8 to 1 m above the floor, or on a student's table; the relevant measuring standards should be followed, and the locations of measurements documented. For thermal environment measurements, a height of 0.6 m is considered to represent the conditions experienced by a seated child. Illuminance, as indicated earlier, should be measured at two locations, which differs from the original protocol for TAIL [18]. Instruments must not be placed in ventilation ducts, near heating sources, or in direct sunshine. To avoid seasonality bias, measurements should be made in both winter and summer seasons, although this is not considered mandatory; this recommendation does not apply to radon measurements. In each season, the assessment should be performed during one school week, from Monday to Friday, except for temperature and relative humidity, for

<figure><img src="image_6.png" /><figcaption>**Fig. 5.** Typically measured parameters characterizing the visual environment in schools and the number of studies in schools where they were measured, as reported in the literature review by Tran et al. [21].</figcaption></figure>

<footnote>a WHO guidelines for indoor air quality (2010) [19];</footnote>

<footnote>b Directive 2008/50/EC, 2008;</footnote>

<footnote>c Standard EN 16798-1: 2019 [7] assuming non-low polluted building (for ventilation rate) and CO₂ emission rate of 20 L/h per person (for CO₂);</footnote>

<footnote>d WHO Air Quality Guidelines 2021 [42];</footnote>

<footnote>e French Agency for Food, Environmental, and Occupational Health & Safety (2013) [45];</footnote>

<footnote>f EU Directive 2013/59/EURATOM;</footnote>

<footnote>g Nordic classification system of mould assessment [8].</footnote>
<table><thead><tr><th>Component</th><th>Original TAIL</th><th>schoolTAIL*</th><th>Nationwide survey of schools in France (2013-2017)</th></tr></thead><tbody><tr><td>T<br>(Thermal environment)</td><td>Air temperature</td><td>Air temperature</td><td>Air temperature</td></tr><tr><td>A<br>(Acoustic environment)</td><td>Background sound pressure level</td><td>Background sound pressure level<br>Reverberation time</td><td>Background sound pressure level</td></tr><tr><td>I<br>(Indoor air)</td><td>Benzene<br>Formaldehyde<br>Radon<br>PM<sub>2.5</sub><br>Visible mould<br>Relative humidity<br>CO<sub>2</sub><br>Ventilation rate</td><td>Benzene<br>Formaldehyde<br>Radon<br>PM<sub>2.5</sub><br>Visible mould<br>Relative humidity<br>CO<sub>2</sub><br>Ventilation rate<br>NO<sub>2</sub></td><td>Benzene<br>Formaldehyde<br>-<br>PM<sub>2.5</sub><br>Visible mould<br>Relative humidity<br>CO<sub>2</sub><br>-<br>NO<sub>2</sub></td></tr><tr><td>L<br>(Luminous – visual environment)</td><td>Daylight factor<br>Artificial illuminance</td><td>Daylight factor<br>Artificial illuminance: horizontal on the desks<br>Artificial illuminance: vertical on the blackboard</td><td>-<br>Artificial illuminance: horizontal on the desks<br>Artificial illuminance: vertical on the blackboard</td></tr></tbody></table>

*Shaded are additional parameters to be rated in schools compared to the original TAIL for offices and hotels; Bolded are parameters whose measurement protocol in schools was modified compared to the original TAIL

<caption>**Fig. 6.** Comparison of the components included in the original TAIL scheme [18], in the schoolTAIL (proposed in the present paper), and measured during the school campaign in France used to examine schoolTAIL.</caption>

<caption>**Table 4**</caption>

The ranges of daylight factor and illuminance defining the quality categories of luminous in the rating scheme for schools (schoolTAIL); the quality categories are represented by colours, green being the highest.

<table><thead><tr><th>Parameters</th><th>Green (Category I)</th><th>Yellow (Cat. II)</th><th>Orange (Cat. III)</th><th>Red (Cat. IV)</th></tr></thead><tbody><tr><td>Daylight factor<sup>*</sup></td><td>≥ 5.0 %</td><td>≥ 3.3 %</td><td>≥ 2.0 %</td><td>If other quality levels cannot be achieved</td></tr><tr><td>Total illuminance (horizontal on students' desks and vertical on blackboards)<sup>*</sup></td><td>≥ 500 lx</td><td>≥ 300 lx</td><td>≥ 200 lx</td><td>If other quality levels cannot be achieved</td></tr></tbody></table>

which measurements can continue for one month, and for radon, which requires a measurement period of two months. Table S1 in the Supplementary Data presents the full protocol.

## 4. Examining the feasibility of the schoolTAIL rating scheme

The data from the French nationwide school survey, conducted between 2013 and 2017 [22,50], was used to assess the feasibility of the TAIL rating scheme for schools. This survey provides one of the most comprehensive sets of measurements in schools to date, even though the reverberation time, radon concentration, and ventilation rate were not included in the measuring protocol, and the daylight factor calculation could not be performed at this scale. Consequently, the performance of the schoolTAIL rating scheme could only be evaluated using 10 out of the 14 parameters (Fig. 6). The levels of the 10 parameters assessed in the French survey are shown in Table 5. Although only 10 parameters were used, the size of the data set still makes this validation pertinent.

