**1. Introduction**

Indoor air quality (IAQ) is essential to create healthy and comfortable spaces for users in which to carry out their daily activities, especially considering that people spend more than 90% of their time indoors [1]. There is a large volume of published studies describing the role of IAQ in the health of building users [2,3]. When the use of the building is academic, the importance of IAQ is higher because poor indoor air for long academic periods can have a negative impact on both the students' health and intellectual performance [4,5].

The European ventilation standard (CEN) [6] establishes mechanical ventilation as the strategy to guarantee the minimum outdoor airflow to maintain adequate IAQ. In Spain, regulations established mandatory mechanical ventilation in educational buildings in 2007 [7]. A total of 90% of the compulsory educational buildings in Spain were built before 2007 when mechanical ventilation systems were not mandatory [8]. In the case of higher educational centers such as universities, the share of buildings constructed before ventilation systems were mandatory remains unknown. However, it could presumably be higher since they are often located in historical buildings. Therefore, educational buildings in Spain are mainly naturally ventilated. Ventilation (single-sided or cross ventilation) is only promoted by the occasional opening of windows when users consider it necessary, based on no objective IAQ indicator [9]. Thus, maintaining acceptable IAQ during the whole academic period remains a challenge [10]. This manual control of the openings is even more limited during the cold season since it causes uncomfortable airflows and a considerable drop in the classroom hygrothermal conditions.

**Citation:** Poza-Casado, I.; Gil-Valverde, R.; Meiss, A.; Padilla-Marcos, M.Á. Impact of Air Infiltration on IAQ and Ventilation Efficiency in Higher Educational Classrooms in Spain. *Sustainability* **2021**, *13*, 6875. https://doi.org/ 10.3390/su13126875

Academic Editor: Vincenzo Torretta

Received: 20 May 2021 Accepted: 15 June 2021 Published: 18 June 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**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 this scenario, air infiltration through cracks and other unintentional openings in the building envelope is the only mechanism that can provide the continuous renovation of the indoor air. For this reason, it is important to determine the airtightness of the building envelope, since it impacts directly on air infiltration, caused by pressures gradient [11].

This paper addresses the indoor conditions of a higher educational building with natural ventilation in the Continental climate area of Spain, considering different parameters such as temperature, relative humidity (RH), CO2, TVOC, and PM, assessing the ventilation performance through the analysis of the ventilation efficiency caused by air infiltration.
