Technische Universität Berlin

Continuous ventilation, air filters – how should we deal with aerosols indoors?

Professor Dr. Martin Kriegel is head of the Hermann Rietschel Institute at TU Berlin. The focus of his research includes the spread of aerosols: “Depending on the room conditions, special room airflows are created. The resulting spread of aerosols is best understood and assessed when the scientific background is clear,” explains Kriegel.

FAQs about aerosols and SARS-CoV-2 on YouTube (English subtitles)


What are aerosols and how big are they?

The term aerosol is a kind of made-up word. Aerosols are solid or liquid particles carried by air. The size of the particles we refer to as aerosols is not defined. It is rather a question of whether the particle moves through the air or falls to the ground. If airflow is slow, such as in enclosed spaces, then aerosols are generally smaller than 10 micrometers. Outside where the wind is stronger, sand and raindrops are also carried a great distance by air. In this case, they would also be aerosols.

What are droplets?

Droplets are liquid particles with significant, dominant gravitational pull. Droplets measuring 50 micrometers or larger quickly fall to the ground in normal enclosed spaces. They are also visible to the healthy human eye.

How do aerosols behave in the air?

Different forces act on small particles. Diffusion refers to the independent movement of tiny particles which occurs regardless of airflow. Because small particles also have mass, they are subject to gravity and fall to the ground. Another force affecting small particles is the air movement in the room which carries the particles with it. In still air, where there is no air movement in a room, a particle measuring 10 micrometers or more falls 3mm every second to the ground. However, the air speed in a room is on average 50mm to 200mm per second. A comparison between the rate of sinking and room air speed shows that the air speed is faster. Particles whose movement primarily depends on air speed are called aerosols.

What factors determine how aerosols move in the air?

How aerosols move depends on the movement of the air. There are two different sources of air movement in a room. One is heat and the other is forced air movement. Forced air movement occurs, for example, when windows are opened or as a result of ventilation or air conditioning units, fans, or movement of people in the room.

Air rises at heat sources. If air flows upwards in a room, air elsewhere in the room must flow downwards. Air movement can vary greatly within a room. As a rule, there is always movement in all three spatial directions. Humans are a strong source of heat in a room and cause the greatest movement of air.

If the air flows against an object or wall, do the aerosols stick to that surface?

Generally, no. Aerosols only come into marginal contact with surfaces as air flows around any objects or along a wall. Because aerosols follow the flow of air and have very little inertia, they move together with the air and also flow around the object or along the wall.

Can aerosols that have landed on surfaces end up circulating in the air again?

Ordinarily no. If these minuscule particles stick to a surface, the adhesive forces are usually so great that a very large, countering force is needed for them to re-enter the air. Such a force occurs, for instance, when you drag your feet on the ground or rub your fingers on an object. In these cases, the particles could once again enter the airflow.

How does the size of liquid aerosols change in the air?

Aerosols and droplets from breathing air are liquid particles. They are made up of water, proteins, and salts among other things. They tend to be warmer than the surrounding air. As a result, some of the water they contain evaporates very quickly. Within a few seconds of being discharged from someone’s mouth or nose, the size of the droplets and aerosols reduces by about half. This means that larger droplets can quickly become aerosols. The particles quickly assume the temperature of the ambient air, causing the evaporation process to slow. Because aerosols move with the air and do not have any significant movement of their own, this process continues until a balance is reached and their size hardly continues to change. The remaining evaporation depends on the humidity of the room. The drier the air, the more liquid will evaporate. The wetter the air, the less liquid will evaporate. Once the liquid has completely evaporated, a so-called droplet core remains measuring approximately 0.3 micrometers.

How big are aerosols that enter the air from respiratory passages?

This depends on the activity of the person in question. When breathing, only particles smaller than five micrometers are expelled into the air. These are exclusively aerosols. When speaking or singing, larger particles which often originate in our mouths, for example through wet speech, are discharged into the air. However, 99 percent of these are still smaller than five micrometers. Yelling, coughing, or sneezing cause a significantly higher number of larger particles measuring over five micrometers. Nevertheless, there is still a far greater number of small aerosols than the larger droplets.

How many aerosols are produced by our respiratory air?