Because the data used for assessing schoolTAIL were only from France, they were compared with the results of measurements made by the European SINPHONIE project that investigated indoor air quality

<footnote>* Standard EN 17037+A1 (2021)[48].</footnote>

<footnote>** Standard EN 12464-1 (2021)[49].</footnote>
<caption>**Table 5**</caption>

The distributions of schoolTAIL parameters in the classrooms (N = 602) from the nationwide school survey in France (2013–2017).

<table><thead><tr><th>TAIL parameter</th><th>number of classrooms</th><th>mean</th><th>Standard deviation</th><th>10th percentile</th><th>25th percentile</th><th>median</th><th>75th percentile</th><th>90th percentile</th></tr></thead><tbody><tr><td><strong>T – Thermal environment</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Air temperature (°C) during occupied periods<sup>a,b</sup></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>In the heating season</td><td>237</td><td>23.1</td><td>1.8</td><td>20.8</td><td>21.9</td><td>23.1</td><td>24.3</td><td>25.3</td></tr><tr><td>Outside the heating season</td><td>68</td><td>24.7</td><td>2.1</td><td>22.0</td><td>23.1</td><td>24.6</td><td>26.2</td><td>27.5</td></tr><tr><td><strong>A – Acoustic environment</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Noise level (dB(A)) during unoccupied periods<sup>a</sup></td><td>541</td><td>31</td><td>4.0</td><td>26</td><td>28</td><td>30</td><td>34</td><td>39</td></tr><tr><td>Reverberation time (s)</td><td colspan="8">Not assessed</td></tr><tr><td><strong>I – Indoor air quality</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Benzene (µg/m³)</td><td>583</td><td>1.4</td><td>0.9</td><td>0.5</td><td>0.9</td><td>1.2</td><td>1.7</td><td>2.3</td></tr><tr><td>Formaldehyde (µg/m³)</td><td>602</td><td>21</td><td>12</td><td>9.2</td><td>13</td><td>19</td><td>28</td><td>36</td></tr><tr><td>NO₂ (µg/m³)</td><td>598</td><td>9.0</td><td>10</td><td>&lt;LOD</td><td>&lt;LOD</td><td>5.5</td><td>12</td><td>22</td></tr><tr><td>Radon (Bq/m³)</td><td colspan="8">Not assessed</td></tr><tr><td>PM₂.₅ (µg/m³)</td><td>573</td><td>21</td><td>11</td><td>11</td><td>14</td><td>18</td><td>24</td><td>31</td></tr><tr><td>Mould (presence of visible mould)</td><td>599</td><td colspan="8">No visible sign: 97 %; Visible signs of mould (surface not assessed): 3 %</td></tr><tr><td>Relative humidity (%) during occupied periods<sup>a,b</sup></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>In the heating season</td><td>237</td><td>40</td><td>9.9</td><td>28</td><td>33</td><td>39</td><td>46</td><td>54</td></tr><tr><td>Outside the heating season</td><td>68</td><td>49</td><td>10</td><td>36</td><td>42</td><td>50</td><td>57</td><td>62</td></tr><tr><td>CO₂ (ppm) during occupied periods<sup>a,b</sup></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>In the heating season</td><td>238</td><td>1,291</td><td>742</td><td>508</td><td>707</td><td>1,123</td><td>1,671</td><td>2,309</td></tr><tr><td>Outside the heating season</td><td>70</td><td>1,016</td><td>618</td><td>455</td><td>545</td><td>816</td><td>1,305</td><td>1,876</td></tr><tr><td>Ventilation rate (L/s/person)</td><td colspan="8">Not assessed</td></tr><tr><td><strong>L – Luminous environment</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Daylight factor (%)</td><td colspan="8">Not assessed</td></tr><tr><td>Artificial illuminance (lux)</td><td>595</td><td>447</td><td>215</td><td>193</td><td>291</td><td>420</td><td>571</td><td>742</td></tr><tr><td>Horizontal (on the desks)</td><td>560</td><td>366</td><td>326</td><td>102</td><td>161</td><td>270</td><td>459</td><td>743</td></tr></tbody></table>

LOD: Limit Of Detection.

<caption>**Table 6**</caption>

The distributions of IEQ parameters in the schools from the European SINPHONIE project (winter 2011–2012).

