## 4 Conclusions  
The natural ventilation scheme of an Uruguayan primary school classroom was quantitatively assessed by both estimating ventilation rates and airborne transmission risk.  
Ventilation rates were studied in controlled situations without occupants and during normal use by fitting a box model by measuring CO\({}_{2}\) concentrations. It was found that the classroom has the potential of reaching very high ventilation rates when crossed-ventilated. Nonetheless, during normal occupation this only happens during short periods. Consequently, daily mean ACH values remain low. This may happen due to thermal discomfort of the occupants, which prevents them from operating the windows.  
Transmission risk for SARS-CoV-2 was assessed via a Wells-Riley approach. Mean monthly risk was estimated through CO\({}_{2}\) measurements by using a modified Rudnick-Milton model. Risk was shown to steadily decline towards spring. This is compatible with the thermal discomfort hypothesis, as warmer outside air would allow for more frequent high ventilation rate events.  
The relationship between ventilation rates and airborne transmission risk was explored by using a modified Gammaitoni-Nucci model. It was shown that for constant ventilation rates similar to the daily mean values determined for the classroom, transmission risk was predicted to be much higher than what was estimated via the Rudnick-Milton model. By studying periodic ventilation schemes with the Gammaitoni-Nucci model, it was shown that this could be explained by the short but intense ventilation events observed. It is therefore concluded that periodic ventilation is an extremely valuable strategy for reducing transmission risk, which allows for a compromise with thermal comfort in naturally ventilated spaces during wintertime. Nonetheless, periodic ventilation might need complementary risk reduction strategies to further reduce risk values.  
Additional ongoing work is aimed at extending the measurement campaign, including more classrooms of the same school building and other variables such as temperature and relative humidity. In the near future, we intend to include more schools. Furthermore, data analysis could be complemented with other models. For example, the performance of box models could be evaluated via Computational Fluid Dynamics (CFD). CFD would also allow to assess the influence of the airflow pattern in the airborne infection risk. In addition to this, taking into account the relevance of the thermal comfort and its influence in the ventilation, Building Energy Management (BEM) models will be used  
Conceptualization, A.V., A.P.G., M.D. and M.M.; methodology, A.V., A.P.G., M.D. and M.M.; software, A.V. and A.P.G.; validation, A.V.; formal analysis, A.V., A.P.G., M.D. and M.M.; investigation, A.V., A.P.G., M.D. and M.M.; resources, M.D. and M.M.; data curation, A.V.; writing--original draft preparation, A.V.; writing--review and editing, A.V., A.P.G., M.D. and M.M.; visualization, A.V.; supervision, M.D. and M.M.; project administration, M.D. and M.M.; funding acquisition, M.D. and M.M. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Unidad de Extension of the Facultad de Ingenieria, Universidad de la Republica (Llamado a Proyectos de Extension 2021, _COVID-19 y ventilation en escuelas_).  
Not applicable.  
**Data Availability Statement:** Raw data sets can be provided upon reasonable request to the authors.  
**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.