The magazine Nature had already addressed the issue last February with a long article and an accompanying editorial: the risk of coronavirus contagion through surfaces is almost zero, despite the efforts still focused on cleaning and sanitization. With an appeal: to pay more attention to the main contagion route, the so-called air transmission (aerosol), that is the large and small respiratory droplets that carry viral particles emitted by people who sneeze, cough, speak and breathe and that can remain for a long time in a closed, poorly ventilated area. Now a new report published by the Centers for Disease and Control Prevention (CDC) of the United States that analyzed the various studies available has concluded that the transmission of the coronavirus through contact with a very rare contaminated surface: one case in ten thousand.

The first tips

At the beginning of the pandemic, when nothing was yet known about this new virus, experts began to report possible dangers of contagion through contaminated surfaces: touching them and then putting your hands on your nose and mouth could be a way for the virus to enter. Thus began the non-stop cleaning (initially also of roads). Companies have equipped themselves with accurate disinfestations and even the train and metro wagons are periodically sanitized. Sanitation has become a business. After all, deep cleaning makes people feel safer because it is something you can see. But after so many months it is now much clearer that the coronavirus is a respiratory virus, it is transmitted through small droplets that float in the air indoors. There are no studies proving infection through contact with contaminated surfaces. The fact that the virus remains on a surface does not automatically mean infection. A rather large amount of viral particles must be deposited on that table or on that handle, which then ends up on a hand that finally touches a face. Not so automatic. And above all, the virus must still be infectious (and not inert as it happens after a certain period of time).

When to use soap and water and disinfectants

Now the CDC report sheds some clarity on a subject that has long remained controversial. The researchers estimated the probability of contagion with quantitative studies of microbial risk assessment. The results of this research suggest that the risk of Sars-Cov-2 infection via the fomite route is low and generally less than 1 in 10,000, which means that any contact with a contaminated surface has less than 1 in 10,000 chance. to cause an infection, scientists conclude, suggesting soap and water or detergent to clean the surfaces. The use of disinfectants for surface hygiene has proven to be effective in prevent secondary transmission of Sars-Cov-2 between an infected person and other people within the family. There is however little scientific support for the routine use of disinfectants in community settings, both internal and external, to prevent transmission from Sars-Cov-2 from fomites. In most situations, to reduce the risk of contagion just clean the surfaces using soap or detergent. Disinfection recommended only in internal environments of the community in which a suspected or confirmed case of Covid-19 has occurred in the last 24 hours. The new US cleaning guidelines do not apply to healthcare facilities, which may require more intensive cleaning and disinfection.

Where you get infected: closed environments

But if only one person in two thousand becomes infected through contaminated surfaces and one in a thousand becomes infected outdoors, as revealed by an Irish research, how can they be explained? the millions of infected people in the world? It is now known that closed environments are the ones most at risk of contagion, in particular bars, restaurants, gyms and hotels as concluded a research published in Nature that analyzing the movements of 98 million Americans has identified these as places where the risk of coronavirus infection is highest. The critical places are small closed rooms with limited ventilation, especially with a long residence time remember Giorgio Buonanno, full professor of Environmental Technical Engineering 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, especially parties when speaking out loud or singing without a mask.

The solution

The solution? Not necessarily to close them, and it also applies to schools: you can manage the risk of contagion with adequate ventilation and aeration strategies. In a medium-sized classroom it is possible to completely change the air by opening the windows (ventilation) in 10-20 minutes, but with 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 which can be moved to various environments. It is important to know that today engineers are able to estimate the risk and manage any indoor environment, intervening on ventilation, exposure times and crowding, concludes Buonanno.

April 8, 2021 (change April 8, 2021 | 20:03)

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