Making buildings healthier

Protection against the spread of infectious aerosols.


Condair Group, a world leader in the humidification and dehumidification sector, has released a new whitepaper entitled “Making Buildings Healthier”. It contains information on how building managers can protect occupant health with a holistic approach to controlling their indoor environments. The purpose of this document is to provide an overview of these factors and to promote dialogue between building managers, users and health and safety officers, enabling the appropriate package of health protection measures to be considered.


A good air ventilation system, maintaining a correct level of humidity, filtering the inside air, proper lighting and the correct choice of construction materials can help us protect ourselves against the transmission of infectious diseases.



The SARS-CoV-2 pandemic has focused public attention on the risks of transmitting viral infections inside buildings. The contributing factors that have been known for some time have now been placed in the spotlight, emphasizing the influence that elements such as fresh air, temperature, minimum relative humidity and even sunlight have on the spread of viruses.


People are the center of attention – How buildings can help us protect our health


Buildings were originally constructed to protect us from a hostile environment. However, thanks to the relentless search for energy-saving methods and operational efficiency, plus the use of high-tech artificial materials, buildings can now make us sick. Perspectives from the Covid-19 pandemic have shown how vulnerable we are when we inhale polluted indoor air.

In recent years, advances in construction technology have allowed buildings to become more and more energy efficient, but also more airtight. From optimizing the space used and the density of the number of residents, to high-tech insulation and air conditioning systems that achieve outstanding energy results and help optimize costs, everything has been carefully calculated. However, the consequences of these modern trends on the health of people who spend most of their time inside buildings are rarely taken into account.


The air quality inside the buildings is largely unregulated.


The indoor air quality is paramount to protecting people’s health. In recent years, an increasing number of scientific documents have proven the impact that the air inside buildings has on immunity and the spread of respiratory infections. A healthier indoor climate in offices, schools, hospitals and care centers, for example, would simultaneously be extremely beneficial for business, health services and the national economy. However, there is still a lack of comprehensive standards for the factors that influence indoor air quality.


Lessons learned from Covid-19.


The SARS-CoV-2 pandemic has focused public attention on the risks of viral transmission of viruses and respiratory infections inside buildings. The contributing factors that have been known for some time have now been placed in the spotlight, emphasizing the influence that fresh air, temperature, minimum relative humidity and even light have on the spread of viruses. New technologies, such as UV-C irradiation, have also been proposed, although this technology is still being studied.

Healthier buildings are the result of several factors, for some buildings these approaches may be inapplicable or technologically unfeasible. The purpose of this document is to promote dialogue between building managers, users and health and safety officers, allowing the appropriate package of health protection measures to be considered.




“Healthy buildings – healthier people”


Respiratory infections cause huge losses of productivity and generate considerable medical costs that must be borne by both businesses and society. The catastrophic consequences for the economy were demonstrated during the quarantine period due to the coronavirus. Influenza infections alone are responsible for more than 500 million cases of disease worldwide each year. People are especially at risk in buildings where many people are present indoors or when they suffer from pre-existing poor health conditions.



1) Wyon, D.P. ‘The Effects of Indoor Air Quality on Performance and Productivity.’ IndoorAir, U.S. National Library of Medicine, 2004
(2) Statista Research Department, Germany, 2020
(3) Haverinen Shaughnessy et al., ‘Association between substandard classroom ventilation rates and students’ academic achievement’, 2011
(4) Ritzel, G. ‘Sozialmedizinische Erhebungen zur Pathogenese und Prophylaxe von Erkältungskrankheiten’, 1966
(5) ‘Report on the burden of endemic health care associated infection worldwide’, World Health Organisation, 2011



Making buildings healthier:

1 Reimagining buildings
2 Factors for healthier buildings
3 Open-plan office with lots of employees
4 Risk group: Older people and those with pre-existing conditions


TRANSMISSION VECTORS – Direct contact – Droplets –  Airborne aerosols


Enclosed spaces are infectious


Viral respiratory infections are transmitted almost exclusively from person to person indoors. In the industrialized world, people interact, work, sleep, and travel indoors for nine-tenths of their lives. The routes of transmission in these spaces are by direct contact, indirect contact, airborne particles and aerosols.

Directly transmitted viruses spread through direct contact with the skin or mucous membranes. If someone sneezes in the hand, for example, the viruses will stick to the surface of the hand. If this person shakes hands with other people, the virus can enter the mucous membranes through the mouth, nose or eyes. Indirect transmission of the virus involves the transfer of pathogens deposited on an inanimate object, such as door handles or other surfaces that are touched by several different people.


