It is not absolutely clear how the coronavirus spreads - whether primarily via droplets or via aerosol in respiratory air. When a person infected with the coronavirus coughs, speaks or sneezes, a spray consisting of droplets of different sizes and aerosols is generated, which penetrates the room air and then spreads.
All of these droplets and aerosols contain potential viruses. Professor Dr. Martin Kriegel, head of the Hermann-Rietschel-Institut at TU Berlin, is researching how these particles behave - whether and when they sink to the ground, how far they spread, if they remain in the air and where they sediment.
“We are conducting a number of projects to research how long pathogens remain in the air under a wide range of conditions,” says Kriegel. Professor Kriegel’s “Contamination Control” team is performing its research using two clean rooms, several room airflow labs and a research operating theater. To better understand the corona pandemic, the researchers are examining to what extent the spread of the virus is affected by the composition and size distribution of the particles in exhaled air (aerosol). The term aerosol refers to the smallest fluid or solid particles (these could be viruses, either alone or combined with saliva or even dirt or particulates) in a gas, most commonly air. The size of the particles ranges from a few nanometers (a millionth of a millimeter), through to several micrometers (one thousandth of a millimeter). To provide a comparison: The breadth of a human hair is 100 micrometers.
“It seems that as far as the coronavirus is concerned, both infection via droplets and airborne transmission via aerosols apply,” Kriegel explains. In the case of droplet infection, the virus particles in a droplet of saliva come into contact with the mucus membranes of another person. While in airborne transfers, the viruses – tied to the smallest fluid particles – enter our respiratory passages. Decisive for the behavior of viruses in air are the size of the carrier aerosols as well as the indoor climate, the air exchange rate and the way in which a room is aired. “Larger particles sink faster to the ground. Smaller particles follow the air flow and can remain longer in the air,” Kriegel explains.
The spread of the mixture of particles, saliva and air produced by speaking coughing or sneezing occurs in two stages. Firstly the process of coughing/speaking/sneezing produces a spray which penetrates the room air, increasingly mixing with it. What happens next to this spray depends on a number of factors, such as its speed, turbulence, the temperature difference between the spray and the surrounding air as well as the difference in humidity. A number of studies have shown that speaking/coughing/sneezing produce particles of 0.01 μm to 1500 μm. “Once the spray has fully mixed with the room air, the process of spreading begins,” explains Kriegel. “The smaller particles largely follow the room airflow, whilst larger particles sink successively to the ground. The fact that the largest particles are only emitted by sneezing is often overlooked. Almost all aerosols produced by speaking and coughing are small.
Working in a number of projects, the research team measured the sedimentation rate of particles of various size categories. Almost all small particles (0.5 to10 μm) are still present in air after twenty minutes. Little or no sedimentation of these particles can be detected. More than 50 percent of medium-sized particles (3 to10 μm) can still be detected in the air after twenty minutes. “A further study shows that under certain conditions even larger droplets (>60 μm) can spread a long way through a room. This is the case if the particles are propelled from a source of warmth (such as a human body). They rise, spread horizontally and only then start to sediment. Possible horizontal air movements strengthen the spreading effect,” says Kriegel.
To examine the possibility of recommencing normal working life, the research team simulated the spread of particles in an office where four people are working and where there is no air conditioning system. “Our tests show that small particles of less than 50 μm spread extensively through a room and remain in the air for a long time without the use of an air conditioning system. By contrast, the spread of particles of between 5 and 20 μm is less extensive in a room where air conditioning is present and the majority of particles are discharged,” concludes Kriegel.
“The decisive questions, which we are examining in our interdisciplinary projects, are how large do SARS-CoV-2 particles have to be to remain infectious and the extent to which the dwell time of particles of this size can be influenced by air intake and exhaust systems or even by simply airing a room. The indoor climate also plays a role, as aerosols become much smaller as a result of humidification, which in turn alters their behavior. What we can conclude is that pathogens remain in rooms for hours at typical air exchange rates in residential and office buildings. The sink rate and the process of air renewal take a very considerable time. As such, any increase in the supply of fresh air is to be recommended.”