This depends on the activity of the person in question. When breathing, we discharge on average 50 particles measuring less than five micrometers per second. When speaking, this number is about 200 per second. And when singing, about 3000 particles per second. However, the number can greatly vary. Everyone is different. Someone could discharge 150 particles when breathing, 400 when speaking, or 6000 when singing.

Viruses and aerosols

How many viruses are present on a single aerosol/droplet?

This has not yet been scientifically determined. Medical doctors presume that a virus is present on every one out of ten aerosols. However, there are also estimates claiming that every aerosol carries a virus.

Can I measure aerosols and viruses?

It is possible to measure the size and number of particles in the air. You can do this, for instance, using a laser particle counter. Because there are millions of particles in room air (fine dust, shed skin flakes, fibers, etc.) that do not originate from respiratory air, it is not possible to only measure the amount of respiratory air aerosols in a normal room. Such measures can only be undertaken in special cleanrooms, such as those at TU Berlin’s Hermann Rietschel Institute.

The number of viruses in room air cannot be directly measured. To do so, an air sampler is needed which can only be analyzed in a special lab.

How big are viruses?

Viruses are approximately 0.1-0.2 micrometers.

How long do viruses survive on aerosols?

This depends on the specific conditions. There is no conclusive opinion from medical researchers. Scientists at Charité-Universitätsmedizin Berlin, who we work together with, currently assume viruses continue to live for up to three hours.

How long do viruses live on surfaces?

This depends on the specific conditions. There is no conclusive opinion from medical researchers. Scientists at Charité-Universitätsmedizin Berlin, who we work together with, currently assume viruses continue to live for up to 72 hours.

Why can aerosols carrying viruses be so harmful?

Aerosols are so small that they can directly land in the alveoli (air sacs) in our lungs without being stopped by our upper respiratory system to any considerable extent. If these aerosols carry viruses, the viruses acquire nearly unhindered access to our mucous membranes and blood vessels, where they can then significantly multiply and cause an infection.

Is a single virus sufficient to cause an infection?

The exact number of viruses needed to trigger an infection is not yet known to medical science. In the case of SARS-CoV-2, scientists currently believe that a single virus is not sufficient, but rather a certain number of viruses is needed. However, the exact number is unknown. One thing is undisputed, though: The more viruses we breathe in, the higher our risk of infection.


How can I remove aerosols carrying viruses from the air?

The first thing you should always do is let in fresh air so that the concentration of virus-carrying aerosols in our breathing air is as low as possible. The more fresh air in a room, the lower the concentration of virus-carrying aerosols. In general, fresh air ensures good air quality in a room, as there are also other factors which could have a negative impact on us, such as high carbon dioxide concentrations (CO2).

Additionally, it is possible to clean the air by removing aerosols through the use of special filters.

When is air quality considered good?

Good air quality refers to air with a CO2 concentration under 1000 ppm (parts per million). Various air quality guidelines, such as workplace directive ASR 3.6, which is established in the Workplace Ordinance, are also based on this binding value. The more people in a closed room, the higher the concentration of aerosols and CO2 in the room, as every person is continuously exhaling CO2 and aerosols. Outdoor air in Germany contains around 400 ppm CO2.

What is considered a poorly ventilated room?

A room is poorly ventilated when the CO2 level is over 1000 ppm. A concentration of CO2 measuring 2000 ppm or higher is considered hygienically critical.

How long does the window have to be open to ensure good air quality?

It is difficult to conclusively answer this question from a scientific perspective. How much air flows through a window depends on the temperature difference inside and outside, wind speed and direction, and the size of the window’s opening.

The existing ventilation guidelines provided by the German Environment Agency or workplace directives state that we should completely open a window (inrush airing) for several minutes every 20 minutes. During warmer seasons, windows can/should be kept permanently open. When possible, transverse airing is also important, in other words, opening windows and/or doors opposite each other.

How much fresh air is needed?

In order to become infected, a certain number of viruses is needed. However, this figure is still unknown to medicine. By allowing fresh air into a room, you ensure that less viruses are present in the air. The time you spend in a room also determines how many viruses you inhale. The question of the amount of fresh air needed thus also depends on the duration of your stay. The size of the room also plays a role, as the viruses distribute themselves over the entire room.

An estimate of the amount of fresh air needed can be made by specialist engineers.