<table><thead><tr><th>TAIL parameter</th><th>n classrooms</th><th>mean</th><th>Standard deviation</th><th>10th percentile</th><th>25th percentile</th><th>median</th><th>75th percentile</th><th>90th percentile</th></tr></thead><tbody><tr><td><strong>T – Thermal environment</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Air temperature (°C) during occupied periods</td><td>4,927</td><td>21.5</td><td>2.1</td><td>17.0</td><td>20.0</td><td>22.0</td><td>23.0</td><td>24.0</td></tr><tr><td><strong>A – Acoustic environment</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Noise level (dB(A)) during unoccupied periods</td><td colspan="8">Not assessed</td></tr><tr><td>Reverberation time (s)</td><td colspan="8">Not assessed</td></tr><tr><td><strong>I – Indoor air quality</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Benzene (µg/m³)</td><td>4,923</td><td>4.5</td><td>5.9</td><td>0.5</td><td>1.1</td><td>2.4</td><td>4.8</td><td>11</td></tr><tr><td>Formaldehyde (µg/m³)</td><td>4,967</td><td>15</td><td>10</td><td>5.9</td><td>7.6</td><td>11</td><td>18</td><td>28</td></tr><tr><td>NO₂ (µg/m³)</td><td colspan="8">Not reported in [23]</td></tr><tr><td>Radon (Bq/m³)</td><td>4,066</td><td>230</td><td>828</td><td>30.0</td><td>55.6</td><td>114</td><td>188</td><td>316</td></tr><tr><td>PM₂.₅ (µg/m³)</td><td>4,715</td><td>45</td><td>35</td><td>17</td><td>22</td><td>38</td><td>51</td><td>86</td></tr><tr><td>Mould (presence of visible mould)</td><td colspan="8">Not reported in [23]</td></tr><tr><td>Relative humidity (%) during occupied periods</td><td>4,927</td><td>43</td><td>14</td><td>26</td><td>36</td><td>40</td><td>50</td><td>66</td></tr><tr><td>CO₂ (ppm) during occupied periods</td><td>4,897</td><td>1,581</td><td>904</td><td>656</td><td>918</td><td>1,370</td><td>1,939</td><td>3,073</td></tr><tr><td>Air change rate (h⁻¹)</td><td>4,781</td><td>0.6</td><td>0.7</td><td>0.1</td><td>0.2</td><td>0.4</td><td>0.7</td><td>1.7</td></tr><tr><td><strong>L – Luminous environment</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Daylight factor (%)</td><td colspan="8">Not assessed</td></tr><tr><td>Artificial illuminance (lux)</td><td colspan="8">Not assessed</td></tr></tbody></table>

<footnote>a The percentiles are calculated from the raw continuous data recorded during the school week in each classroom.</footnote>

<footnote>b Measurements are missing in one school during the heating season and in two schools outside the heating season.</footnote>
<caption>**Table 7**</caption>

The quality of the thermal environment (T) according to the schoolTAIL scheme in 308 schools in the nationwide survey of schools in France; the brackets show the number of schools.

<table><thead><tr><th>% of schools (n)</th><th>Category I</th><th>Category II</th><th>Category III</th><th>Category IV</th><th>Missing data</th></tr></thead><tbody><tr><td>All seasons</td><td>4.2 % (13)</td><td>10.4 % (32)</td><td>33.1 % (102)</td><td>51.3 % (158)</td><td>1.0 % (3)</td></tr><tr><td>In the heating season</td><td>0.4 % (1)</td><td>8.4 % (20)</td><td>37.4 % (89)</td><td>53.4 % (127)</td><td>0.4 % (1)</td></tr><tr><td>In the non-heating season</td><td>17.1 % (12)</td><td>17.1 % (12)</td><td>18.6 % (13)</td><td>44.3 % (31)</td><td>2.9 % (2)</td></tr></tbody></table>

(IAQ) in 114 schools from 23 countries in 2010–2012 [23], which examined the extent to which the data from France represented the IEQ data in other countries. The data collected in the SINPHONIE project on temperature, relative humidity, benzene, formaldehyde, PM2.5, and CO₂ showed no noticeable differences when compared with the results of the French survey (Tables 5 and 6).

## 4.1. The quality of the thermal environment (T)

Table 7 presents the rating of the thermal environment in schools in the French survey according to the schoolTAIL scheme. Only 4.2 % of the schools achieved the highest quality rating (Category I), while 51 % were rated in the lowest quality (Category IV). Category I was achieved more frequently in the non-heating season.

Fig. 7 shows box plots representing the distribution of every air temperature measurement in each classroom during occupied hours according to the season and the corresponding thermal quality category according to the schoolTAIL scheme. The highest air temperature recorded in classrooms in either season was about 33 °C. The lowest air temperatures recorded in classrooms during the heating season were around 13 °C and 16 °C outside the heating season. Because low and high temperatures can lead to poor overall quality (Category III or Category IV), using the schoolTAIL scheme led to the rapid identification of schools with poor thermal conditions.

Figs. S2 and S3 in the Supplementary Data provide more details on how the air temperature evolved throughout the day. They show the median air temperature in all classrooms at each time step within a specific quality category. Regardless of the season, there was a systematic increase in temperature during the day as residual heat was building up during operating hours in the classroom. Consequently, the peak temperature and the variation of the air temperature throughout the day had the largest impact on the quality classification of the thermal environment (Fig. 6). It is interesting to note that the lowest variations in temperature occurred in the heating season during which median air temperature differed very little between quality categories, probably because temperatures in classrooms were controlled, while outside the heating season the median temperature differed considerably between the lowest and the highest quality category.

