Why is there a lot of talk about aerosol transmission?
Covid-19 is not as contagious as measles but studies and various tests are multiplying which confirm that the transmission of the new coronavirus occurs not only through droplets (heavy droplets that, due to a larger size, fall to the ground by gravity within two meters) but also through tiny droplets (aerosols) exhaled by infected people when they cough or sneeze but especially when they talk, sing, scream, breathe. Furthermore recent studies suggest that the aerosols can move several meters and fill a room for long periods while maintaining a high infectivity charge. Spacing and masks defend against larger droplets and are indispensable for this mode of contagion but alone they are not enough to counteract transmission by air.
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What are the places where coronavirus spreads most easily?
“The critical places are closed environments with reduced dimensions and limited ventilation, especially with a long residence time” he recalls Giorgio Buonanno, full professor of Environmental Technical Physics at the University of Cassino and at the Queensland University of Technology in Brisbane (Australia). In fact, it has been seen in numerous studies around the world that Sars-CoV-2 spreads especially in those closed environments where many people gather: weddings, churches, gyms, restaurants, public transport, choirs, bars, slaughterhouses, prisons, parties especially when speaking out loud or singing without a mask.
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What can happen with the cold season?
Now that we face the winter season, the risks of contagion, even by air, increase (just like with the flu and other respiratory viruses) because more time is spent indoors and the concentration of infectious particles increases without the presence of a adequate ventilation. It is conceivable that the spread of the virus in Italy last summer, after the effect of the long lockdown, slowed down both for the longer time spent outdoors and for the greater ventilation in closed environments with the windows open. In this way the mode of transmission by air has been curbed.
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What is meant by ventilation?
In a medium-sized classroom it is possible to completely change the air by opening the windows (ventilation) in 10-20 minutes but in the cold season it is not always feasible. The ideal would be to work with controlled mechanical ventilation systems: in the case of recirculation, the use of HEPA filters is recommended. When mechanical ventilation is not feasible because it requires major renovations, you can think of portable air purifiers that can be moved to various environments.
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What if there is a positive in an enclosed space such as a classroom?
The risks increase when the teacher is infected because he speaks longer and louder to be heard, so emits 100 times more than normal breathing. A potential sick pupil in comparison speaks very sporadically and is much less dangerous. If in a class of 150 cubic meters 25 students spent 5 hours with a sick teacher explaining for two hours, without taking any precautionary measures against aerosols, up to 12 students could be infected. Every single individual would have a 15% chance of becoming infected. “To have an acceptable R0, that is less than 1, only two students could stay in that room,” explains Buonanno.
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Can the risk of airborne infection be reduced in the classroom?
The University of Cassino has published a tool for the estimation of the risk of contagion due to airborne transmission of Sars-CoV2 in closed environments. “If everyone wore masks, a maximum of 7 students could be infected (5 less than the basic situation) and in that room, again to have an R0 lower than 1, 3 people could remain while the individual risk of infection would be halved. With forced ventilation (a total air change every 20 minutes) only 4 people would risk contagion and to lower the R0 below 1 in those conditions, 5 people could remain in class. At school, it is even better than forced ventilation to equip teachers with a microphone (so they would not be forced to speak loudly): only 1.4 people would risk infection and 9 people could remain in that room (with 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 an infectious teacher would drop drastically to 0.4 (less than one infection) and up to 30 students could stay in that room while maintaining a R0 lower than 1: only with all these precautions could the R0 index be adequately controlled by keeping it below 1.
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Is it possible to estimate the risk even on a bus?
On board an urban bus, with 80 passengers, one change of air per hour (on the vehicles the air changes are actually even higher), a residence time of 30 minutes and subjects standing up and talking, in presence of an infected passenger could infect a maximum of 2 people. To lower R0 below 1 on that bus 54 people could get on. Wearing the masks the problem is eliminated: a maximum of 0.7 people could be infected and the bus could accommodate up to 107 passengers while keeping R0 below 1. The increase in forced ventilation with an air change every 10 minutes can also help : the result is similar to that obtained by wearing the masks. If all the measures are taken together, the maximum number of infected people will be 0.4 and up to 193 passengers could go on board. “Public transport, unlike what one might believe, are not particularly risky places for transmission via aerosol in the case of short exposure times” explains Buonanno. But with such high incidence rates these days it is easy for more than one infected individual to get on board a public transport, in which case the risks double and triple.
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So are spacing and masks not enough?
«Spacing and masks are a necessary condition (especially to defend against droplets, the largest droplets and from short distance contagion) but they are not sufficient for airborne contagion in closed environments. Today we know how to estimate the ventilation conditions, crowding and exposure times in closed environments to better manage the risk of contagion ».
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31 October 2020 (change October 31, 2020 | 18:51)
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