Scientists still disagree on the exact routes that the SARS-CoV-2 virus spreads itself. Via infections from droplets in the air, or from smudges or even from aerosols (the exhaled air) in the air that we breathe. Various virologists support the theory that the infection is only caused by droplets. They assume that it’s impossible to become infected in a supermarket, for example, but only through direct contact with an infected person. In the meantime, however, there is growing evidence that the coronavirus lingers much longer in the air we breathe than was previously thought.
When someone coughs, talks, or sneezes, they discharge a jet of droplets and aerosols of varying sizes that are dispersed into the air. If a person is infected with corona, all of those droplets and aerosols potentially contain viruses. Researchers at TU Berlin have now carried out various projects to determine whether and how quickly these particles sink to the ground, how far they spread, remain in the air, or where they settle.
“We are studying the length of time that pathogens stay airborne under a wide range of conditions in several projects,” says Prof. Dr. Martin Kriegel, head of the Hermann Rietschel Institute at TU Berlin. He and his “Contamination Control” team are using two research clean rooms and several airflow laboratories, as well as a research operating theater to examine, among other things, the extent to which the spread of the virus hinges on the composition and size distribution of the particles contained in exhaled air. The particle size ranges from a few nanometers (a millionth of a millimeter) to several micrometers.
Dispersion dependent on size and peripheral conditions
“It appears that both droplet infections and airborne transmissions, i.e. via aerosols, are relevant when it comes to the coronavirus,” Martin Kriegel notes. Droplet infection occurs via viruses that are transferred from a saliva droplet directly to the mucous membranes of another person. In contrast, in the case of infection via aerosols, virus particles enter the respiratory tract directly. The size of the carrier aerosols is decisive for the behavior of airborne viruses. However, the researchers also take into account the indoor climate, the air circulation rate, and the way in which the rooms are ventilated. “Larger particles tend to drop to the ground faster. Smaller particles follow the airflow and can remain in the air for a long time,” Martin Kriegel states.
According to the scientists, spreading it across a room occurs in two stages. First, coughing/speaking/sneezing creates a jet of particles between 0.01 μm and 1500 μm. These permeate the air in the room and then merge with it more and more. The trajectory of this jet depends on various peripheral conditions, such as velocity, turbulence, temperature differences between the jet and the ambient air, and differences in humidity. “Once the jet becomes fully mixed with the ambient air, it then diffuses,” Martin Kriegel explains. “The smaller particles largely follow the airflow in that space. Whereas larger particles gradually fall onto the floor. The fact that people only discharge very large particles whenever they sneeze is often ignored. Tiny aerosols are almost exclusively dispersed when we speak or cough normally.”
Amount of time particles need to settle
That is why the researchers have also measured the so-called sedimentation time. Which is the amount of time different size classes of particles need to settle. It turned out that tiny particles (0.5 to 3 μm) were still present almost in their entirety in the air even after a 20-minute measurement period. None or only a minuscule amount of particle sedimentation could be detected. Of the medium-sized particles (3 to 10 μm), more than 50% were still present in the air after 20 minutes. And another study showed that larger droplets (larger than 60 μm) can still, under certain circumstances, spread far across a room. “This is the case, for example, if the particles are emitted in the upward flow of heat sources (e.g. from a person). They rise, then spread horizontally, and start to settle after that. Any horizontal air movements further intensify the dispersion effect,” says Martin Kriegel.
In view of the fact that more and more people will quit their home office soon to return to their regular offices as a result of the easing of restrictions, the researchers have simulated particle dispersion in an imaginary office with four people. With and without mechanized ventilation. “This shows that it is precisely smaller particles under 50 μm that spread far into a room without any mechanized ventilation. And stay there for a long time. By comparison, particles between 5 and 20 μm spread less extensively in a room equipped with mechanized ventilation and are largely dissipated,” Martin Kriegel sums up.
Crucial questions that the scientists want to clarify next are: “How large do SARS-CoV-2 particles have to be in order to still be infectious? And how can targeted air conditioning systems or even simply ventilating rooms influence the length of time these particles remain present?” Kriegel adds. “The indoor climate also plays a role here, because the aerosols very quickly shrink when they evaporate and then they behave in a different way. In principle, it can be said that these pathogens remain in a room for hours in residential and office buildings which have typical air circulation rates. The speed at which they settle takes a very long time, as does the air renewal process. Therefore, generally speaking, any increase in the supply of fresh air would be very sensible.”