## 1 Introduction  
The World Health Organisation (WHO) declared on 11 March 2020 that COVID-19 disease had become pandemic [1]. Since then, the WHO has been constantly reporting on the outbreak and providing global guidelines for its control [2]. This has been a global challenge in air quality (AQ) research [3; 4; 5], especially on its prevention modes, and detection and protection systems [6; 7]. For example, research has been performed on barriers to transmission, infection, and vaccines [6; 8]. Many researchers are still working on these and other issues. Among them, the importance of air renewal to achieve adequate IAQ and avoid health risks can be highlighted. The WHO identified direct contact with people suffering from the disease as an obvious source of contagion. Therefore, the social distance was delimited, the use of masks was made compulsory, and other types of actions were taken such as eye protection and use of biohazard clothing for professionals directly exposed (for example, hospital workers) [9; 10]. In addition, general applicable options that are not dependent on the decision of the individual were also sought during this pandemic. For instance, closing public spaces, restricting seating capacity in places of assembly [11], and increasing indoor air renewal in public buildings [12; 13] stand out. [PERSON] et al. [14] summarised a series of recommendations in a decalogue related to health strategies and pandemic challenges, which can be a great reference for addressing the future times ahead. On the other hand, two means of transmission of this type of disease were established: surface and airborne. To combat the first mode, surfaces that have been in contact with secretions or particles from sick people must be treated to inactivate the virus [15]. In addition, cleanliness of hands and face have been increased to reduce the likelihood of infection [16]. Concerning airborne transmission, the spread can occur through \"droplets\" [17; 18], which are larger than 100 micrometres in size, and aerosols, which are smaller than 10 micrometres in size. Research has proven that contagion can involve aerosols from distances of up to 10 m [11; 17]. Therefore, how to reduce aerosol concentration in indoor spaces has been explored [19; 20]. In this context, aerosol monitoring can be performed by means of characterisation systems [21]. Apart from being infected with the COVID-19 disease, exposure to air pollutants can affect different organs of people [22], damaging the respiratory, nervous, urinary, and digestive systems, among others. International standards and regulations [23; 24; 25; 26; 27; 28; 29] establish the following parameters for determining air quality (AQ):  
* Physical parameters: Wet bulb temperature (T) and relative humidity (RH) to assess thermal comfort as well as ambient particulate matter concentration (PM\({}_{2.5}\) y PM\({}_{10}\)). * Chemical parameters: Carbon dioxide (CO\({}_{2}\)), carbon monoxide (CO), formaldehyde (HCHO), ozone (O\({}_{3}\)) and volatile organic compounds (TVOCs). * Biological parameters: Bacteria and fungi. * Air renewals (based on metabolic CO\({}_{2}\) concentration). It can be noted that these parameters are mostly assessed in outdoor environments using stations with specific instrumentation. The European Environment Agency (EEA) and the European Commission provide information on AQ [30] throughout Europe [31]. Clean air is a mixture of gases [32]. The percentage of CO\({}_{2}\) corresponds to 0.035% among the different elements. However, excess CO\({}_{2}\) is considered to be one of the main sources of environmental pollution. Unfortunately, measurement of CO\({}_{2}\) concentration is not available at all stations. To reduce harmful emissions in outdoor and indoor environments [33], many countries have established strict policies to improve AQ and comfort. In the current context, they are also serving to identify areas with a potential danger of contagion in the COVID-19 pandemic.