## 1 Introduction
In indoor environments, CO\({}_{2}\) displaces oxygen and exposes users to the effects of hypoxia [34; 35]. As proposed by [PERSON] and [PERSON] [36], the ideal indoor concentration can be established close to 350 parts per million (ppm) in environments with special requirements and 500 ppm in normal activity environments. Nevertheless, other reference values are provided in specific standards and regulations because it is considered an indicator that relates AQ and comfort level to CO\({}_{2}\) concentration [37; 38]. It can be noted that in the context of COVID-19, CO\({}_{2}\) concentration [39; 40; 41; 42] from occupant respiration has been used as an indicator of IAQ. As a baseline, the limit values set by the US Environmental Protection Agency (EPA) for CO\({}_{2}\) concentrations (in ppm) and the corresponding AQ are shown in Table 1. In the same vein, the Technical report \"CEN/TR 16798-2\" [43] indicates that a value of 400 ppm can be assumed as the average outdoor concentration. \begin{table}
\begin{tabular}{c c} \hline \hline
**CO\({}_{2}\) (ppm)** & **AQ** \\ \hline
340–600 & Good \\
601–1000 & Moderate \\
1001–1500 & Unhealthy \\
1501–5000 & Hazardous \\ \hline \hline \end{tabular}
\end{table} 
Table 1: AQ related to the pollutant CO\({}_{2}\). In living spaces, the main focus of CO2 increase is of metabolic origin, which is related to the type of activity [44; 45]. As a result, the more effort a person makes, the more exhalations are produced, and the more CO2 is emitted. The WHO has classified activities according to the \"intensity of physical activity\", relating them to the metabolic equivalent unit (MET). Table 2 provides a classification of activities with the range of physical intensities in METs. In this regard, [PERSON] et al. conducted research [46] using low-cost devices, concluding that an increase in activity results in an increase in MET concentration and exhaled metabolic CO2. COVID-19 is currently a major global concern. [PERSON] and [PERSON] confirmed that CO2 co-exhaled with aerosols containing SARS-CoV-2 by COVID-19 can be used as an indicator of indoor SARS-CoV-2 concentrations [47]. Other researchers have pointed out that group immunity is expected by the end of 2024 [48] thanks to the vaccination process, but broader strategies must be put in place that can respond to future diseases. Therefore, buildings should aim to act smarter [49], renovating indoor air to achieve better quality. These activities are in accordance with the sustainable development goals (SDGs) of the 2030 Agenda [50]. Although the need for AQ control in public, residential, and business buildings has been analysed in many studies [36; 51; 52], the price of the systems required for this purpose has been a barrier to implementation. This applies mainly to low-income households [53] and socially disadvantaged environments [54]. The Center for Disease Control and Prevention (CDC) has published extensive research on aerosol concentrations in different situations. In unventilated shared-use spaces, CO2 concentration is high [36] and infections are higher. This context is unchanged even if a social distance greater than two metres or six feet is maintained [11; 55; 56]. Although there are other methods to reduce pollutants such as filtration with HEA filters [57], ultraviolet radiation treatments, or disinfection with ozone [58], they were not considered because they are beyond the scope of this research. In this context, ventilation has become a fundamental solution [59; 60] for the reduction in indoor pollutant concentrations and for the improvement of IAQ [61]. In some cases, the way to achieve air renewal is by means of natural ventilation (cross, outdoor, indoor) [33; 62; 63]. In other cases, a forced ventilation system must be used [64]. In all cases, pollutant concentrations are reduced if air is renewed. These lead to a decrease in CO2 concentration in indoor spaces [65].