The direct and indirect impact of relative humidity on indoor health has been scientifically recognised and proven by studies conducted over the past few decades. In this chapter, you will find a selection of two relevant scientific papers.

The summary of the key findings provides a clear overview for non-medical professionals and offers a sound basis for justifying the benefits of a minimum humidity level to protect building occupants and for implementing this in new building design or renovation projects.

The effect of relative humidity on respiratory health is divided into direct and indirect influences: the direct influence affects the body’s defence mechanisms, which prevent pathogenic particles from entering the respiratory tract and multiplying there. These include the filtering function of the nose, the self-cleansing processes of the respiratory tract mucosa, immune responses, as well as other defence reactions and degradation processes. In the literature sources evaluated, it is generally accepted as a ‘ ’ that low relative humidity leads to an impairment of the body’s own cleansing function of the mucous membranes and the body’s defence mechanisms. The most frequently cited optimum relative humidity for immune defence lies between 40 and 60 per cent.

Figure: Transport mechanisms of inhaled particles in the respiratory tract. (Source: RWTH Aachen)

Indirectly, relative humidity also affects the survival time of pathogens and the suspension time of germ-laden droplets. In general, the ambient climate influences the risk of infection: Temperature and humidity affect the properties of aerosols. Viruses are transported in aerosols, which consist of small liquid droplets, dissolved salts and proteins. Under dry conditions, aerosols shrink more quickly, become lighter and remain suspended in the air for longer. In addition, temperature and humidity affect the stability and survival time of viruses. However, depending on the virus type, the influence of relative humidity is analysed differently in the literature. For example, influenza viruses are inactivated to the greatest extent at moderate relative humidity, whilst coronaviruses are inactivated to the greatest extent at moderate to high relative humidity. High relative humidity generally reduces the suspension time of particles and aerosols in the air; however, it should be avoided due to the risk of microbial growth and mould formation.

Figure: Influence of the ambient climate on the suspension time of aerosols and the survival time of viruses. (Source: RWTH Aachen)

Particles present in the air can cause eye irritation and conjunctival infections. An intact tear film protects the surfaces of the eyes from environmental influences and prevents eye problems and discomfort. Analyses of scientific studies show that at low humidity levels, tear film production decreases whilst the blink rate increases significantly. Compared to an ideal humidity range of 50–80 per cent, the cell density of the conjunctival goblet cells, where the tear film is formed, decreases sharply at low relative humidity levels. Literature reviews also reveal statistically significant correlations between dry air and skin impairments. Dry indoor air can significantly impair the skin’s protective function. Particles can penetrate a brittle and cracked epidermis, causing inflammation and dermatoses. Chronic skin conditions can worsen. Significant skin impairments are noted in the literature for relative humidity levels below 50 per cent.

Figure: The red markings indicate the range of high viral stability at different relative humidity levels. (Source: RWTH Aachen)

Based on its literature review, RWTH Aachen University concludes that maintaining a moderate level of relative humidity can significantly reduce health problems affecting the respiratory tract, eyes and skin, as well as the associated absenteeism. According to the researchers, indoor relative humidity should therefore ideally be maintained between 40 and 60 per cent. At the same time, the scientists point out that relative humidity must be considered individually for each of the criteria examined. This applies in particular to the inactivation of various virus types (see Fig. 3). The recommendation of a specific target range can therefore only ever represent a compromise.

Overall, however, the compromise of a moderate range of relative humidity results in the fewest adverse effects on humans and their health.

Video: Interview with Dr Kai Rewitz on the findings of the RWTH Aachen literature review.

The research was carried out at the Centre for Virtual Engineering (ZVE) of Fraunhofer IAO in Stuttgart and was further supported by an online survey conducted in collaboration with the Facility Management degree programme at HTW Berlin.

For the study, three DRAABE NanoFog Evolution direct room humidifiers were installed in a section of the ZVE, ensuring a constant minimum relative humidity of around 40 per cent. The survey of office users took place over several months, during which the humidifiers were switched on and off periodically. The survey results were compared with other areas of the building where no additional humidification was in use.

Image: Condair direct room humidifiers in use at Fraunhofer IAO

When asked about their perception of humidity, there are significant differences in how people rate the various scenarios: when humidification is active, the humidity is never perceived as being too low. If humidification is not active or is not present, over 40% of office users are bothered by the air being too dry. On average, the difference between humidified and non-humidified office spaces was around 16% relative humidity. Increasing the humidity results in a positive feeling among office users and greater satisfaction with the working environment. Over 50% of respondents also describe the indoor climate as very refreshing.

Figure: Too low a level of humidity is perceived as disruptive (Source: Fraunhofer IAO)

The mucous membranes of the respiratory tract perform an important protective and self-cleansing function. The viscosity of the mucous membranes plays a key role in determining how quickly pathogens are cleared from the body. The faster this happens, the lower the risk of illness.

Clinical studies show that a sufficiently high level of humidity of at least 30% is necessary for the airways to clear themselves quickly. The results of the study confirm the impact of dry air on the mucous membranes: 54% of employees in offices without air humidification agree with the statement that they frequently experience dry airways at work. In workplaces with air humidification, complaints of dry airways are reduced by over a third to 35%.

Figure: Dry airways (Source: Fraunhofer IAO)

Particularly for professions that require a great deal of speaking, the constant use of the voice carries a high risk of developing voice disorders. Generally, a minimum relative humidity of 40 per cent is recommended for professions involving a high volume of speaking.

The Fraunhofer study also confirms a clear trend here: voice problems can increase when relative humidity is low: In working environments without air humidification, just under a third (29 per cent) frequently suffer from voice problems. If, on the other hand, the air is continuously humidified, only 18 per cent report frequent voice problems.

Figure: Voice problems (Source: Fraunhofer IAO)