Droplet transmission


The most common types of infection are: short distance transmission by particles and longer distance airborne transmission by aerosols. Viral particles from an infected individual are inhaled by another person, then enter the mucous membranes of the upper respiratory tract. This phenomenon is called particle or aerosol mediated transmission, depending on the size of the particles. During breathing, coughing, speaking or sneezing, infectious viruses present in a person’s respiratory system can be emitted in drops of saliva and mucus. These drops come in different sizes and quantities. In medical terms, a “drop” is a particle with a diameter greater than 5 μm (micrometers). These larger particles remain in the air only for a short time: after only a few seconds, these particles then fall to the floor or other surfaces. The particles are transmitted only up to a distance of about 1.5 to 2 m. However, they can also be spread by contact with contaminated surfaces – such as when these surfaces are touched and viruses then come in contact with the mucous membranes by touching your face with your hands.


Airborne transmission of aerosols


Particles with a diameter of less than 5 μm can travel long distances through the air before infecting humans. This route is considered airborne transmission of aerosols. Some of these aerosols may actually contain very little liquid and may be mostly solid matter. Due to their low mass, these aerosols have the potential to evade the influence of gravitational force and remain in the indoor air for several hours. Even if the indoor air remains relatively still, small infectious aerosols can spread through the air in large spaces over a long period of time.







Airborne transmission: The spreading of viruses by airborne aerosols is crucially dependent on the indoor climate in buildings. Air changes per hour, temperature and relative humidity are relevant factors for reducing the risk of infection.







Personal precautions


To protect against viral spread indoors, there are a number of precautions that can be taken, depending on the transmission vector. For contact and large droplet transmission, precautions such as good hand hygiene, sneezing into the crook of one’s elbow, observing distancing, and wearing a mask covering the nose and mouth are all very effective means of reducing the risk of infection. These precautions are ineffective, however, for airborne transmission of tiny areosols.


Indoor climate as a factor


Beyond precautions involving proper hand and surface hygiene, the spread of aerosols inside buildings requires the identification of indoor climate factors that can be controlled to mitigate the risk of transmission. Relevant factors are those that directly affect the capabilities of viral aerosols to shrink, remain infectious and spread through the airborne route. Optimised ventilation with plenty of fresh air reduces the risk of SARS-CoV-2 infection by diluting and removing infectious viral aerosols, for example, while excessively low levels of humidity mean that viruses may stay viable and travel farther in small aerosols.


Transmission pathways:

1 Viruses in indoor spaces
2 Person to person
3 Droplet and airborne
4 Pathways in the workplace
5 Schools and day nurseries
6 Particle travel between surfaces and air




Tackling infectious aerosols with fresh air


To stay healthy, we know we need to spend as much time outside as possible and breathe fresh air whenever we can. The same principle applies indoors: the more fresh air inside, the lower the concentration of viral particles. Filters and proper ventilation are also important for removing infectious particles and contaminated air. Bringing as much fresh air into the room as possible is an effective method for removing viral aerosol particles from indoor spaces. As the proportion of fresh air rises, the viral aerosol particles in room air is increasingly diluted.


Ventilation from windows


The simplest option is to open a window. The volume of air entering through an open window depends on the temperature level, wind speed / direction and the angle at which the window is opened. The general recommendation is short but ample ventilation, by windows being fully opened for several minutes at least every hour. This air exchange will be most effective when two windows opposite each other are opened at the same time. However, there are limitations to the efficiency of using windows for ventilation. In summer, the temperature difference between the outside and the inside air is often too low and the air exchange is minimal. In winter, heat loss and sharp drops in relative humidity are arguments against the constant use of open windows.


Mechanical ventilation


Ventilation and air conditioning systems can regulate in a controlled way the required volume of fresh air used inside and outside the room. The air recirculation rate is an important parameter: an ‘air recirculation per hour’, for example, means that the volume of fresh air introduced per hour is the same as the volume of the room. As air recirculations increase, the risk of infection decreases. The ideal air recirculation rate depends on the use of the building and the number of people inside. It should be noted that higher recirculation rates can lead to an increase in energy consumption and a decrease in relative humidity levels. Checking CO2 levels (the concentration of carbon dioxide in the air) is a practical way to determine whether an occupied room is well ventilated or not. Air quality is considered good when the CO2 concentration is less than 1,000 ppm (parts per million).




Specialized filters can also remove even the smallest aerosols from the air. The use of filters is especially recommended for ventilation and air conditioning systems in which air is frequently recirculated. Different classes of filters are available, which are effective at filtering different particle sizes. High quality MERV filters (class 13 or better) and HEPA filters are designed to hold more than 99% of particles up to 0.3 μm in diameter (micrometers). Their efficiency is limited for smaller particles.





