Air and Covid-19

Covid-19 is changing our expectations of indoor environments. Buildings can play a central role in the spread of disease: Increasing evidence shows that the virus spreads primarily indoors. 

Coronavirus infections occur predominantly in family environments and workplaces. In addition to preventive measures such as masks and distance rules, good ventilation is therefore also important. Good ventilation of indoor spaces with fresh air is mandatory for good and healthy indoor air. But how does the necessary fresh air get into our buildings? 

With the help of smoke and carbon dioxide, flow conditions are simulated in a replica classroom.
© German Aerospace Center

Today, conventional window ventilation is reaching its limits. The narrative of energy efficiency and sustainability as well as the dense construction of new buildings and sealing measures in the context of renovations, have reduced the uncontrolled natural air exchange to a minimum. As a result, fresh air has to be supplied, usually without the knowledge of the occupants, by means of complex building technology. 
However, even new standards such as that of Minergie, which are supposed to enable living and working comfort of occupants through a high-quality building envelope and systematic air renewal, do not solve the problem of fresh air supply in times with and after Covid-19. 
The modern indoor society should think some basic thoughts about the food air.One year after the Covid-19 outbreak, there is consensus that the virus is spread primarily through close human-to-human contact. However, some uncertainty remains about the relative importance of the different modes of transmission of SARS-CoV-2, the virus that caused  Coronavirus disease 2019 (Covid-19). 

Even after a year of the virus and ongoing vaccination efforts, the situation worldwide is far from under control. The Robert Koch-Institute’s current situation report for Germany, dated 15.03. 2021 (the date of this report), states:

“The high nationwide case numbers are caused by mostly diffuse events with numerous clusters, particularly in private households, but increasingly also in daycare centers, schools and professional settings.” (1)

An essential prerequisite for effective control of Covid-19 is knowledge of transmission routes and risk situations.

There has been evidence and documented studies for months that this virus can remain airborne for longer periods and greater distances than originally thought. In addition to close contact with infected people and contaminated surfaces, the spread of Covid-19 can also occur via airborne particles indoors, possibly beyond the 2 m range suggestedby some recommendations for social – read: spatial – distancing.

Therefore, in addition to preventive measures such as masks and distance rules, good ventilation is also important. Good ventilation of indoor spaces with fresh air is mandatory for good and healthy indoor air. But how does the necessary fresh air get into our buildings?

Indoor Society

On average, people spend more than 90% of their time indoors, i.e. in homes, workplaces, offices or on public transport. It is therefore obvious that the majority of Covid-19 patients are infected with the virus in these indoor spaces.

For years, scientists have been sounding the alarm that our distance from nature and its resources is linked to a variety of chronic health problems such as allergies, asthma, depression, irritable bowel syndrome and obesity. More recently, experts in a variety of fields have examined why buildings, even those designed to be as germ-free as possible, are vectors for disease, not the least of which is Covid-19. The relative lack of airflow and sunlight is an obvious problem; temperature, humidity and indoor air pollution also play a role. (2) Studies suggest that indoor air is up to five times more polluted than outdoor air. (3)

Transmission Strategies of the Virus

Coronavirus can be transmitted via droplets in direct contact – but also via aerosols, particles suspended in the air that are smaller than five micrometers. (4) Such transmissions are also possible in certain situations over greater distances like, for exmaple,  when many people congregate in inadequately ventilated indoor spaces.

It is considered certain that most corona infections are transmitted via droplets when an infected person comes too close to other people. Larger droplets sink quickly to the ground. Aerosols, on the other hand, are much smaller and can remain suspended in the air for several hours, accumulating indoors. When inhaled, they also reach the deep portions of the lungs. However, how often people become infected via aerosols is not yet clear and needs to be further investigated in studies.

Infections caused by aerosols are unlikely to occur outdoors, as the tiny particles are rapidly dispersed in fresh air and diluted to such an extent that they no longer pose a danger. This is because a certain amount of virus is required for an infection and there is no room for it on individual tiny aerosols.

The situation is different indoors: The smaller a room is, the more infectious aerosols are distributed in the air and inhaled by other people in the room. Anyone who spends several hours in a room with an infected person without a mask is taking a certain risk. This is because when a person releases aerosols, they disperse fine mist. Therefore, when several people spend time indoors, there is an increased risk of infection. Numerous factors play a role in the likelihood of infection occurring indoors and these factors can vary greatly from case to case: including the number of people present, the activity of the people, the volume of the room, the air exchange rate, the air flow, the type of ventilation available (window ventilation, ventilation technology) and any filters used. (5)

Duration of Infection

The infection duration calculation model developed by U.S. researchers (6) uses factors such as UV radiation (sunlight), humidity and temperature to indicate how long it takes for 90% of all SARS-CoV-2 viruses to decay.

These data suggest that the potential for aerosol transmission of SARS-CoV-2 is dependent on environmental conditions, the influence of relative humidity, simulated sunlight and chemical structure (suspension matrix – note 7). For the survival of SARS-CoV-2 in aerosols, sunlight appears to be an important factor influencing the risk of disease transmission through aerosols.