Since most French classrooms have no cooling systems but only heating systems, the temperature during the heating season was more controlled. However, during heating season, the median temperature was rather higher (> 23 °C), thus the thermal environment quality was

<figure><img src="image_7.png" /><figcaption>Fig. 7. Distribution of air temperatures (°C) measured in occupied classrooms according to the season and the quality category in the schoolTAIL scheme.</figcaption></figure>
<caption>**Table 8**</caption>

The quality of the acoustic environment (A) according to the schoolTAIL scheme in 308 schools in the nationwide school survey of schools in France; the number of schools is in the brackets.

<table><thead><tr><th></th><th>Category I</th><th>Category II</th><th>Category III</th><th>Category IV</th><th>Missing data</th></tr></thead><tbody><tr><th>% of schools (n)</th><td>29 % (90)</td><td>32 % (99)</td><td>18 % (57)</td><td>16 % (49)</td><td>5 % (13)</td></tr></tbody></table>

generally poor (categories III and IV). During non-heating season, there were more variations across the day, thus the poor thermal environment quality (categories III and IV) resulted from the variations across different temperature categories' thresholds.

## 4.2. The quality of the acoustic environment (A)

Table 8 presents the quality of the acoustic environment in schools in the French survey. Only background sound pressure levels were used because RT was not available; therefore, the rating of the quality categories presented in this section is incomplete according to the schoolTAIL scheme.

The results show that 29 % of schools were classified in the highest quality category and 16 % in the lowest.

Fig. 8 shows the distribution of the measured background sound pressure levels ($LA_{eq}$) in 295 schools (average of $LA_{eq}$ in classrooms), according to the categories defining the quality of the acoustic environment and the surrounding traffic levels, reported as “high” or “medium to low” using the governmental website (https://www.geoportail.gouv.fr). The $LA_{eq}$ was significantly higher in schools located near areas with high traffic levels than in those located near areas with medium to low traffic levels (Wilcoxon rank-sum test; $p < 0.0001$). Similarly to what was observed for the thermal environment, the schoolTAIL scheme could differentiate the schools depending on their acoustic conditions.

<caption>**Table 9**</caption>

Indoor air quality (I) according to the schoolTAIL scheme in 308 schools in the nationwide survey of schools in France; the brackets show the number of schools.

<table><thead><tr><th>% of schools (n)</th><th>Category I</th><th>Category II</th><th>Category III</th><th>Category IV</th><th>Missing data</th></tr></thead><tbody><tr><th>Benzene</th><td>80.2 % (247)</td><td>17.2 % (53)</td><td>0.0 % (0)</td><td>1.0 % (3)</td><td>1.6 % (5)</td></tr><tr><th>Formaldehyde</th><td>76.9 % (237)</td><td>23.1 % (71)</td><td>0.0 % (0)</td><td>0.0 % (0)</td><td>0.0 % (0)</td></tr><tr><th>NO<sub>2</sub></th><td>65.0 % (200)</td><td>21.7 % (67)</td><td>0.0 % (0)</td><td>13.0 % (40)</td><td>0.3 % (1)</td></tr><tr><th>PM<sub>2.5</sub></th><td>0.3 % (1)</td><td>22.1 % (68)</td><td>0.0 % (0)</td><td>75.6 % (233)</td><td>2.0 % (6)</td></tr><tr><th>Visible mould</th><td>94.2 % (290)</td><td>5.5 % (17)</td><td>0.0 % (0)</td><td>0.0 % (0)</td><td>0.3 % (1)</td></tr><tr><th>Relative humidity</th><td>14.3 % (44)</td><td>50.3 % (155)</td><td>28.2 % (87)</td><td>6.2 % (19)</td><td>1.0 % (3)</td></tr><tr><th>CO<sub>2</sub></th><td>1.0 % (3)</td><td>8.1 % (25)</td><td>21.8 % (67)</td><td>69.1 % (213)</td><td>0.0 % (0)</td></tr><tr><th>Overall IAQ</th><td>0.0 % (0)</td><td>1.6 % (5)</td><td>5.2 % (16)</td><td>93.2 % (287)</td><td>0.0 % (0)</td></tr></tbody></table>

## 4.3. The quality of indoor air (I)

Table 9 presents the indoor air quality (I) measured in schools in the French survey. We did not have data for ventilation rates and radon, so the rating of the quality categories presented in this section is incomplete according to the schoolTAIL scheme.

The results show that more than 90 % of the schools achieved at least the second-best quality level, Category II, for benzene, formaldehyde, and mold. However, most schools only achieved the lowest quality levels (Category III or IV) for $CO_2$ and $PM_{2.5}$. The quality categories for relative humidity and $NO_2$ differed significantly between the schools. Table 9 also shows the overall quality of IAQ, which was in the lowest category for 93 % of the schools; overall IAQ was calculated according to the method described in section 3.5.

