What happens when particles stop moving

How coronaviruses spread in indoor air


Like droplets and aerosols, aerosols
Aerosols are mixtures of solid or liquid particles in a gas mixture such as air. The tiny particles can float in it for a long time. Behavior, whether and when they sink to the ground, how far they spread, remain in the air or where they sediment, is currently being investigated at the TU Berlin, for example.

How exactly the CoronaCorona
Virus is spread - whether mainly via a droplet infection or rather via aerosols in the breath has not yet been conclusively clarified. When a corona patient coughs, speaks or sneezes, a jet of droplets and aerosols of different sizes is generated, which penetrates the room air and spreads there. All these differently sized droplets and aerosolsAerosols
Aerosols are mixtures of solid or liquid particles in a gas mixture such as air. The tiny particles can float in it for a long time. potentially contain viruses. How these particles behave, whether and when they sink to the ground, how far they are distributed, remain in the air or where they sediment, is a research topic of Prof. Dr. Martin Kriegel, head of the Hermann Rietschel Institute at the TU Berlin.

"In various projects we are investigating the dwell time of pathogens in the air under a wide variety of conditions," says Martin Kriegel. His team is responsible for these experiments "Contamination Control“Two research clean rooms, several indoor air flow laboratories and a research operating room are available. In connection with the corona pandemic pandemic
A pandemic is a widespread disease that affects entire countries or continents.
If, for example, the genetic information of two different influenza viruses mixes in an intermediate host (e.g. pig), a new type of virus appears with as yet unknown properties. This so-called subtype can spread quickly because people do not have antibodies against this pathogen, either naturally or as a result of a vaccination. The annual flu vaccination covers new variants of the influenza virus (i.e. slight changes in the surface structure), but not completely new subtypes. If a pandemic breaks out, a vaccine against the new subtype must therefore be developed quickly and / or an antiviral drug must be used across the board.
The scientists are investigating to what extent the spread of the virus depends on the composition and size distribution of the particles within the exhaled air (aerosol aerosol
Aerosols are generally understood to be a mixture (or a multiphase system) of gases (especially air) with irregularly distributed solid particles (e.g. dust, fiber dust, smoke) or with liquid components. Dusts are mixtures of irregularly distributed solid particles in gases that result from mechanical work or whirling up. Fiber dusts are mixtures of irregularly distributed fibers (of organic or inorganic origin - for example asbestos) in gases. Fumes are the finest mixtures of solid particles in gases that arise from thermal processes (e.g. welding smoke or metal oxide smoke) or from combustion processes (e.g. soot or fly ash).
) depends. An aerosol consists of the smallest, liquid or solid particles (these can be viruses, for example, individually or in combination with saliva or soot, fine dust
Fine dust is considered toxic and, according to estimates by the EU Commission, costs many Europeans their lives every year. With a size of less than 10 µm fine dusts can float in the air (so-called airborne dusts) and can be inhaled by humans, so that the fine dust particles can get into the lungs and pass into the body via the alveoli. So they advance into the liver, among other things.
etc.) in a gas, usually air. The particle size ranges from a few nanometers - i.e. a millionth of a millimeter - to several micrometers. For comparison: a human hair is around 100 micrometers thick.

"For the corona virus it appears that both droplet infections and airborne transmission via aerosols are relevant," says Martin Kriegel. In the case of a droplet infection, the virus particles reach the mucous membranes of another person in a droplet of saliva. In the case of airborne transmission, the viruses - bound in the smallest liquid particles - get into the respiratory tract. The size of the carrier aerosols is decisive for the behavior of viruses in the air, but so is the indoor climate, the air exchange rate and the way in which ventilation is carried out. “Larger particles sink to the ground faster. Smaller particles follow the air flow and can remain in the air for a long time, ”says Martin Kriegel.

The spread in the space of the mixture of particles, saliva and air, which is produced when speaking, coughing and sneezing, takes place in two steps. First of all, coughing / speaking / sneezing creates a jet that penetrates the room air and increasingly mixes with it. The course of the incoming jet depends on various boundary conditions such as speed, turbulence, the temperature difference between the jet and the ambient air and the difference in humidity. It is known from various studies that speaking / coughing / sneezing particles from 0.01 micrometers to 1500 micrometers occur. "After the jet has been completely mixed with the room air, it is distributed," explains Martin Kriegel. “The smaller particles largely follow the flow of air in the room, while larger particles gradually fall to the floor. Often ignored is the fact that humans only emit very large particles when they sneeze. Normal speaking and coughing almost exclusively generate small aerosols. "

In various projects, the scientists measured the so-called sedimentation time (deposition time) of particles of various size classes. Small particles (0.5 to 3 micrometers) are almost completely present in the air after a measurement time of 20 minutes. A deposit of these particles is not or only slightly noticeable. For medium-sized particles (3 to 10 micrometers), more than 50 percent can still be found in the air after a measurement time of 20 minutes. “Another study shows that even larger droplets (> 60 micrometers) can spread widely in space under certain circumstances. This is the case, for example, if the particles in the buoyancy current are emitted by heat sources (for example by a person). They rise, spread out horizontally, and only then begin to deposit. Any horizontal air movements increase the spreading effect even more ”, says Martin Kriegel.

In connection with the resumption of everyday working life in an office occupied by several people, the scientists also simulated the spread of particles in an office occupied by four people with and without mechanical ventilation. “This shows that smaller particles below 50 micrometers in particular can spread widely in the room without mechanical ventilation and linger for a long time. In contrast to this, particles between 5 and 20 micrometers spread less in a room with mechanical ventilation and are largely removed, ”summarizes Martin Kriegel.

“The crucial questions that we will now investigate in interdisciplinary projects are how big SARS-CoV-2 particles have to be in order to be still infectious and how the retention time of this particle size through targeted air intake and exhaust systems or simple ventilation Spaces can be influenced. The indoor climate also plays a role, because the aerosols quickly become smaller through evaporation and then behave differently. Basically, it can be said that with typical air exchange rates in residential and office buildings, the pathogens remain in the room for hours. The rate of descent and the air renewal take a very long time. Any increase in the outside air supply is therefore generally sensible. "

Source: Technical University of Berlin

Photo: © Gesundheitstechnische Gesellschaft e.V: (GG) - Technical-Scientific Association



Image: © Gesundheitstechnische Gesellschaft e.V: (GG) - Technical-scientific association