On the website, scientists have published methods for calculating the amount of fresh air needed for different rooms.

How much fresh air enters through an open window?

It is difficult to conclusively answer this question from a scientific perspective. How much air flows through a window depends on the temperature difference inside and outside, wind speed and direction, and the size of the window’s opening.

What are CO2 sensors?

CO2 sensors continuously measure the level of CO2 in the air. Some models use a traffic light indicator to display the level of CO2 concentration. As a rule, green means: < 1000 ppm CO2, yellow: 1000 to < 2000 ppm CO2 , red: > 2000 ppm CO2. Some models use slightly different CO2 thresholds.

Why should I use CO2 sensors?

By controlling the CO2 value, you can get a realistic impression of how well ventilated a room is. Particularly when ventilating a room by opening a window, we often have a false impression of good air quality. The CO2 indicator can help us to learn suitable ventilation behavior. For this reason, the use of CO2 sensors is strongly recommended in rooms with window ventilation.

Can I achieve good air quality through window ventilation?

Yes and no. Throughout most of the year, this is possible but not guaranteed. Particularly during colder months, it is nearly impossible to produce good air quality through window ventilation while maintaining a comfortable temperature in the room. In the summer, the outdoor temperature is often the same as the room temperature. Additionally, there is often only a light or no breeze so that very little air flows through the open window.

Can a fan placed on a windowsill in an open window help to improve air exchange?

Not necessarily. Air flows through a window in different places. A fan sucks air from all directions and blows it in a certain direction. If the natural airflow direction - caused by the temperature difference and wind - is not taken into consideration and the fan is not facing the right direction and in the correct place, this could lead to lower air exchange or even short circuit flow causing exhausted room air to once again be blown into the room.

What is the difference between window ventilation and a ventilation system?

A ventilation system automatically introduces fresh air to a room. When ventilating through an open window, it is unclear to those in the room, how much air is entering and leaving through the window and how long it must be kept open to attain good air quality. This is similar to a thermostat controlled by a heating system compared to an oven which must be fueled by hand.

Do air conditioning systems spread viruses?

To answer this question, the term air conditioning system must first be clarified. Specialist engineers can differentiate based on the name. There are three initial categories: 100 percent fresh air, X percent fresh air and y percent circulating air, and 100 percent circulating air. In principle, no system is a significant cause of virus spread. As soon as fresh air is introduced to a room, the concentration of aerosols is automatically reduced. Systems which use a percentage of or only circulating air are primarily intended to maintain air temperature (warm, cool) and not improve air quality. All systems blow air into a room at a certain speed, thus causing air flow volume. The air movement originating from people and other sources of heat in the room is generally greater than the amount of air movement caused by ventilation systems. For this reason, aerosols spread quickly throughout a room even without ventilation systems. The air movement caused by air conditioners (ventilation systems) overlaps with this air movement and accelerates the distribution. However, this does not affect the inherent spread of aerosols. Both occur within just a few minutes.

Even if a ventilation system operated on 100 percent circulating air, it generally does not have a decisive influence on the distribution of aerosols. The number of viruses in the room remains the same.

So: No, air conditioners are not virus factories. Systems using fresh air reduce the aerosol concentration in the air and always reduce the risk.

Are recirculating ventilation systems bad?

It is important to differentiate whether the units are located in the room or whether they are centrally installed in the building and collect air from multiple rooms and then redistribute it to all rooms. De-centralized units are neither good nor bad. In the case of central units, viruses may be collected from one room, mixed with the air from all other rooms, and then redistributed to all the other rooms via traditional fine dust filters. During this process, the air is mixed with fresh air. Traditional fine dust filters eliminate approximately 50 percent of the viruses. If, for example, a person in the building is infected, all the virus-carrying aerosols expelled by them will be mixed with a large amount of uninfected air (air from other rooms and fresh air) and filtered. While the viruses will be distributed into all the rooms, the concentration in the air is much much lower compared to the room where the infected person is. In the case of SARS-CoV-2, the risk of being infected this way, is very low.

What are filters?

Filters are used to eliminate particles from the air. There are different classes of filters which effectively eliminate certain particle sizes. Traditionally, fine dust filters which eliminate roughly 50 percent of aerosols are built into ventilation systems. There are also higher quality filters, such as HEPA or ULPA filters. HEPA filters (H12/H14) eliminate 99.9 percent of aerosols resulting in air that is considered particle-free.