The residence of aerosols in the air depends on their size. If the particle is large, it sinks relatively quickly due to gravity; if it is small –typically five microns and smaller – it moves with the air. Indoors, it takes a long time for such particles to land on the ground. Although the concentration decreases significantly after hours, some can remain airborne for up to a day or even several days. (8)

But SARS-CoV-2 may not remain infectious as long as the aerosols are airborne. Several studies have shown that at normal room temperature, the virus remains active for only 1 to 2 hours. Therefore, when one enters an office in the morning, there will still be hardly any dangerous aerosols in the air.

Air Management

The transmission of coronavirus through aerosols, i.e. suspended particles in the air – especially indoors – is therefore increasingly coming into focus.

To reduce aerosol transmission, the viral load in the room must be moved and transported out – for example, through regular ventilation. In everyday life, this may be more decisive than washing hands and disinfecting. The analysis of the room situation or air management therefore plays an increasingly important role. 

This makes it clear that the layout of a building and the design of a room as well as the occupancy and type of heating, ventilation and air conditioning (HVAC) systems, can all have an impact on the potential airborne spread of the virus.

An important approach to reducing indoor concentrations of air pollutants or contaminants, including any viruses that may be present, is to increase ventilation – that is the amount of outdoor air that enters indoor spaces. Ensuring adequate ventilation with outside air can help reduce indoor concentrations of airborne contaminants, including viruses.

As a rule of thumb, the smaller the room and the more people in one place, the more difficult it is to maintain the minimum distance and the poorer the air exchange in a room, the higher the risk of infection. This is especially important to remember when traveling in elevators.

Case Study Elevator

If the elevator is not used after aerosol emission and remains on a floor with the doors closed, the potentially infectious droplets can remain in the cabin air for up to 30 minutes. Thus, even if a person acutely infected with SARS-CoV-2 has already exited the elevator, potentially infectious particles may remain. The saliva droplets of a mildly ill Covid-19 patient can contain between a thousand and a billion RNA copies of the coronavirus per milliliter of fluid. Speaking loudly can expel up to several hundred thousand droplets per minute and a single cough can expel several million such droplets. If elevator doors are left permanently open, aerosol persistence is reduced to 2-4 minutes. (9)

These are the findings of a study from Amsterdam, which examined air conditions in hospital elevators. There, current standards for air exchange rates (10) through mechanical ventilation in hospital elevators vary from 6 to 20 air changes per hour. If we assume that aerosol particles are continuously mixed with the supply air, without considering particle deposition, resuspension and stagnant flows, an air change rate of 10 times per hour means a 100-fold reduction in aerosol particles in about 28 minutes. From the experiments, a 100-fold reduction occurs in 24-30 minutes with the doors closed, 12-18 minutes in operation and 3-5 minutes with the doors open. In addition, a notable finding of this study was that the ventilation in all elevators studied automatically shuts off after 1-2 minutes in the idle position – a finding that could easily be extended by reprogramming the action control software.


In general, the more people there are in an indoor space, the greater the need for ventilation with outside air. In other words, the ventilation rate should be based on the number of people staying in an indoor space (and a few other factors). In fact, the Centers for Disease Control and Prevention CDC of the U.S. Department of Health and Human Services has found that “indoor spaces are riskier than outdoor spaces, where it can be more difficult to keep people apart and there is less ventilation.” (11)

“In very basic terms, it can be stated that at typical air exchange rates in residential and office buildings, pathogens remain in the space for hours. Sink rates and also air renewal take a very long time. Any increase in outside air supply is therefore generally sensible,” says the head of the Hermann Rietschel Institute, the Institute for Energy Technology at the Technical University of Berlin, Martin Kriegel. (12)

The Institute of Fluid Mechanics and Aerodynamics at the University of the German Armed Forces in Munich has investigated the extent to which mobile indoor air purifiers can effectively reduce an indirect SARS-CoV-2 infection risk through aerosols. (13)

In their report, they state that mobile, high-performance room air purifiers with special filters can keep the aerosol concentration in rooms of small and medium size at a low level. Thus, the indirect risk of infection can be greatly reduced by these devices, even with closed windows and without a suitable ventilation and air-conditioning system. (14)

In non-medical buildings where ventilation systems use recirculated air (e.g., office buildings, restaurants, hotels, shopping malls, retirement homes and schools), high-efficiency particulate filtration (the highest grade approved by the AHU) should be used or the recirculation rate should be reduced or, if possible, set to zero. (15)

The purpose of air conditioning and ventilation systems is to provide “thermally comfortable” environments for people and often to improve the quality of circulating indoor air. (16) Studies examining different levels of indoor temperature and relative humidity in the transmission of SARS- CoV-1, H1N1 and MERS-CoV viruses show that lower temperatures and relative humidity (below 21° C, below 40% relative humidity) increase survival of coronaviruses and influenza on dry surfaces. (17) For example, SARS-CoV-1 can survive for at least two weeks in an air-conditioned environment where the temperature is typically 22 to 28° C and relative humidity is usually controlled at 30 to 60%. (18)

The models known so far now allow statements to be derived quantitatively. 
The lower the risk of infection:

  • The larger the volume of the indoor space,
  • the fewer people stay in the room,
  • the shorter the time people spend in the room,
  • the fewer aerosol-forming activities such as loud talking, shouting, singing, etc. take place,
  • when masks that fit and filter as well as possible are worn and
  • if the highest possible air exchange rate can be achieved, ideally by means of a room air conditioning system with outside air supply.


Covid-19 is changing our expectations of indoor environments. Buildings can play a central role in the spread of disease: It is increasingly clear that the virus spreads primarily indoors. 

Coronavirus infections occur predominantly in family environments and workplaces. Open-plan offices, where many people spend long periods of time, are places where infection is quite likely. It can be reduced with hygiene rules and keeping a distance, but it cannot be ruled out. With the air exchange rates practiced so far in residential and office buildings, the pathogens can remain in the room for hours. The sinking rate and also the air renewal take a very long time. A permanent outside air supply with fresh air is therefore generally advisable. In addition, factors such as indoor temperature and relative humidity play an important role. (19)

Part 2 explores the issue of indoor climate in hospitals and its health effects on patients.



2) This includes new standards such as the Minergie standard, which is intended to ensure living and working comfort for building occupants by means of a high-quality building envelope and systematic air renewal. 
The Minergie standard promises a tight and well-insulated building envelope, which should guarantee low heat loss. In addition, there is automatically controllable ventilation with heat recovery for good air quality and environmentally friendly heating.
However, as a result of regulation problems and construction defects (due to incorrect construction elements and inadequate execution, among other things), there is sometimes a respectable gap between the claim and the built reality.


4) See, among others: Thick air (Part 2) – Indoor climate and SARS-CoV-2

5) In an April 2020 study, Chinese researchers found that all but one of the identified outbreaks occurred in an indoor environment, confirming that indoor sharing poses a major SARS-CoV-2 infection risk.
Hua Qian, Te Miao, Li Liu, Xiaohong Zheng, Danting Luo, Yuguo Li (2020), Indoor transmission of SARS-CoV-2.
See also:


7) A suspension is a heterogeneous, immiscible mixture of a liquid and a solid finely dispersed therein. This is distributed or slurried in the liquid. A suspension is therefore a dispersed solid phase in a continuous liquid phase.

8) This is confirmed by the following two studies:
The results of this study suggest that transmission of SARS-CoV-2 via aerosols and fomites is plausible because the virus can remain viable and infectious in aerosols for hours and on surfaces for up to days (depending on the infectious material excreted).
SARS-CoV-2, the agent of the Covid-19 pandemic, can be transmitted via droplets in the air or by contact with surfaces on which droplets have settled. This study examined the ability of SARS-CoV-2 to survive in the dark, at two different relative humidities and in artificial saliva, a clinically relevant matrix. SARS-CoV-2 was found to be stable in the dark in a dynamic small particle aerosol under the four experimental conditions tested and viable virus could still be detected after 90 minutes. The decay rate and half-life were determined. Decay rates ranged from 0.4 to 2.27% per minute and half-lives from 30 to 177 minutes for the different conditions.

9) Van Rijn C. et al. (2020). Reducing aerosol transmission of SARS-CoV-2 in hospital elevators, Indoor Air, Int. Journal of Indoor Environment and Health.

10) Air exchange rate: see, among others.
Thick Air (Part 2) – Indoor Climate and SARS-CoV-2
See also part 2 of this BLOG «Indoor climate in hospitals and health effects for patients».




14) This with with F7 + H14 filter combinations. 
See the report of the Fraunhofer Institute for Building Physics from 26.02.2021.
However, the authors of the Bundeswehr study (note 13) add (p. 22):
“To ensure that the room air purifier does not become a virus slinger, the H14 filter should be heated up by the device to about 100° C once a day for about 30 minutes to destroy the viruses in the filter and to counteract the formation of biofilms, bacteria and fungi without harmful chemical additives or UV-C radiation.”


16)  Page 6

17) Otter JA, Donskey C, Yezli S, Douthwaite S, Goldenberg SD & Weber DJ (2016). Transmission of SARS and MERS coronaviruses and influenza virus in healthcare settings: the possible role of dry surface contamination. J Hosp Infect 92, 235-250. 

18) Chan KH, Peiris JSM, Lam SY, Poon LLM, Yuen KY & Seto WH (2011). The Effects of Temperature and Relative Humidity on the Viability of the SARS Coronavirus. Adv Virol 2011, 734690. 



Cite this blog post
Jean Odermatt (2021, April 8). Air and Covid-19. Learning from Corona. Retrieved May 18, 2024, from

Jean Odermatt

Prof. Jean Odermatt is sociologist and interior designer. He is a professor emeritus at Bern the University of the Arts (HKB). His research focus is on how interior design can promote the process of recovery.

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