Fig. 9 presents the distribution of $CO_2$ concentration in every classroom, according to the season and their schoolTAIL quality category. The median $CO_2$ concentrations were higher in classrooms with a low-

<figure><img src="image_8.png" /><figcaption>**Fig. 8.** Distribution of the background noise level (dB(A)) measured in 295 schools according to the nearby traffic conditions and the quality category in the schoolTAIL scheme. For each category, the boxplot on the left represents schools with medium to low traffic, and the boxplot on the right represents schools with high traffic.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 9.** Distribution of CO₂ concentration (ppm) measured in occupied classrooms according to the seasons and quality categories in the schoolTAIL scheme.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 10.** Distribution of NO₂ concentrations (µg/m³) measured in schools in different locations characterized by high or medium-to-low traffic; colours show the quality categories according to the schoolTAIL scheme.</figcaption></figure>

quality category. CO₂ concentration was higher in classrooms investigated during the heating season.

Figs. S4 and S5 in the Supplementary Data present the evolution of the measured median CO₂ concentration over the day. Regardless of the season, CO₂ concentration peaks were observed in the morning, around 10 am, and then in the afternoon, around 2 to 3 pm.

Fig. 10 and Fig. 11 show the NO₂ and PM₂.₅ concentrations in schools (average concentration across the investigated classrooms) depending on the traffic conditions around the schools. No statistically significant

<figure><img src="image_11.png" /><figcaption>**Fig. 11.** Distribution of PM₂.₅ concentrations (µg/m³) measured in schools in different locations characterized by high or medium-to-low traffic; colours show the quality categories according to the schoolTAIL scheme.</figcaption></figure>

### Table 10

The quality of the luminous environment (L) according to the schoolTAIL scheme in 308 schools in the nationwide survey of schools in France; the brackets show the number of schools.

<table><thead><tr><th>% of schools (n)</th><th>Category I</th><th>Category II</th><th>Category III</th><th>Category IV</th><th>Missing data</th></tr></thead><tbody><tr><td>Horizontal on desks</td><td>6.0 % (18)</td><td>36 % (112)</td><td>23 % (72)</td><td>33 % (103)</td><td>1.0 % (3)</td></tr><tr><td>Vertical on blackboards</td><td>4.0 % (11)</td><td>13 % (40)</td><td>23 % (70)</td><td>57 % (177)</td><td>3.0 % (10)</td></tr><tr><td>Overall luminous quality</td><td>1.0 % (1)</td><td>18 % (56)</td><td>31 % (97)</td><td>49 % (151)</td><td>1.0 % (3)</td></tr></tbody></table>

difference in PM₂.₅ concentrations was observed between schools located close to high or medium-to-low traffic (Wilcoxon rank-sum test; *p* = 0.065), but significantly higher concentrations of NO₂ were measured in schools located close to areas with a high traffic level (Wilcoxon rank-sum test; *p* < 0.001).

### 4.4. The quality of the luminous environment (L)

Table 10 presents the quality of the luminous environment in schools. We did not have data for the daylight factor so the rating of the quality categories presented in this section is incomplete according to the schoolTAIL scheme.

The results show that about half of the schools (49 %) had

### Table 11

Overall quality of indoor environment according to the schoolTAIL scheme in 308 schools in the nationwide survey of schools in France; brackets show the number of schools.

<table><thead><tr><th></th><th>Category I</th><th>Category II</th><th>Category III</th><th>Category IV</th><th>Missing data</th></tr></thead><tbody><tr><td>% of schools (n)</td><td>0 % (0)</td><td>0 % (0)</td><td>1 % (5)</td><td>99 % (303)</td><td>0 % (0)</td></tr></tbody></table>
<figure><img src="image_12.png" /><figcaption>Fig. 12. Quality of the components of the indoor environment in the schools used for the feasibility study of the schoolTAIL rating scheme.</figcaption></figure>

insufficient levels of illuminance and were rated in the lowest quality category (Category IV), while only one school was rated in the highest quality category (Category I). The luminous quality was better on the students' desks than on the blackboards.

## 4.5. Overall quality of indoor environment

Table 11 presents the overall quality of the indoor environment in the schools in the French survey (calculated according to the method described in section 3.5), while Fig. 12 summarizes the quality of each specific component/domain of indoor environmental quality. Because it is based on only 10 out of 14 parameters, the rating is incomplete according to the schoolTAIL rating.

No school achieved either of the two highest quality categories, Category I or II; 99 % of the schools were in the lowest quality category, Category IV; yet schoolTAIL was able to identify which aspects of IEQ should be improved.

For the four IEQ components and the overall IEQ (Table 12), and for each parameter, most schools showed the same rating across classrooms.

<table><caption>**Table 12**
Comparison of ratings of the schoolTAIL components and overall IEQ between classrooms from the same school.</caption><thead><tr><th></th><th>Thermal</th><th>Acoustic</th><th>IAQ</th><th>Lighting</th><th>Overall IEQ</th></tr></thead><tbody><tr><td>Number of schools with 2 or 3 investigated classrooms and no missing data</td><td>274</td><td>235</td><td>281</td><td>277</td><td>281</td></tr><tr><td>Number (%) of schools with the same rating across classrooms</td><td>164 (60 %)</td><td>177 (75 %)</td><td>236 (84 %)</td><td>146 (53 %)</td><td>268 (95 %)</td></tr><tr><td>Number (%) of schools in which at least one classroom was rated differently</td><td>110 (40 %)</td><td>58 (25 %)</td><td>45 (16 %)</td><td>131 (47 %)</td><td>13 (5 %)</td></tr></tbody></table>

Indoor air quality was the most homogeneous component within schools, while artificial lighting was the least homogeneous.