How effective are filters at eliminating aerosols?

Traditional fine dust filters eliminate roughly 50 percent of aerosols. HEPA (H13/H14) filters remove 99.99 percent of aerosols resulting in air that is considered essentially particle-free.

Can the air be cleaned with filters?

Yes, this is possible. However, it is first and foremost recommended to ensure that the air quality is good, as this alone is proven highly effective at reducing the risk of infection. Good air quality is a basic right of all humans and applies both indoors and outdoors. Nevertheless, you must always keep in mind: The longer you spend indoors in a room, the higher your risk, as the air will never be 100% clean. Even if filters are used, this level of purity cannot be attained in normal rooms. This is only possible in special cleanrooms, such as at the TU Berlin Hermann Rietschel Institute.

Can ventilation systems be refurbished with HEPA filters?

No, because the ventilation systems are not equipped with the fixtures needed for the filter to be installed. Even if this were possible, the performance of the ventilation system would be impacted by the installation. Generally, this causes less air to be transported.

Are there special ways to direct airflow in a room and prevent aerosols from spreading?

Yes, there are. These are implemented when an absolute absence of particles is required. This includes, for instance, rooms for the production of medicines or computer chips and special rooms in health care.

Does it matter how and where the air in a room comes from as long as it is fresh air?

Yes and no.

Yes: Fresh air is a positive thing. When airing out a room by opening a window, opening two opposite windows is better than only opening windows on one side of the room. Ventilation systems blow fresh air into a room and discharge the exhausted air. Specialist engineers and system manufacturers must adhere to the applicable rules for planning and construction to ensure that fresh air is being supplied.

No, because the position of the additional air being delivered into the room and of the extracted air from the room affects the effectiveness of the ventilation. The concentration of aerosols throughout a room is rarely the same. Recirculation areas (poorly ventilated spaces) can occur where the aerosols accumulate. Alternatively, there are areas of a room which are ventilated well. This all depends on the room airflow, which is influenced by many different factors. As a rule, only special experts from the field of room airflow can provide further details.

What type of ventilation is recommended for normal offices, classrooms, or event rooms?

A distinction is made between mixed ventilation and displacement ventilation (layered ventilation). In mixed ventilation, room air is mixed by the presence of heat sources (people) and the type of air injection (generally via air outlets in the ceiling). Ideally, the aerosol concentration should be the same everywhere in the room. The aerosol concentration is diluted through the introduction of fresh air.

In displacement ventilation, the fresh air is introduced close to the ground creating a sort of fresh air lake. The air rises near heat sources (for instance people) and collects under the ceiling, where it is sucked out. This type of ventilation is more effective than mixed ventilation, since the air quality around people is better.

Is it possible to completely free rooms of viruses?

Yes and no.

Yes – in very special cleanrooms, viruses that have entered the air can be transported out of the room in a matter of seconds before they can spread out in the room.

No – in a normal environment (office, classroom, movie theater, etc.) these types of complex ventilation systems are not possible.

How long does it take for a room to be cleared of virus-carrying aerosols if it is not in use?

This depends on the amount of fresh air, the size of the room, and the time. Air exchange rate refers to the combination of the amount of fresh air with the room’s volume. In mechanically ventilated offices, the air exchange rate is approximately double. This means that the room air is exchanged with fresh air twice every hour. This can be somewhat confusing as this does not directly provide information about the aerosol concentration. If the air exchange rate is double, it takes about 2.5 hours until the aerosol concentration has been reduced by 99 percent.


Can wearing mouth-nose protection/a face mask protect me from aerosols?

No, not considerably. Aerosols are so small that they may pass through the material but a larger percentage enter on the sides. So you still breathe them in despite wearing mouth-nose protection. FFP2/3 masks are needed to prevent you from breathing in aerosols. But, face masks are effective at blocking larger droplets and the aerosols are at least redirected. This prevents aerosol clouds from coming into contact with a person opposite you. This is why it is useful and important to wear masks if you cannot keep sufficient distance from others.

Do I protect others from my aerosols by wearing mouth-nose protection/a face mask?

