## 2 Materials and Methods  
### Measurement Campaign  
A measurement campaign was conducted in two elementary schools in Uruguay, both of which operate in the same building, one during the morning shift (between 8:00 and 12:00) and another one during the afternoon shift (between 13:00 and 17:00). The building has no central heating or ventilation systems and relies entirely on natural ventilation.  
A single second grade classroom in the first floor of the school building was selected. It has a surface area of 6 m \(\times\) 8 m and a height of 3.5 m, which yields a total net volume of 168 m\({}^{3}\). It has four double casement windows facing west, which communicate directly to the school's outside playground. On the opposite wall, facing east, there is a door and an awning window, which communicate with a central corridor. Figure 1 further defines the geometry of the classroom.  
The selected classroom has a nominal occupation of 30 children plus a teacher for both shifts. The populations are stable, that is, the students are always the same. There is a half hour break between 10:00 and 10:30 in the morning shift and between 15:15 and 15:45 in the afternoon shift.  
Two different CO\({}_{2}\) concentration datasets were obtained. First, controlled tests were performed. These consisted of artificially generating CO\({}_{2}\) in the unoccupied classroom by mixing calcium bicarbonate with acetic acid in a bucket. The windows and the door were closed during generation and two ceiling fans were turned on to ensure the air was fully mixed. After a relatively high concentration was attained, CO\({}_{2}\) generation was stopped and the fans were turned off. Some windows and the door were opened, with different configurations for each test. The room was emptied and the decrease in CO\({}_{2}\) concentration was registered. For this test, two Aeroqual 500 monitors with calibrated 0-5000 ppm CO\({}_{2}\) heads were used (monitors AQ1 and AQ2) [28]. These registered a measurement every minute. Additionally, two AirCO\({}_{2}\)ntrol 5000 CO\({}_{2}\) monitors were used (monitors AC1 and AC2) [29]. These were calibrated by comparison with the Aeroqual instruments and registered a measurement every 5 s.  
The purpose of the controlled tests was twofold. Firstly, they were conducted to infer what ventilation rates could be expected. Secondly, they enabled assessing different ventilation patterns and associating them with specific ventilation rates.  
The second dataset consisted of daily measurements (one per minute) in the classroom during normal school activities between August and December. In this case, monitors AQ1 and AQ2 were positioned at the front and back of the classroom, respectively. Monitor AQ1 was mounted on the side of the teacher's desk and monitor AQ2 was positioned over a storage cabinet. Installing two monitors allowed for a primary evaluation of the perfect mix hypothesis, which will be introduced in the following section. This is rarely addressed in the available literature and, to the best of our knowledge, has not been previously evaluated in works dealing with ventilation and risk of airborne transmission.  
Finally, a Davis Instruments Vantage Pro2 weather station was installed on the roof of the school during some months of the measurement campaign, in order to characterise the outdoors conditions.  
Figure 1: Studied classroom. (**a**) shows the west wall and (**b**) shows the east wall. All dimensions are in m.