The Venn diagram in Fig. 13 shows which components (T, A, I, or L) influenced the rating of the overall IEQ in school buildings – when two or more ellipses overlap, the corresponding components were in the same category. Results in red circles show that in 76 schools (25 %), the overall rating of the indoor environment was determined by the quality category of the thermal environment (T) and indoor air quality (I). In 64 schools (20 %), it was determined by the quality category of the luminous environment (L) and indoor air quality (I). Indoor air quality (I) was the component that most often determined the overall schoolTAIL rating, either alone (in 54 schools, 18 % of the total number of schools) or in conjunction with other components that were rated in the same category (in 237 schools, 77 %). These results underline diversity in the IEQ conditions in schools and document that schoolTAIL can discriminate the quality rating based on different parameters.

In the Sankey plot (Fig. 14), the proportion of schools is shown in each of the quality categories I-IV that are colour-coded in the five vertical columns, one column for each component, and one for the overall quality level. In each column, the quality categories to which a greater number of schools were assigned appear above those categories to which fewer schools were assigned ("n/a" indicates the schools where no measurements were made). The width of the grey tracks connecting quality categories in adjacent columns indicates the number of schools assigned to both categories thus connected. It can be seen that the schools assigned to Category IV (shown in red) for the thermal component (T, the first column) were assigned to many different quality categories for the acoustic component (A, the adjacent column) and that this was also the case for schools assigned to Category IV for the indoor air quality component (I, the third column), and so on. The large proportion of schools assigned to Category IV for the indoor air quality component was clearly the main contributor to the large proportion of schools assigned to Category IV for the overall indoor environmental quality in schools.

In summary, the application of the schoolTAIL rating scheme to
<figure><img src="image_13.png" /><figcaption>Fig. 13. The components of the schoolTAIL rating scheme that affected the overall rating of indoor environmental quality in the French schools (N = 308).</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 14. Sankey plot showing quality categories of each component and overall quality according to the schoolTAIL scheme in 308 schools from the nationwide survey in France (2013–2017).</figcaption></figure>

measurements carried out in schools in France showed that the school-TAIL rating scheme performed well. The findings can be summarized as follows:

* The quality of thermal environment (T) was mainly determined by the variations in median temperature during the heating season and the increasing median level during the daya in the non-heating season.

* The quality of the acoustic environment (A) differed depending on the location of the schools and was lower for schools located close to heavy traffic.

* The concentrations of CO₂ and PM2.5 mainly determined indoor air quality (I). CO₂ concentration was slightly higher during the heating season.

The rating was affected by the concentration of NO₂ resulting from proximity to traffic.

* The luminous environment (L) quality was determined by illumi-nance, which was higher on students’ desks than on blackboards.

* The overall indoor environmental quality in schools was most often determined by indoor air quality.

* The schoolTAIL rating scheme was able to identify which compo-nents were in need of improvement.
# 5. Discussion

## 5.1. General comments and implications

In the present paper, we adapted the TAIL scheme [18] for rating indoor environmental quality in schools and named the new rating schoolTAIL. We retained the principles used to develop the original TAIL and added a few parameters, and adapted the measuring protocol to ensure its relevance for school buildings. The adaptation of TAIL to school buildings marks an important milestone, as it establishes a standardized method for evaluating IEQ in schools. The proposed schoolTAIL rating scheme is consequently an important step toward developing monitoring and rating schemes that will benefit not only schools but also other buildings where we spend > 85% of our lives [51] and which have become a new human habitat. This is particularly important considering that the newest Energy Performance of Buildings Directive (EPBD) is mandating measurements of IEQ in new and retrofitted buildings [52]: TAIL and schoolTAIL can play an important role in ensuring compliance with this mandate.

Applying the schoolTAIL rating scheme in schools will make it possible to evaluate and benchmark IEQ in school buildings using a similar protocol, compare them with each other as well as with other building types for which the original TAIL rating scheme was developed. The scale of the potential IEQ problems in schools can also be identified using the new rating scheme. The application of schoolTAIL is also expected to stimulate the development of measuring equipment for achieving credible and repeatable monitoring on a large scale. It is expected that such monitoring will bring benefits in the short and long term, both for children and for teachers and other school personnel, and that it will result in considerable improvements of IEQ in schools. Achieving this goal is especially crucial when aiming at environments with reduced risks for health and well-being, facilitating rather than obstructing the learning process. Finally, the application of schoolTAIL is expected to raise the awareness of IEQ among new generations and its importance for healthy living and a high quality of life.

