## 1 Introduction
It can be noted that the current trend for effective AQ control is to promote natural ventilation through strategies combining large external openings [66] with intelligent ventilation [67], which also allows optimizing energy consumption by increasing its efficiency [68; 69]. Several studies have analysed methods and models to determine the air renewal required to achieve acceptable IAQ parameters [70; 71; 72; 73]. In all of them, the amount of exhaled CO2 has emerged as a critical factor in establishing the number of air renewals. In addition, different standards indicate the number of air renewals recommended in the diverse living spaces of buildings according to their use and occupancy. Among them, those set by ASHRAE [74; 75] and the national legislation in Spain [76], which comes from the Directive 2010/31/EU of the European Parliament and of the Council of 19 May 2010 on the energy performance of buildings, stand out. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Classification** & **Description** & **MET** \\ \hline \multirow{2}{*}{Soft} & Activities that do not require excessive effort and are normally at & \multirow{2}{*}{\(<\)3} \\  & rest (sleeping, watching TV, driving, eating,... ). & \\ \hline \multirow{2}{*}{Moderate} & Activities that accelerate the heart rate in a sensitive way & \multirow{2}{*}{3–6} \\  & (housework, dancing, construction work and painting,... ). & \\ \hline \multirow{2}{*}{Intensive} & Activities that require greater effort and cause rapid or & \multirow{2}{*}{\(>\)6} \\  & accelerated breathing, involving activities that increase the heart & \\ \multicolumn{1}{c}{} & rate (aerobics, sports, or moving weights of more than 20 kg,... ). & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Activity classification according to metabolic equivalent level (MET). Monitoring IAQ, and more specifically CO\({}_{2}\) concentration in buildings, can be realised using professional (commercial) equipment or clonic devices. These clonic systems are increasingly being used due to their reliability and prestige in the electronics market. [PERSON] et al. [77] carried out a research focused on commercial equipment and clonic devices used in Spain for the measurement of CO\({}_{2}\) during the COVID-19 pandemic. As a noteworthy result of this research, it must be noted that the price of commercial instruments on the market ranges from EUR 75 for the cheapest to EUR 400 for the most expensive, although they include other functionalities to measure relative humidity, temperature, and particle size. In addition, they also evaluated clonic devices. They showed that the measurement error varies over a spectrum ranging from 9% to 15% in environments with 500 ppm CO\({}_{2}\) and from 7% to 12% when concentrations are closed to 700 ppm CO\({}_{2}\). On the one hand, professional equipment are electronic devices designed and commercialised by manufacturers intended for a professional audience (laboratories and health and safety services), which can monitor any physical or chemical parameter of AQ. They can be classified according to their portability, autonomy, and number of parameters monitored. In addition, these systems have the advantage that their probes and sensors are calibrated in metrological laboratories with controlled atmospheres. They also allow data to be stored and sent instantly to other devices. However, the high weight of some of them, due to the high number of measuring sensors, is a disadvantage for their portability. On the other hand, clonic measurement devices are composed of electronic components and open-sourced software. The scalable configuration of clonic devices makes it possible to customize and increase the number of features of professional equipment. To begin with, a motherboard must be selected. Raspberry Pi and Arduino are two of the manufacturers offering more competitiveness in this field. According to studies reviewed, 37.5% of clonic devices are based on Arduino microcontrollers and 35% on Raspberry Pi ones [78].