Corona transmission: air conditioning systems as virus throwers?

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Corona transmission: air conditioning systems as virus throwers?





© AFP
In many countries, public buildings, but also private apartments, are equipped with air conditioning systems, like here on a house facade in Moscow.


In a restaurant in China, nine people are said to have been infected because the air conditioning system distributed the Sars CoV-2 pathogens. The virus could also have spread via ventilation on a cruise ship.

Air conditioning systems as virus throwers?

Reports like this from China show how little is known about the transmission of the coronavirus. At the end of January, dozens of people were sitting in a restaurant in the Chinese city of Guangzhou. The restaurant has five floors and is ventilated by air conditioning. There are no windows. A few weeks later, ten guests developed Covid-19. They sat at different tables and belonged to three families who otherwise had no contact with each other.

Researchers reconstructed that the virus was probably transmitted from a single infected person in the restaurant. They excluded other infection routes than visiting the restaurant at the same time. However, this “patient 0” did not only infect people who were sitting at his table. The scientists assume that the air conditioning system played a crucial role in this case. The air flow could have transported the viruses significantly further than when speaking, coughing or sneezing. The experts are therefore calling for the distance between tables to be increased further, for air conditioning systems to be better monitored and for external ventilation to be improved.


The upper sketch shows the possible air movement due to the air conditioning in a restaurant in Guangzhou, the lower sketch shows the seating plan of the families. The researchers assume that person A1 was the carrier of the virus.


© Guangzhou Center for Disease Control and Prevention
The upper sketch shows the possible air movement due to the air conditioning in a restaurant in Guangzhou, the lower sketch shows the seating plan of the families. The researchers assume that person A1 was the carrier of the virus.


But it would be premature to conclude from the study from China that all ventilation systems are dangerous. “There are technical differences in air conditioning systems and the possibility of filtering, all of which should be considered,” says epidemiologist Rafael Mikolajczyk from the University of Halle. “With coronavirus, we are more likely to assume a droplet infection, where the transmission takes place in the immediate vicinity rather than via aerosols that are carried over longer distances.” Initial studies were able to prove that infectious viruses can still be in the air even after hours. However, it is still unclear whether this viral load is sufficient to infect humans. What experts agree on: In principle, good ventilation in rooms actually helps to reduce the risk of infection. They assume that the virus in a Berlin club or bar in Ischgl, for example, infected so many people because there was hardly any fresh air in the premises.

The Federal Environment Agency (UBA) also says that what matters most is the type of air conditioning system and its maintenance. It is customary on cruise ships not to completely discharge the exhaust air from the interior to the outside via the central ventilation systems, but rather to allow it to continue circulating in the rooms. This could lead to the transmission of the pathogen. That would substantiate the suspicion that the cruise ship Diamond Princess, which was quarantined in Japan with hundreds of corona infections, could have infected numerous passengers through the central ventilation system with the virus. When Sars broke out in 2003, hundreds of infections in a block of flats in Hong Kong were probably due to a defective exhaust system.

This danger can also be seen at the Federal Environment Agency: incorrect planning or inadequate maintenance of ventilation systems could result in “incorrect currents”. In this way, the exhaust air from one part of the building could inadvertently enter another area as supply air. The UBA says: “In such cases, the spread of viruses via the air conditioning system cannot be ruled out.” If a system only ventilates a single room, works properly and is serviced regularly, there is no increased risk of infection.

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