The proposed schoolTAIL rating scheme does not include all parameters that could be used to describe IEQ in classrooms. However, it is just a first version of the scheme, and it can be further improved once we learn more about its utility and when our knowledge about IEQ in classrooms and its effects on children has been extended. An important prerequisite for increasing the number of parameters in schoolTAIL is the development of more sophisticated instrumentation for their measurement, provided that they can be deployed at a reasonable cost.

The proposed schoolTAIL does not include learning as the outcome. This change necessitates a fundamental re-evaluation of the principles that underpin the development of IEQ standards, which was not the purpose of the present work.

Although the proposed version of schoolTAIL is not a complete solution, the results presented in this paper show that it can already be a very useful tool for identifying which conditions in schools should be improved, even though the validation was not performed with all fourteen parameters included in the scheme.

New parameters relevant for classrooms were included in the proposed schoolTAIL compared with the original TAIL rating scheme [21], because they had been shown to affect children's health, well-being, and learning. The ease of use and affordability of the measurement of potential new parameters were considered, which may encourage schools to use it voluntarily. Two new parameters that were added to the existing TAIL rating scheme include reverberation time and $NO_2$ concentration. Reverberation time can affect verbal communication between teachers and students, which is crucial in schools, especially for young students whose vocabulary and grammatical skills are still developing. It predicts speech intelligibility, which is difficult to measure directly. $NO_2$ has been shown to be a risk factor for asthma in children.

The schoolTAIL differs from the recently proposed IEQ assessment

tool for schools, called CHAT:IEQ [53]. While the objectives of developing CHAT:IEQ were similar to the ones used to develop schoolTAIL, there are some essential differences in the assessment methods proposed by the two schemes. CHAT:IEQ combines arbitrary expert ratings with measurements. In other words, it combines the approach of IEQ-Compass [13] and schoolTAIL. Furthermore, credits are awarded for every IEQ component, as is typically done in certification schemes based on the subjective decisions of experts. The most important difference is that schoolTAIL ratings are based solely on measurements carried out in school buildings, without arbitrary judgments made by experts, which may not be consistent across countries or even regions. This is also the core advantage of the original TAIL rating scheme [18], which can be classified as performance metric.

This paper demonstrates the effectiveness and usefulness of the schoolTAIL rating scheme. The scheme identifies which IEQ parameters in schools require improvement, allowing resources to be allocated appropriately (Fig. 12). The findings show that not all schools require the same solutions (Fig. 12) and that the location of the school may determine how high IEQ can be obtained. These are very important observations, exemplified, for instance, in Fig. 10, where traffic conditions significantly impacted $NO_2$ concentrations in classrooms. This feature is another important benefit of using the schoolTAIL rating scheme, as it benchmarks existing conditions in schools and identifies solutions, rather than claiming that a universal solution is appropriate for all school buildings.

The proposed rating scheme stipulates a standard protocol, ensuring the measurements are comparable. It also provides a useful method for communicating and disseminating the findings. Fig. 12 shows the quality of the different IEQ components in the schools. This fig. provides a basis for immediate action, which is particularly important for authorities and decision-makers, as well as crucial for school management, parents, and their children at each school level.

Arguments may be raised about the inequality in the number of parameters for each of the four IEQ components included in the schoolTAIL rating scheme, as there is one parameter for the thermal environment, two for the acoustic environment, two for the lighting environment, and nine for indoor air quality. However, a strict selection process was followed when identifying those 14 parameters, and they represent a reasonable and well-funded combination of parameters that can be used to characterize the effects of IEQ on health and well-being in schools. They also reflect the existing guideline values from regulations, international standards, and established bodies, such as the WHO, which define the different quality categories. Finally, their measurement is relatively straightforward and reliable.

The rating method for IEQ remained unchanged during the adaptation of the TAIL rating scheme to schools: the worst-rated IEQ parameter determines the rating of each IEQ component, and the worst-rated IEQ component determines the overall IEQ rating. This may be considered too strict, but the approach does make it impossible to conceal an IEQ issue in a classroom or school behind an average value, thereby avoiding potential trade-offs. It encourages schools to improve every parameter and component with equal effort to obtain a high overall rating. This is why the principle of using the worst parameters has been sustained in schoolTAIL. More discussion on this issue is provided by Wargocki et al. [18].

The schoolTAIL scheme was validated using measurements that had been obtained previously, and some schoolTAIL parameters had not been measured, e.g., radon. For some parameters, the measurement protocol did not fully comply with the schoolTAIL recommendations. For example, because the particle counters were installed in the same space as the sound level meter during the school week, the sound from the particle counters could affect the background noise levels, so the acoustic environment was evaluated based on measurements taken over the weekends preceding the sampling week, when the particle counters were not in operation. Additionally, some data were missing. For example, outdoor air temperature was not measured, although it is
needed when assessing the thermal environment outside the heating season in schools without any mechanical cooling, according to the EN 16798-1 standard. Instead, the outdoor temperature recorded at the nearest weather station was used. In summary, the validation of the schoolTAIL rating scheme presented in this paper was conducted using only 10 out of 14 parameters, with some compromises made to account for differences in measurement protocols.

