The coronavirus pandemic has worked to focus public attention on the risks posed by viral infections in buildings. Contributory factors that have been discussed for some time have now been placed centre stage, emphasising the influence that fresh air, temperature and humidity, but also light and materials, can have on the spread of viruses. Indoor air quality in particular is of prime importance for protecting the health of building users. In recent years, an ever-increasing body of scientific evidence has been documenting its impact on the immune system and the spread of respiratory infections. Viral respiratory infections are transmitted almost exclusively from person to person indoors. The most common route of infection is airborne transmission at close range via droplets and at longer range via aerosols: viral particles from an infected individual are breathed in by another person, and these then enter mucous membranes in the upper respiratory tract. Depending on the particle size, this is called droplet transmission or aerosol transmission. Because of their small size, aerosols are extremely light. Viral aerosol particles can spread through the air in large spaces over a considerable period of time. Relevant factors for their spread are the movement and humidity of the air, which have a direct impact on the range of aerosols, their ability to remain suspended in the air and their infectiousness.

Viral respiratory infections are transmitted almost exclusively from person to person indoors. The most common route of infection is airborne transmission at close range via droplets and at longer range via aerosols: viral particles from an infected individual are breathed in by another person, and these then enter mucous membranes in the upper respiratory tract. Depending on the particle size, this is called droplet transmission or aerosol transmission. Because of their small size, aerosols are extremely light. Viral aerosol particles can spread through the air in large spaces over a considerable period of time. Relevant factors for their spread are the movement and humidity of the air, which have a direct impact on the range of aerosols, their ability to remain suspended in the air and their infectiousness.

Getting as much fresh air into the room as possible is the most effective method for removing viral aerosol particles from indoor spaces. As the proportion of fresh air rises, this increasingly dilutes the viral aerosol particles in room air. Air conditioners can move the required volumes of fresh air into the room and stale air out of the room in a controlled manner. The air exchange rate is an important parameter here: the higher the air exchange rate, the lower the risk of infection. The ideal air exchange rate depends on building usage and the number of people present indoors. It should be noted that higher rates of exchange can lead to an increase in energy consumption and a lowering of relative humidity levels. Air quality is considered to be good when the concentration of CO2 is less than 1,000 ppm (parts per million).

The airborne transmission and viability of viruses is also significantly influenced by the relative humidity. The lowest risk of transmission is at a minimum humidity of 40 to 60%. Because the mucous membranes are self-cleaning, this is also the range in which the human immune response is most effective. Aerosols essentially consist of water, salts and proteins. At a relative humidity of under 40%, aerosols lose their water content and dry out. This produces dry aerosols, which are smaller and lighter, and which can remain airborne in the room for longer. Air flows and the movements of people in the room also mean that dry aerosols are raised from surfaces again more quickly, and can therefore spread further. Apart from its effect on suspended particles, humidity is also hugely important in terms of the infectiousness of these pathogen-rich droplets. At less than 40% relative humidity, aerosols dry out to the extent that the salts they contain crystallise out. This works to protect the viruses and they remain infectious for longer. When breathed in, the crystallised salts then dissolve again in the moist environment of the respiratory tract. The viral particles, which are 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, aerosols only dry out to an extent where salt concentrations rapidly increase but salts do not crystallise out, and the viruses contained in these aerosols do not survive.

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 pathogenic microorganisms can remain viable. Natural UV-A and UV-B light is absent from our buildings, since window glass absorbs and reflects up to 100% of UV radiation. However, LED lighting that can reproduce UV-A and UV-B can be used to simulate full-spectrum sunlight within a building. This would reduce the propagation of pathogens while also boosting our immune system.

Alongside technical systems, the ability of a building to protect against infectious diseases also depends on its usage and facilities. In recent years, many buildings have tended to adopt the kinds of layouts that emphasise openness and transparency. However, these positive moves to facilitate teamwork 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. 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, since dust remains trapped in fibres and cannot be swept up again into the air. Organic fabrics also store water while helping to reduce noise levels in the room. 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.

Healthier buildings that protect against respiratory infections are the result of many factors, some of which work in synergy with one another. For specific buildings, some approaches will also be unsuitable or technically unfeasible. For company managers, facility managers, employees and occupational health and safety officers, it is important to seek dialogue now, with a view to putting together the right package of health protection measures for existing buildings effectively, sustainability and in good time.

A systematic risk assessment consists of seven steps. First, the work areas are defined. These can be individual workplaces or departments, such as call centres or customer service. The potential health hazards and exposures are then summarised and subsequently assessed. Allowance should be made here for how likely a hazard is and what consequences result from it. In the case of humidity, measurements over an extended period of time are essential. In the next step, the TOP principle is used to define measures to improve health protection – primarily through technical solutions. The most important element of the risk assessment is the written documentation and follow-up: once it has been implemented and its impact verified, the process is continuously updated.