Air that is too dry and too hot is not healthy



Professor Akiko Iwasaki

Professor of molecular, cell and developmental biology at Yale University, and research scientist at the Howard Hughes Medical Institute (USA).


“A low level of humidity is one of the reasons for the seasonal occurrence of flu outbreaks. The world would be a healthier place if the humidity of the air in all of our public buildings were to be kept at around 40 to 60% relative humidity.”




The fact that waves of colds and influenza infections occur during the colder months in particular is largely dependent on a number of seasonal factors that affect the indoor climate. These are related to the air temperature, as well as a drop in relative humidity. Even in summer, however, the air-conditioning units used for cooling can cause the air circulating in these interior spaces to dry out – making life much easier for viral aerosols.


Relative humidity makes the difference


If a building were to be hermetically sealed off from the outside world, the absolute humidity inside would be constant and unchanging. However, relative humidity is the key factor for properly assessing the humidity situation. Relative humidity describes the saturation percentage of air with water vapour and is affected by the air temperature. Warm air can hold a greater amount of water than cold air. Air will always attempt to absorb water in the form of water vapour until it reaches maximum saturation. This is why relative humidity falls when air is heated, although the absolute humidity remains the same


Buildings in winter


When the indoor air is heated and the windows are then opened or fresh air is introduced into the building by a mechanical system, this air will begin to dry. The colder the outside air, the lower its water absorption capacity – and the drier it becomes. If this cold, dry outside air is introduced into the building, the relative humidity will drop rapidly as this air is continuously heated. The air then tries to restore balance: if no humidification systems are installed, the air will try to become saturated by extracting moisture from any materials, structures and human bodies present.


Proper ventilation and heating


Before installing a humidification system, it is important to check air recirculation rates and temperatures. The proportion of fresh air inside should be reduced to the minimum necessary – especially in winter. Windows that are permanently open and with excessive high air exchange rates should be avoided to prevent air drying. Also, indoor spaces should not be overheated: an ideal temperature here is between 20 and 22 ° C.





Viruses prefer dry air.


Airborne transmission and virus survival are also significantly influenced by the relative humidity of the indoor air (-1). The lowest transmission risk is achieved with a relative humidity of 40% to 60%. At the same time, this is the humidity level in which the response of our immune system is most effective. Relative humidity decisively affects the ability of the viral aerosols to remain suspended in the indoor air. Unlike larger and heavier infectious particles produced by coughing or sneezing, which fall to the ground after a few seconds, lighter and smaller aerosols can remain suspended in the air for hours.


Dry aerosols remain in the air longer


Aerosols are made up of water, dissolved salts and proteins. At a relative humidity below 40%, aerosols cannot retain this water and therefore dry out. This phenomenon therefore produces dry aerosols, which are smaller and lighter and can float in the indoor air for a longer time. Unlike larger particles, their lower water content also makes them less contagious and so they cannot fuse together so easily. The airflows and movements of the people in the room also mean that dry aerosols are lifted faster from the surfaces and can therefore continue to spread (-2).


Viruses survive longer in the absence of an adequate level of humidity


Apart from its effect on suspended particles, humidity is also hugely important for the contagiousness of these pathogen rich droplets. At less than 40% relative humidity, aerosols dry out so much that the salts they contain crystallise. These salts protect the viruses and they remain infectious for longer. When breathed in, the crystallised salts dissolve once more in the moist environment of the respiratory tract. The viral particles, still infectious, are released onto the mucous membranes, where they can trigger an infection. If relative humidity is within the optimum range of 40 to 60%, however, particles only dry out to an extent where salt concentrations rapidly inactivate viruses rather than protecting them.


1 Miyu Moriyama, Walter J. Hugentobler, Akiko Iwasaki: ‘Seasonality of Respiratory Viral Infections, Annual Review of Virology’ (2020)
2 W. Yang et al, ‘Dynamics of Airborne Influenza A Viruses Indoors and Dependence on Humidity’, PLoS ONE, Issue 6 (2011)



Use additional humidification

With a humidification system, any building can now keep relative humidity within the safe range of 40 to 60%, using an approach that is both hygienic and energy-efficient. Depending on building conditions and requirements, centralised systems can be installed in the ventilation and air-conditioning system or local, direct room humidification systems can be used.