## 5.2. Future work

Applying the schoolTAIL rating scheme on other datasets, in other countries and climate regions will broaden its applicability and provide insights for its potential evolution. Future development may reduce the number of parameters to be measured, for example, by systematically eliminating correlated parameters or adding new parameters suggested by new scientific evidence or emerging technical capabilities. The schoolTAIL rating scheme currently provides a snapshot of IEQ at a specific moment in one or two seasons. In the future, if technical capabilities allow it, the schoolTAIL rating scheme (similar to the original TAIL rating scheme) could evolve to be based on real-time, low-cost IEQ monitoring, providing a more comprehensive evaluation over time. At the classroom level, a real-time indicator could alert occupants whenever IEQ parameters exceed some thresholds, enabling prompt mitigation. At the school level, it would provide stakeholders with insights for prioritizing improvements in IEQ.

Evaluating the overall IEQ rating using a different principle might be a useful way to assign priorities among schools. An example of a different rating of overall IEQ is shown in Fig. 16. In this approach, each quality category of the component is assigned a rank from 1 (the highest) to 4 (the lowest), and the ranks are summed for each school. Then they are converted to % where 4 is 100% (the highest quality level), and 16 is 0% (the lowest quality level). As shown in Figs. 15 and 16 for the schools investigated in the Paris metropolitan area, this method provides improved discrimination of the overall quality of IEQ in schools, while still providing the information on the quality of each component. Further studies are needed to investigate this approach (see also Fig. S6 in the Supplementary Data).

Future developments of the TAIL rating scheme might incorporate the responses of building occupants, i.e., their subjective evaluations. With the currently proposed objectively measured parameters, the inclusion of subjective ratings would provide a more complete evaluation of indoor environmental quality and improve discrimination between

<figure><img src="image_15.png" /><figcaption>Fig. 15. Overall indoor environmental quality in schools with the original TAIL colors scheme (Paris intra-muros area).</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Fig. 16. Overall indoor environmental quality in schools with the alternative scoring in 100% (circled numbers), where 100% is the highest quality and 0% the lowest (Paris intra-muros area).</figcaption></figure>

buildings in terms of their IEQ, taking into account the variability in occupants' responses, preferences, and expectations.

## 6. Conclusions

The original TAIL rating scheme for assessing IEQ in offices and hotels was successfully adapted to school buildings. The new rating scheme is called schoolTAIL and is ready for deployment in schools.

Two new parameters were added when adapting TAIL to schoolTAIL: reverberation time (RT) and nitrogen dioxide ($NO_2$) concentration. Other minor modifications were made to the original rating scheme, including an update of the guideline values for PM2.5 following the 2021 revision of the WHO air quality guidelines and a protocol for assessing artificial illuminance in classrooms that includes measurements on blackboards as well as worksurfaces. All other parameters and protocols remained unchanged from the original TAIL rating scheme.

A validation of schoolTAIL was carried out using measurements from a French survey of 308 schools from 2013 to 17. The results show that schoolTAIL was able to identify potentially invisible problems in school buildings. For example, it highlighted recurring trends in classrooms, such as an afternoon increase in classroom temperature, elevated $PM_{2.5}$ and $CO_2$ concentrations, and the influence of nearby traffic. The results also underscored the challenge of maintaining consistently high quality across all four IEQ components, signalled by the low overall IEQ levels. SchoolTAIL was shown to be an excellent indicator of school problems, providing a deeper understanding of the entire school building stock.

The results demonstrate that the schoolTAIL rating scheme and the original TAIL scheme are practical and expedient, both having common parameters defining IEQ. Although based on the data from one country, the results indicate a strong potential for the applicability of schoolTAIL in other countries as well. The results create an incentive for adopting the rating scheme, also because its future applications are expected to lead to its further development and improvements, and could be used to provide real-time information on IEQ.

## Declaration of generative AI and AI-assisted technologies in the writing process

During the preparation of this work, no generative AI or AI-assisted technologies were used.
### CRediT authorship contribution statement

**Minh Tien Tran:** Writing – review & editing, Writing – original draft, Methodology, Formal analysis, Data curation. **Wenjuan Wei:** Writing – review & editing, Supervision, Methodology, Conceptualization. **Claire Dassonville:** Writing – review & editing, Methodology. **Christophe Martinsons:** Writing – review & editing. **Pascal Ducruet:** Writing – review & editing. **Mickaël Derbez:** Writing – review & editing. **Corinne Mandin:** Writing – review & editing, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization. **Valérie Héquet:** Writing – original draft, Supervision, Methodology, Conceptualization. **Pawel Wargocki:** Writing – review & editing, Writing – original draft, Supervision, Methodology, Conceptualization.

### Funding

This research was co-funded by the French Agency for Ecological Transition (ADEME), contract number TEZ20-047, and the Scientific and Technical Centre for Building (CSTB).

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

This research was co-funded by the French Agency for Ecological Transition (ADEME), contract number TEZ20-047, and the Scientific and Technical Centre for Building (CSTB). We thank Prof. David Wyon for comments and careful proofreading of the manuscript.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.enbuild.2026.116942.

### Data availability

The authors do not have permission to share data.

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