**4. Discussion**

In pre-COVID cases, the lack of ventilation is evident. In some situations, there is almost none, except when the windows and doors are rarely opened during the midday break. Only in the scenario pre-COVID-19 Level 1◦A was the intention of a routine indoor air renewal perceived; thus, even if ventilation is scarce, it is enough to maintain adequate TVOC concentration for most of the time. In this classroom, the average indoor CO2 was 34% below the other classrooms, but even so, during 81% of the week's school time, or for 4 out of the 5 teaching hours, CO2levels exceeded the limit set by Spanish regulations.

From the tests performed on the scenarios (A–D'), the most important aspects of the impact of ventilation on the monitored contaminants were analysed:


Regarding the indoor temperature conditions throughout the school day, it was observed that scenarios C and D' were the most favourable, with values of just 18.7% and 11.0% of the time outside the acceptable temperature range.

One last aspect to consider for any natural ventilation strategy to be valid is its feasibility to be lasting over time. In other words, it must be compatible with the daily routine of the classroom. The surveys carried out revealed that the punctual opening entailed a grea<sup>t</sup> difficulty because, in order to be effective (cross-ventilation), coordination between two classrooms was needed. Therefore, it would be a solution with little scope. Additionally, to achieve the durability of the protocol, Geelen et al. (2008) [22] pointed out the need for a didactic package attached to the scenario of application, for both a COVID-19 or post-COVID-19 scenario. This may have failed in Scenario A, as reflected in the results.

This study was limited to a case study, so it would be necessary to evaluate its applicability to other educational centres in order to achieve a larger sample and representativeness.