Mucous membranes: our first line of defense


We humans are not entirely defenceless in the face of attacks from viruses and bacteria: the response mounted by our immune system will decide whether or not we become ill and – if we do – the speed of our recovery. We are protected from infection by the self-cleaning mechanisms used by the mucous membranes in our airways. The surfaces of these mucous membranes are covered by fine motile hairs (cilia), which move freely within a fluid secretion (saline layer). Covering this is a sticky gel layer, on which most of the viral, bacterial and pollutant particles breathed in remain stuck. As long as the cilia remain highly motile, they can transport the secretions together with these microorganisms towards the larynx, where this secretions can then be swallowed or coughed out.


A weakened immune system


As the relative humidity decreases, the pathogen removal system becomes less efficient (3). At lower levels of relative humidity, the saline layer begins to dry out. This results in the collapse of the cilia, which therefore lose their motility. The increased viscosity of the mucous membrane blocks the flow of mucus and the risk of infection increases due to viruses that invade the cells of the mucous membrane. Once the relative humidity has dropped to 20%, this self-cleaning process stops completely. Experiments have shown that the fastest transport rate of the pathogen – and therefore the lowest risk of infection – is at 45% relative humidity.


Damage to mucous membranes


When the air is too dry, two other mechanisms also have a direct impact on the immune system and hinder the effectiveness of our adaptive immune response. Epithelial cells form a physical barrier beneath the layer of the mucous membrane, which prevents viruses from entering host cells. Breathing very dry air affects these cells and therefore affects the regeneration processes used by the epithelia of the respiratory tract (lung cells). Secondly, low relative humidity can also reduce the formation of interferon in lung tissue. Interferons trigger the production of proteins that fight invasive viruses and thus prevent them from multiplying (3).






Without a sound set of data, it is difficult to decide which particular parameters need to be changed to achieve a healthier indoor climate. Buildings are made healthier and more productive by systems that consistently collect data on the relevant air quality parameters and suggest actions to take. Sensor systems and monitoring solutions can be integrated into any building with very little effort. Excessive CO2 levels, excessive heating, very low humidity levels and air pollution with fine particles and volatile organic compounds are health hazards that also lead to reduced productivity. Without proper measurements, the underlying causes of “sick building syndrome”, days lost due to illness, or factors that lead to the rapid spread of respiratory infections are difficult to identify.


Prevention with the help of building automation


To ensure continuous monitoring and quantification of air quality, sensors and monitoring systems can now be easily retrofitted in any building. The systems are implemented either as an integrated part of building automation or as a simpler, independent solution. Usually, the relevant parameters – such as temperature, CO2 concentration, humidity and VOC levels – are measured using a multifunctional sensor system contained in just a single unit.. When coupled with motion sensors, fully integrated systems can even detect the number of people using a particular enclosure. Fresh air, temperature and humidity are automatically adjusted to the optimal parameters before the indoor air begins to become a danger to the health of people present.


Certified air quality


Proof of managed indoor air quality by continuous sensor measurement is an important requirement for many types of building certification programs. The leading standards for building sustainability and health are the US LEED program, the UK BREEAM assessment method, the German DGNB and the international WELL certificate. The WELL Building Standard is the first assessment system that concentrates on one single objective, namely: designing buildings and interior spaces to ensure that they have a positive influence on the health and well-being of their users. Fulfilling the requirements for monitoring as set out by these standards usually requires collecting statistics on ventilation performance and the resulting improvements to indoor air quality that this achieves. These standards also stipulate various exposure limit values and benchmarks in terms of air exchange rates, concentrations of particulate matter and ozone, VOC emissions and relative humidity.






Natural light is healthy


Maximizing sunlight makes people healthier. One reason for this is the formation of vitamin D in response to sunlight exposure. Daylight is a free resource that can be actively applied in buildings to protect the health of employees while increasing productivity. However, the important components of UV-A and UV-B sunlight are blocked by glass windows.  Our bodies trigger the production of healthy levels of vitamin D in response to UV-B rays from the sun. Investigations have shown that the higher the levels of vitamin D in blood, the lower the likelihood of contracting a respiratory infection. Each incremental increase of just 10 nmol/l (nanomoles) reduces the risk of illness by 7% 1. The lack of sunlight and the fact we spend most of our time shut away in buildings contribute to the seasonal occurrence of respiratory infections in autumn and winter.


Letting the sun into the building


Sunlight also plays an important role as an active line of defence against viral infections. The UV component of sunlight stimulates the body’s immune system on the one hand, while also enhancing the formation and mobility of the natural killer cells that tackle viruses and bacteria. Sunlight also reduces the period during which many pathogenic microorganisms can remain viable. Investigations conducted on flu viruses show that the time taken for half of the viral particles to become inactive drops rapidly in sunlight from around 32 minutes to under 3 minutes. Natural UV-A and UV-B light is absent from our buildings, since window glass (and thermally insulating glass in particular) absorbs and reflects up to 100% of UV radiation. UV-LED lighting, which can reproduce both UV-A and UV-B light, makes it possible to simulate full-spectrum sunlight within a building. This would reduce the propagation of pathogens while also boosting our immune system.