No, not considerably. Approximately 90% of aerosols escape via the sides of your mask into the air. But, face masks are effective at blocking larger droplets and the aerosols are at least redirected. This prevents aerosol clouds from coming into contact with a person opposite you. This is why it is useful and important to wear masks if you cannot keep sufficient distance from others.

What advantage do face shields/visors offer?

Face shields primarily protect you from larger droplets. However, face shields are not as effective as blocking droplets as traditional face masks (mouth-nose protection).

Which mask will protect me and others from being infected via our airways?

FFP masks (FFP2/3) effectively protect the person wearing them when properly used. This means the mask is pressed close against your face so that air cannot escape through the sides. Some FFP masks have a valve to ease exhaling. Air and thus aerosols escape through the valve. When you inhale, the valve automatically closes, so that you breathe through the fabric, which is very effective in preventing aerosols from passing through.

What kind of protection do plexiglass partitions offer in an office, at the register, etc.?

This type of barrier is most effective at blocking droplets. It is ineffective at blocking aerosols. Depending on the airflow in the room, they can have both a positive or negative effect on the distribution of aerosols.

Risk assessment

What is the risk of being in a room indoors compared to being outdoors?

Aerosols accumulate in the room air in enclosed spaces. The aerosol concentration can be reduced by introducing fresh air to the room. Aerosol clouds exhausted by people disperse quickly outdoors and are picked up and distributed by the wind. Even if there is only a slight breeze, the air speeds outside are much higher than indoors.

The risk indoors depends on how many infected persons are present, how long they are in the room, the room size, amount of fresh air introduced to the room, and the duration of the stay of a healthy person who is constantly inhaling the virus-carrying aerosols. In theory, the risk can be calculated as demonstrated by scientists in their publications. However, decisive data is missing to provide actual calculations: the number of viruses on an aerosol and the number of viruses needed to cause an infection. Thus far, this data has not been provided by medicine.

Essentially, the facts can be summarized as follows:

  • The higher the amount of fresh air, the lower the risk.
  • The bigger the room, the lower the risk.
  • The shorter the stay, the lower the risk.
  • The number of people in a room does not play a role in the importance of adhering to the AHA rules (German acronym for minimum distance, hygiene, mouth-nose protection). Given the current level of infection, we can currently assume a maximum of one infected person in a room.

What is the risk of infection in an airplane?

Enough fresh air is introduced to the cabin to sufficiently keep the CO2 level in an acceptable range. A large portion of the air introduced to the cabin is circulated air that has been filtered through HEPA filters and is thus considered particle free. The entire amount of air (fresh air + filtered circulated air) that is virus-free is very high compared to a typical large-room office in a building. The concentration of virus-carrying aerosols is very low.

The close proximity of people to one another, however, increases the risk of infection. The risk of infection increases when sitting in the immediate vicinity of an infected person. Thus, it is very important that passengers wear masks.

What is the risk of infection in a train carriage?

Sufficient fresh air is introduced to the people in the space to keep the CO2 low. The amount of fresh air relative to an infected person is very high, for instance compared to a typical open-plan office in buildings.

The close proximity of people to one another, however, increases the risk of infection. The risk of infection increases when sitting in the immediate vicinity of an infected person. Thus, it is very important that passengers wear masks.

What is the risk of infection in a movie theater or theater?

If the room is mechanically ventilated, sufficient fresh air will enter the room to keep the CO2 concentration and thus aerosol concentration low. The amount of fresh air in relation to an infected person is quite large compared to rooms in other buildings.

The proximity of people to one another, however, increases the risk of infection. The risk of infection increases when sitting in the immediate vicinity of an infected person. Thus, it is very important that passengers wear masks.

What is the risk of infection in a classroom?

90 percent of classrooms are not equipped with a ventilation system so that they are almost exclusively ventilated through open windows. Because window ventilation does not guarantee fresh air in the room and windows are generally not opened frequently enough, spending time in these rooms carries a great degree of risk. In order to achieve better ventilation behavior, the use of CO2 sensors is recommended to ensure sufficient fresh air is being introduced.

The close proximity of students to each other in the room also increases the risk of infection. If someone sits near an infected person, their risk of infection increases. Thus, it is very important that students and teachers wear masks.

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