# Ventilation Strategies to Ensure Thermal Comfort for Users in School Buildings: A Critical Review
## 2 Materials and Methods
### Selection Criteria  
Once the above criteria were established, the search for articles was conducted using the keywords specified. In total, 277 articles were obtained: 186 from the pre-COVID-19 period and 91 from the post-COVID-19 period. Of the 186 pre-COVID-19 articles, 50 were found in the SCOPUS database and 136 in WOS. For the post-COVID-19 articles, 32 were found in SCOPUS and 59 in WOS.  
Subsequently, an analysis was carried out in order to eliminate duplicate articles from these two databases. After excluding duplicates, 163 articles from the pre-COVID-19 period and 74 from the post-COVID-19 period were selected, so that a total of 237 articles were selected according to the initial search criteria.  
To support the selection process, a co-occurrence map was generated using VOSviewer (Figure 2), which revealed five main thematic clusters: thermal comfort (green), natural ventilation and architectural design (red), indoor air quality (blue), educational context (yellow), and school building retrofitting (purple). VOSviewer is developed by Leiden University, located in Leiden, The Netherlands. This mapping allowed for a clearer understanding of the thematic structure within the dataset and was used to guide the identification of the most relevant articles--those situated at the intersection of the main clusters and addressing the four core concepts: ventilation, school, thermal comfort, and strategy. From here, a filtering process began, initially selecting articles based on their titles. The titles of all extracted articles were reviewed, selecting those whose titles include aspects relating to the objectives set out in this review. This selection yielded 56 articles from the pre-COVID-19 block and 24 from the post-COVID-19 block, for a total of 80 articles.  
Figure 1: Methodology: search criteria.  
From these 80 articles, a new selection was carried out based on the abstracts. All the abstracts of the chosen articles were reviewed, and those which refer to ventilation strategies in schools and include thermal comfort were selected. According to the exclusion criteria followed in this part, articles focusing on other topics and failing to address users' thermal comfort or discuss ventilation strategies were excluded. Articles meeting the exclusion criteria were classified by topic, and several recurring themes were identified. In Table 1, the themes identified and the number of articles associated with each are listed. As can be seen, one of the most recurring themes is the study or modification of the building envelope to improve thermal comfort conditions (46% of articles), since one of the keywords used, \"strategy\", also refers to forms of retrofitting. After applying the aforementioned selection and exclusion criteria, 20 pre-COVID-19 and 9 post-COVID-19 articles were selected, for a total of 29 articles.  
Finally, a selection based on the content of the article was conducted. The 29 articles selected by abstract were analyzed, identifying articles that meet the following requirements:  
\begin{table}
\begin{tabular}{l c} \hline Causes of discomfort & 6 \\ Changes or study of the building envelope/building retrofitting & 22 \\ Energy use & 8 \\ New simulation models & 2 \\ Air quality analysis & 9 \\ User perception & 1 \\ \hline \end{tabular}
\end{table}
Table 1: Main topics of excluded articles.  
Figure 2: Keyword co-occurrence map generated by VOSviewer with five main thematic clusters: thermal comfort (green), natural ventilation and architectural design (red), indoor air quality (blue), educational context (yellow), and school building retrofitting (purple).  
After reviewing the 27 articles selected, an additional 8 were excluded, as they did not provide the necessary information to meet the objectives of this study. Consequently, the analysis focuses on the results from the remaining 19 articles, which examined ventilation strategies in school buildings, while also considering thermal comfort.