A biological boost


Light is a stimulus that controls our hormones, which in turn regulate our biological clock and ultimately decide how productive, attentive and focused we are during the day. In addition to natural light, dynamically controlled lighting systems can adjust color temperature and lighting intensity to people’s needs, thus ensuring that light has a stimulating or relaxing effect.






Microbes belong in buildings


Surprisingly, over-sanitizing things is detrimental to our immune system. Our buildings must allow interaction with the good microbes present in our environment. Choosing the right materials is important to suppress the microbes that make us sick, while increasing the health of building users by exposing them to healthy microbes. Our immune system continuously interacts with its environment and can distinguish between harmless and harmful microbes. Harmless microbes – “old friends” that have accompanied humans for millennia – support the body’s immune response and limit the spread of pathogenic microorganisms. However, in many buildings, this multitude of good microbes is increasingly absent, resulting in a higher incidence of infectious diseases and allergies.


Stress caused by dry air


Demands for better energy efficiency has brought one high-tech material after another into our buildings and also resulted in a rise in average temperatures. To produce an airtight building shell, insulation and interior fit-outs increasingly use steel, glass and various plastics, all of which have an impact on moisture levels. Unlike natural building materials such as tiles, plaster, clay or wood, industrial synthetic materials are smooth and non-porous, and are unable to absorb water or nutrients. In particular, our beneficial ‘old friends’ microbes cannot survive in the dry and nutrient free environment created by these industrial materials. With no competitors for water and nutrients, pathogenic, multi-resistant microorganisms can propagate unopposed. As microbes are subjected to greater levels of stress and the diversity of these microorganisms is reduced, resistance to substances such as antibiotics can develop more easily.


A healthy mix of materials


Buildings need to be understood as living ecosystems, able to achieve a balanced diversity of microorganisms. To ensure this, porous-free, smooth synthetic materials should be used sparingly and only for surfaces that are frequently touched and therefore need to be routinely cleaned – such as handrails, door handles, taps and keyboards. For walls, ceilings and furniture, natural materials with porous surfaces are preferred, offering an amenable environment for diverse communities of microbes. On these natural surfaces, water and nutrients are in plentiful supply for bacteria and viruses. With our ‘old friends’ in the majority, they suppress pathogenic microorganisms. With the exception of hospitals, surfaces should be cleaned with detergents and chemicals only in exceptional circumstances: soap and water is perfectly adequate.





There are several ways to achieve a healthier environment


Alongside technical systems and structural materials, the ability of a building to protect against infectious diseases also depends on its usage and facilities. The ways in which space is partitioned in buildings affects the intensity of person-to-person contact and therefore the spread of microbes. Floor coverings and plants can also have a positive effect on air quality. The specific use of a building will influence its floorplan and layout. One crucial factor here is the degree of social interaction required between building users. Office buildings will have other requirements than public buildings with high levels of footfall or facilities such as schools and day nurseries. Whatever the type of building, the risk of transmission for pathogenic organisms increases when many people share the same space.


Spatial arrangement


The number of interconnected rooms, doorways and hallways influences communication and movements within a particular building. In recent years, many buildings have tended to adopt the kinds of layouts that emphasise openness, transparency and spaciousness. However, these positive moves to facilitate teamwork and personal interaction also have the effect of increasing the risk of transmission: spacious rooms with large numbers of people are proven to encourage the diversity and occurrence of microbes. The spread of pathogens can be contained, however, by reducing the number of high-occupancy rooms, and ensuring a mix between open and closed spaces.


Choosing a floor covering


The choice of floor covering can also influence indoor air quality. Unlike hard floors, coverings like rugs and carpets reduce levels of fine particulates within a room. Textile floor coverings trap dust particles in their fibres and prevent resuspension into the air. Organic fabrics also store water molecules while helping to reduce noise levels in the room.


Green is clean


Plants filter impurities out of the air and boost microbe diversity while producing oxygen. Under the influence of light, photosynthesis removes carbon dioxide from the air: the plant retains the carbon and the oxygen is released into the room. Plants are also capable of releasing up to 90% of water they are given into the air, which means they are also moderate contributors to humidity.





Facility managers and occupants can use this checklist to take note of the current situation and find out to what extent their building protects against the spread of infections and where improvements can be made. The checklist aims to promote dialogue among stakeholders, to identify the need for outside consulting services and surveys.


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