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Several studies claim that small droplets (aerosols) from sneezing or coughing can be a cause of Covid contagion. This is what Il Corriere della Sera reports in an October 31 article.
“The critical places are closed spaces of reduced dimensions and limited ventilation, especially with a long residence time.“Says Giorgio Buonanno, Senior Lecturer in Technical Environmental Physics at the University of Cassino and at the Queensland University of Technology in Brisbane (Australia).
Schools, or rather classrooms, are also closed places with a high risk of transmission.
Then there is another factor to take into account: we are facing the winter season, which means keeping the windows closed for longer. The opposite situation to that of last spring, when with the first wave of the spread of the coronavirus we are heading towards the spring-summer season, and when you are more outdoors or even indoors, the windows are usually kept open. Therefore, even closed places have the possibility of being ventilated more frequently.
In a medium-sized classroom, Il Corriere reports, it is possible to completely change the air by opening the windows in 10-20 minutes, but in the cold season it is not always possible for the reasons mentioned above.
But what are the risks if there is something positive in the classroom?
The risk is certainly higher if it is the teacher who is infected. The teacher speaks more than a student and in a loud voice, so he emits 100 times more than normal breathing. On the other hand, students speak sporadically.
The reported example also clarifies the number of potential infected in a school classroom: in a 150 cubic meter class with 25 students who follow the lesson for 5 hours with a positive teacher explaining for two hours, without taking any precautionary measures against aerosols, up to 12 students could be infected.
“If everyone wore masks, a maximum of 7 students could become infected (5 less than in the previous example) and in that room, again to have an R0 lower than 1, 3 people could stay while the individual risk of infection would be reduced to half. With forced ventilation (a total change of air every 20 minutes) only 4 people would be at risk of contagion and of lowering the R0 below 1 under these conditions, 5 people could stay in class ”. This is the result of a study by the University of Cassino, which has published a tool to estimate the risk of infection by airborne transmission of Sars-CoV2 in closed environments.
“At school, it is even better than forced ventilation to equip teachers with a microphone (so that they are not forced to speak loudly): only 1.4 people would be at risk of contagion and 9 people could stay in that room (with an individual risk of infection equal to 2.7%). If we put all the devices together: masks, forced ventilation, microphone, the maximum number of infected people in the presence of a contagious teacher would drop drastically to 0.4 and even 30 students could remain in that room keeping an R0 lower than 1: only with all these precautions, the R0 index could be adequately controlled by keeping it below 1 ”.
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