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The case of three Chinese families who may have contracted the coronavirus in the same restaurant has brought the role of air conditioning to the limelight. Researchers from the Center for Disease Control in Prevention in Guangzhou have examined the three families. All family members were infected with the coronavirus and were in the same restaurant the same night. One of the families had previously traveled from Wuhan and brought the virus from there to Guangzhou. The other families were sitting at the neighboring tables at a sufficient distance, there was no direct contact between the family members. Therefore, the researchers assume that this air conditioning system in the restaurant could also have had an impact on the transmission.
“Health authorities are assuming that the transmission will take place through larger drops that fall to the ground just two meters from the source of the garbage. The recommended hygiene and distance rules are based on this, ”explains Walter Hugentobler of the Institute of Family Medicine at the University of Zurich. At the moment, however, it is not yet known with certainty whether airborne transmission of viruses over longer distances and periods is not possible. “My knowledge of the history of infection transmission and the scientific knowledge of recent years tell me that airborne transmission of respiratory infections is done with certainty. This applies not only to coronaviruses, but also to all viruses and bacteria that transmit upper and lower respiratory tract infections, “the doctor is convinced and explains:” Until the late 1980s, air transmission was not questioned in science. It was even recognized as a primary route of transmission by the US Center for Disease Control. The USA, CDC, discussed at the congresses, but without doubt, although there was no clear evidence to support it. Without any new scientific knowledge to the contrary, airborne transmission was displaced and forgotten in the late 1980s. ”
The speed and intensity with which a virus can spread within a closed room depends largely on factors such as humidity, exchange, and movement. “Air exchange determines the rate at which the amount of virus in the room air is diluted and viruses are ventilated. Air movements determine the direction of spread and the transport range of the virus. However, relative humidity is at least as important. It determines the final size and weight of the droplets and, therefore, their reach and the duration of floating in the air, “says Hugentobler. Moisture has a direct effect on the infectivity of the corona virus. If the air humidity is less than 40 percent, the infectious droplets that are released dry completely because they release the water into the ambient air. Viruses remain contagious, are preserved so to speak, and can spend hours in the air. “If they are inhaled by someone else and moistened in their respiratory tract, a new infection is triggered there and the virus has been successfully transmitted.” However, at an average humidity of 40 to 60 percent, shortly before the drops are completely dry, the viruses in the highly concentrated ingredients of the drops lose their infectivity. and employees are protected from respiratory infection. At this level of humidity, the immune system of the respiratory tract and the self-cleaning system of the nose and bronchi function more effectively, and virus infectivity is dramatically reduced.
It is worth buying a hygrometer to be able to control the humidity inside. This allows to observe the humidity profile for a few days and correct it if necessary. This is where ventilation and air conditioning systems come into play: “If a relative humidity of less than 40 percent is frequently found, you should use a humidifier that has received good marks during the hygiene test. It is also important to avoid excess moisture. If the humidifier does not have a built-in hygrostat, the best way to achieve controlled humidity is to run the humidifier’s power supply through a plug-in hygrostat. It is imperative that the hygiene standards of the humidifier are observed ”, warns the doctor.
When all three families became infected, Chinese researchers assume that the heavy drops could be spread through excess air conditioning. In this case, the vent was placed directly on the table of the infected family so that the drops could be absorbed and spread. Currently, however, this is an isolated case, investigators have not conducted any further experiments to substantiate their assumptions. But Walter Hugentobler also doesn’t rule out transmission through ventilation and air conditioning systems: “By tracing transmission chains during the 2002/2003 SARS epidemic, there were numerous disease outbreaks that could only be explained by air transmission. Transmissions in Hong Kong’s Amoy Gardens high-rise district, as well as in many hospitals and hotels, could only be explained by long-distance air transmissions. Only a small proportion of those affected had had contact with each other. “
When it comes to ventilation and air conditioning systems, the most important thing is correct operation. The air supply volume of ventilation and air conditioning systems must be adapted to the volume of the room and the number of people. It becomes especially dangerous when the air is recirculated. Viruses then pass through the building instead of being carried away by the exhaust air. “In the vast majority of cases, especially if the system is well designed, it will make sense to let it operate to the proper extent. This is especially true when regular ventilation is not possible,” concludes Hugentobler.
