Viruses are travelling farther and surviving for longer
The annual winter waves of flu and colds demonstrate that many viruses thrive more easily during the colder months. The pathogen SARS-CoV-2, trigger of the respiratory illness COVID-19, is also one of these "winter viruses". One of the reasons for this is the low relative air humidity level in Central Europe in winter. When this dry air enters a room and heats up, the relative air humidity level drops further, creating the optimal conditions for viruses to survive. Humidifiers can be used to establish a relative air humidity of 40–60%, which means that the survival period of coronaviruses is considerably shorter.
The small droplets of saliva in which viruses enter the air unprotected evaporate very slowly in winter. As a result, the pathogens are able to travel longer distances. Therefore, the safety distance of 1.5 m from other people may not be enough in winter. What's more, coronaviruses are largely protected against high-energy UV radiation during months when there is little sunshine, whereas this UV radiation kills some of them in summer.
Dry air and lack of vitamin D put strain on the immune defence system
Many people have a vitamin D deficiency during the winter months, which weakens their immune system. In addition, the winter air obstructs mucociliary transport which our nasal and bronchial system uses to stave off a large proportion of viruses. During this process, microscopic cilia transport the pathogens out of the bronchia and nose and into the throat and laryngeal area. From here they enter the stomach where they cannot cause any harm. Mucociliary transport slows down in cold, dry air. In this case too, a humidifier can provide relief as moist air prevents the mucous membranes from drying out. Furthermore, many of the fine droplets measuring less than 1 μm, which can penetrate deep into the respiratory tract, become less infectious when the relative air humidity is high.
The spread of viral illnesses in winter is also increased by social factors. Due to low temperatures, people spend more time in enclosed spaces, which in turn means that the risk of becoming infected with coronavirus is comparatively high.
Air purifiers as a complementary preventive measure
Besides wearing a mouth and nose covering, keeping a minimum distance from others, frequently washing your hands and disinfecting surfaces, thorough airing also contributes towards reducing the viral load in rooms. The high temperature difference between the outdoor and indoor air ensures that air is exchanged very efficiently in winter. On the other hand, it is virtually impossible to maintain a pleasant air temperature due to the entry of cold outdoor air. In school classrooms especially this can lead to problems and affect the students' performance. What's more, the cold outdoor air warms up in the room and as a result becomes even drier – to the benefit of the viruses in the air and to the detriment of the people in the room and their mucous membranes.
Therefore, air purifiers can also be installed to remove a large proportion of the viral load from the room air. These devices should be equipped with efficient filters (such as a HEPA filter) as well as powerful fans that can circulate the room air at least six times per hour. By using an appropriate air filtration device, a low viral load can be ensured even when a room is rarely aired.