**1. Introduction**

The face-to-face return to classrooms in Spain in September 2020 during the global pandemic and after a long period of confinement opened up a debate in society about health security and air quality. The discussion was reinforced when the scientific community began to show evidence of greater transmission by aerosols than by fomites [1]. Poor indoor air quality (IAQ) was already an existing problem in naturally ventilated schools, but awareness was only raised after the COVID crisis. During the past school year, recommendations and protocols have been developed by different institutions and organisations to improve the ventilation performance of classrooms.

The objective of this study is the evaluation of the main protocols presented to improve natural ventilation systems (NVS) in schools, as well as the assessment of their possible adaptations to a post-COVID scenario, through the analysis of a case study.

Before the beginning of this global pandemic (February 2020), measurements were carried out in a nursery and primary school, which yielded worrying results: throughout a winter week of monitoring, the CO2 concentration in classrooms exceeded 1000 ppm during 88.75% of the teaching time, reaching maximum values of 3628.8 ppm. These poor results are in line with those obtained in various previous studies (Table 1).

Inside classrooms, pupils and teachers are commonly the only sources of CO2. Therefore, CO2 is considered a good IAQ indicator, which shows the relationship between ventilation rate and occupancy.

**Citation:** Meiss, A.; Jimeno-Merino, H.; Poza-Casado, I.; Llorente-Álvarez, A.; Padilla-Marcos, M.Á. Indoor Air Quality in Naturally Ventilated Classrooms. Lessons Learned from a Case Study in a COVID-19 Scenario. *Sustainability* **2021**, *13*, 8446. https:// doi.org/10.3390/su13158446

Academic Editor: Tomonobu Senjyu

Received: 15 July 2021 Accepted: 27 July 2021 Published: 28 July 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).



In addition, CO2 has been associated with the presence of other pollutants [4,8]. The presence of pollutants like bioaerosols, Particle Matter (PM), or Total Volatile Organic Compounds (TVOCs) [9] is harmful to a pupil's health and productivity [10–12]. CO2 must be taken into account as a pollutant, too, because, although not injurious at the levels that have been recorded in schools, it can cause adverse effects on the academic performance of the occupants [13].

TVOCs have diverse sources, mainly from the interior. These can come from cleaning products, construction materials or furniture, and, in the case of educational environments, from school materials such as glue, paint, etc. [14].

The approach to improve IAQ in schools is to increase ventilation rates. The first regulatory requirement in Spain for ventilation in schools dates from 1981 [15], which was endorsed in the "Regulation of thermal installations in buildings" (RITE) (1998) [16], and by its subsequent revision in 2007, currently in force with modifications [17].

Since in Castilla y León, 51% of public schools were built before 1980, and the vast majority of these centres lack ventilation systems, which involves addressing natural ventilation performance. This percentage is very similar to the Spanish average [18].

The requirements inside the classrooms are comparable between the different countries, according to Table 2.


**Table 2.** CO2 concentration requirements inside the classrooms in different countries.

1 Value for Category I (educational buildings—new buildings and renovations). 2 Outdoor air concentration was assumed as 400 ppm. 3 Value obtained for up to 20 min.

The uncertainty regarding the real performance of ventilation systems in educational centres has promoted different studies on the improvement of natural ventilation.

Almeida et al. (2015) compared the IAQ between two refurbished and two nonrefurbished classrooms for two months in Spring. All of them had a central heating system with hot-water radiators as terminal units. A mechanical ventilation system (MVS)

provided with CO2 and temperature sensors (indoors and outdoors as a reference value) was implemented in the refurbished classrooms. The results obtained revealed that the natural ventilation system was not able to provide adequate IAQ for the whole school day. The CO2 concentration levels were over 1500 ppm for 20% of the time. On the other hand, while using an MVS a good IAQ was maintained [19].

Also in a mild-climate location, Fernández-Agüera et al. (2019) [20] found that a large part of the schools lacked ventilation systems. Therefore, the air renewal of the classrooms was achieved by means of uncontrolled airflow through the building envelope (air infiltration) or the manual opening of windows. The data showed that, when the windows remained closed, the CO2 concentration reached values over 1000 ppm during 89.3% of the time.

Vasella et al. (2021) [21] studied the impact of an NVS protocol implemented in a hundred schools in Switzerland for four days during the cold season with outdoor temperatures below 15 ◦C (please note that these are similar climate conditions to the ones of the present study). The protocol consisted of the brief opening of doors and windows between different classes and for a longer period at the lunch break when the students had to leave the classroom for its ventilation. The measures implemented included an application to calculate the duration of the brief apertures, and teaching materials to promote the importance of IAQ and ventilation. The protocol implemented reached CO2 concentration levels under 1400 ppm during 70% of the school time, whereas those levels were only achieved during 30% of the time before its implementation, and the mean temperature was between 19.9 ◦C and 20.5 ◦C.

Similar research in the Netherlands carried out in 2008 [22] demonstrated that when protocols have been implemented to raise awareness of the problem among students and teachers, it is possible to achieve a necessary longer-lasting effect. Therefore, before the intervention, the CO2 concentration exceeded 1000 ppm for 64% of the school day, whereas after the intervention, ventilation was significantly improved even though the CO2 concentration still exceeded 1000 ppm for more than 40% of the school day.

Thus, there is enough existing evidence to indicate that it is necessary to control IAQ in schools. However, it is also necessary to evaluate what success it could achieve, and how it could affect the thermal comfort in schools in Castilla y León.

In a scenario in which health had to be prioritised, certain ventilation protocols that compromised thermal comfort were reappraised. To determine the scenarios subject to study, the main mandatory protocols and guidelines published to date were taken into account:

