Thick Air (Part 2) – Indoor Climate and SARS-CoV-2

Preliminary remark of the author 
What does air have to do with design? 

The author is neither an infectiologist nor a specialist in outdoor or indoor air issues. The idea of this three-part blog is rather to present current research results in an increasingly confusing situation around SARS-CoV-2 and to put them into a wider context. The common “silo thinking” contains the risk of losing track of one of the most burning issues of the day. The focus of the following considerations is on the importance of air as an elementary part of a recovery process. These considerations are in line with my earlier blogs, which highlight related issues of colour, noise and light (will follow). Air is the element that is least visible, but its design is fundamental. Air and its treatment in air currents are also subject to design, which is particularly important for the healing process. As a place of healing and convalescence, the air quality in hospitals is of an almost elementary importance. In addition to hygiene and safety, precise ventilation and air conditioning contributes to better indoor air quality, thus also to faster patient recovery and demonstrably higher staff productivity.  

We focus on the currently known and scientifically proven correlations between the SARS-CoV-2 transmission rate and bad air in 3 parts as follows:  

  • Part 1 reflects on the relationship between climate change and air quality 
  • Part 2 focuses on air as one of the 9 basics of “healthy buildings”
  • Part 3 examines the relationship between air quality and healing processes in hospitals. 

The statements are based on the research results that have been made available to date, most of which are still scarce, and require ongoing scientific reviews and modifications. They do not claim the status of conclusive findings, but are intended as food for thought-provoking impulse. 


Human Respiration and Carbon Dioxide (CO2)  

The carbon dioxide (CO2) content of indoor air is primarily a decomposition product of human respiration. With its respiration and evaporation, the human being is a major cause of indoor air pollution. The increase in the concentration of carbon dioxide in indoor air correlates with the increase in the odour intensity of human exhalations. 

For 150 years, the concentration of carbon dioxide has been considered an indicator of indoor air quality. Since 1858, the guideline value of 1,000 ppm carbon dioxide proposed by Pettenkofer has been in force.  

The CO2 concentration in occupied indoor spaces depends mainly on the following factors: 

  • Number of people in the interior 
  • Room volume  
  • Activity of indoor users 
  • The length of time users spend indoors 
  • Combustion processes in the interior 
  • Air exchange or fresh air volume flow  

In addition to human respiration and evaporation, however, other factors play a significant role in the quality of indoor air.

Criteria for Healthy Interiors 

While the outdoor climate is influenced by many factors that are beyond individual control, clear criteria are now known for indoor spaces. 

After 40 years of scientific research, the following 9 criteria for healthy buildings were distilled in the Harvard laboratory: 

  • Ventilation 
  • Air quality 
  • Thermal health 
  • Humidity 
  • Dust and pests 
  • Safety and security 
  • Water quality 
  • Noise 
  • Lighting and views

Description automatically generated
The 9 basics for a healthy building (1) 

While some factors such as better acoustics and lighting are evident, other factors such as humidity, temperature, furniture, carpets and even dust have a massive impact on people’s health and performance. (2)  

In a study of young adults, it was found that any deviation of 1°F (0.55°C) from an optimal indoor temperature is already associated with a 2% drop in performance. In another study, researchers found that in four simulated office activities (text input, addition, proofreading and creative thinking), doubling the rate of outdoor air introduced into an office improves employee performance by 1.7%. It is therefore not surprising that an analysis of the sick leave data of more than 3,000 workers in 40 buildings showed that 57% of all sick leave was due to poor ventilation. 

When rooms are designed according to biophilic design principles (which includes the consideration of the above-mentioned criteria for “healthy buildings”), the occupants have shown  lower blood pressure, lower heart rates and better performance in short-term memory tests. (3) 

As part of many building renovations in the recent past, the building envelopes have been better insulated. In combination with better windows, many buildings today are therefore highly airtight. However, as a result, indoor air pollutants are much less likely to escape outside. If the interiors are insufficiently ventilated, the CO2 concentration increases. This is exacerbated by moisture problems and the formation of mould. Similarly, some toxins found in certain materials or furniture can be found in higher concentrations if they are not ventilated sufficiently. 

Indoor Air Quality and Covid-19 

There seems to be a link between poor air quality and severe Covid-19 processes. People living in areas with poor air or high levels of particulate matter are obviously at risk (see part 1 of this blog series). 

Indoor and outdoor climate have a decisive influence on the infectivity of the coronavirus, especially if they differ significantly during the winter months. 

It has been shown that at a relative humidity of between 20–35%, the risk of contracting an influenza-A- virus is about three times as high compared to an average indoor humidity of 50%. The reasons for this include the faster and wider distribution of fine droplets and the longer life of the viruses in excessively dry air. (4) 

The Physics of the Corona Virus 

For a long time, the main route of transmission of the virus was considered to be direct human-to-human contact: when infected people secrete viruses by sneezing or coughing and these viruses are absorbed by other people through the mucous membranes of the nose. Because these drops are relatively large and heavy, they can only cover very short distances in the air and usually sink to the ground quickly. The recommendation to maintain a minimum distance of 1.5m to 2m (social distance) is based on this assumption. 

Recently, however, outbreaks of Covid-19 have also been recorded, apparently due to the simultaneous presence of many people in one room (choir rehearsals, slaughterhouses, etc.). A safety distance of 1.5m seems to be no longer sufficient when infected and healthy people stay together in one room for a longer period of time. For example, Dutch researchers have now been able to prove that tiny drops of 5 micrometres in diameter – such as those produced when speaking – can float in the air for up to 9 minutes. (5) 

In order to limit the spread of these droplets via the aerosols floating in the air, the researchers  recommend not only continuing to wear masks but also, and above all, ensuring good ventilation indoors. 


Mixtures of tiny particles in the air are considered aerosols. They are tiny little droplets that people emit when they breathe out. Aerosols consist mainly of water, which usually evaporates quickly, and a mixture of viruses and other components that do not evaporate as quickly (e.g. saliva). These remain in the air and spread throughout the room – similar to fine dust. Aerosols can remain indoors for hours or days. 

In aerosols, researchers have now found SARS-Cov-2 viruses that can multiply. They see this as the final proof that Covid-19 can be transmitted via the air. 

Contact transmission, droplet infection – and aerosols: Just a few weeks after the corona outbreak in Europe, scientists found out that the virus probably spreads not only by droplet infection and direct contact, but also over long distances through the air. However, the final proof that airborne viruses can infect cells and multiply was still missing. A team of researchers from the University of Florida has now provided this – technically extremely sophisticated – proof. They have confirmed in experiments that aerosols emitted by corona infected people can contain intact virus particles. This is a first confirmation that SARS-CoV-2 can probably also be transmitted via the tiny suspended particles remaining in the air for a long time. Keeping a safety distance of 1.5m or even2m indoors could therefore give a false sense of security, the study notes. (6)

How exactly the coronavirus is spread – whether mainly via droplet infection or rather via aerosols in the air we breathe – has not yet been conclusively clarified. 

“For the corona virus, it appears that both droplet infections and airborne transmission, i.e. via aerosols, are relevant”, says Martin Kriegel, Director of the Hermann Rietschel Institute, TUB Berlin. 

In a droplet infection, the virus particles in a droplet of saliva get directly onto the mucous membranes of another person. In airborne transmission, the viruses – bound in tiny liquid particles – enter the respiratory tract. The size of the carrier aerosols is decisive for the behaviour of viruses in the air, but also the room climate, the air exchange rate and the way in which the room is ventilated. “Larger particles sink to the ground faster. Smaller particles follow the air flow and can remain in the air for a long time. (…) In principle, it can be said that at typical air exchange rates in residential and office buildings, the pathogens remain in the room for hours. The rate of descent and also air renewal take a very long time. Any increase in the fresh air supply is therefore generally sensible.”, states Martin Kriegel. 

The indoor climate therefore also plays a major role in the question of the size and retention time of the SARS-CoV-2 particles, because the aerosols very quickly become smaller through evaporation and then behave differently. In principle, it can be stated that at typical air exchange rates in residential and office buildings, the pathogens remain in the room for hours. The rate of descent and also air renewal take a very long time. Any increase in the fresh air supply is therefore generally sensible (7).  

In the New England Journal of Medicine, the physician Matthew Meselson from Harvard University confirmed the suspicion regarding aerosols. According to some publications on the length of stay and the infection mechanisms of the new coronavirus, it can be assumed “that aerosols from infected persons can pose an inhalation risk”, at least in closed rooms or poorly ventilated environments. (8)

The few studies known to date worldwide agree that aerosols play a far greater role in virus transmission than previously thought. It is therefore recommended that frequent airing is urgently needed in addition to washing hands. In this way, the concentration of aerosols loaded with virus particles can be kept low. This can also be achieved by modern ventilation systems. However, it is essential that the air is not only circulated, but that 100% of it is supplied from outside. Otherwise, air conditioning systems could exacerbate the problem by spreading existing viruses throughout the room. 

A picture containing indoor, appliance, sitting, kitchen

Description automatically generated
Conventional air conditioning systems simply circulate the available air without supplying 100% fresh air. (Photo: SRF Kassensturz)

“A circulating air air-conditioning unit that does not clean the air but rather dries it out and cools the air – these are all factors that promote aerosol transfer”, states Walter Zingg, specialist in infection prevention and hospital hygiene, Hôpitaux universitaires de Genève HUG, member of the scientific task force of the Federal Council Switzerland (9).  

Air Humidity 

Another aspect that has received little attention so far and could become particularly important in the next flu season is indoor humidity. 

According to a research team from Leipzig and India, the spread of the coronavirus is also influenced by humidity. In dry rooms, it is therefore easier for the virus to spread. 

“If the relative humidity of indoor air is below 40%, the particles emitted by infected persons absorb less water, remain lighter, fly further through the room and are more likely to be inhaled by healthy people”, explained Ajit Ahlawat from the Institute for Tropospheric Research (Tropos) in Leipzig. (10)

The new findings are particularly important for the coming winter season, when millions of people in the northern hemisphere will be staying in heated rooms. Heating the fresh air also ensures that it dries out. In cold and temperate climate zones, the indoor climate is therefore usually very dry during the heating season. This could promote the spread of coronaviruses. 


  1. The element air is one of the 9 basics of “healthy buildings”. 
  2. If SARS-CoV-2 is transmitted via air, it is sufficiently likely that exposure to the virus should be controlled via air. Changes in the operation of buildings, including the operation of heating, ventilation and air conditioning systems, may reduce exposure via air.    
  3. Healthcare facilities should therefore immediately check the actual air supply and exhaust air flows currently set in the ventilation and air conditioning systems and the actual levels of exposure to the virus to ensure that at least standard air volumes are available.   
  4. Recirculation cooling units without fresh air supply (including fans) are problematic because they distribute and circulate the existing room air (and thus potentially viruses) throughout the entire room and do not supply fresh air. It is therefore urgently recommended to switch ventilation units with circulating air to 100% fresh air. 


(1) Healthy Buildings


(3) Allen J.G., Macomber J.D., Healthy Buildings, How Indoor Space Drive Performance and Productivity, Harvard University Press 2020, (11)ISBN 9780674237971

(4) Source: Lowen, A. et al.: PLoS Pathogens (2007) 


(6) Lednicky J.A., et al. 2020 Viable SARS-CoV-2 in the air of a hospital room with COVID-19 patients 

(7) Dauerlüften, Luftfilter – wie umgehen mit den Aerosolen in Innenräumen?



(10) Ahlawat, A., Wiedensohler, A. and Mishra, S.K. (2020). An Overview on the Role of Relative Humidity in Airborne Transmission of SARS-CoV-2 in Indoor Environments. Aerosol Air Qual. Res. (in press). DOI: 10.4209/aaqr.2020.06.0302 


OpenEdition suggests that you cite this post as follows:
Jean Odermatt (October 19, 2020). Thick Air (Part 2) – Indoor Climate and SARS-CoV-2. Learning from Corona. Retrieved July 12, 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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2 Responses

  1. 19/10/2020

    […] Part 2 focuses on air as one of the 9 basics of “healthy buildings” […]

  2. 21/10/2020

    […] Part 2 focuses on air as one of the 9 basics of “healthy buildings” […]

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