**1. Introduction**

People spend about 90% of their time indoors [1]. A large proportion of this time is spent in offices. In Vienna, the share of office workers in 2001 was 28.6% of all employees, and the trend is rising [2]. Moreover, in Germany, a rise in office working places can be observed, as a study shows: In 2020, 71% of all employees in Germany worked at least partly in an office, which means 32 million people, whereas in 2015, it was only about 52% (22.5 million) [3]. Austrian law assumes a normal working time of 8 h per day or 40 h per week [4]. For occupations that are mainly performed in offices, this thus accounts for a share of around 24% of the total weekly time. Due to this amount of time spent indoors, indoor air quality is also increasingly becoming the focus of numerous studies. In many cases, the quality of indoor air is rated as insufficient [5–8]. In addition to the detection of pollutants in indoor air, the temperature and climatic conditions are also the focus of investigations. Temperature and climatic conditions are perceived as the biggest disturbances in office work environment, directly followed by noise pollution [9,10]. Air humidity especially plays a very important role. The occurrence of the following health effects in working spaces is associated with too low humidity: Drying of mucous membranes, colds, eye complaints, skin complaints, and electrostatic charging and discharging [11]. Several studies have shown that the perceived indoor air quality is enhanced by indoor air pollutants, the protective mucous layer in the respiratory tract, and tear films. This results in complaints and diseases of the respiratory tract and eyes [5].

The question of the development of diseases, depending on the relative humidity, was already raised in the 1960s [12]. In this context, a connection between the survival of pathogens and relative humidity was established. Diseases or irritations of the skin, eyes, and upper respiratory tract are often associated with low relative humidity indoors during the cold season [5,13]. Dry, cold respiratory air favors infections of the upper respiratory tract such as colds and throat infections in particular [14,15]. This is probably due to a higher stability of virus particles at low humidity and low temperatures. This has already been shown for rhinoviruses [16], influenza A viruses [17], and numerous other viruses [16], which are typical pathogens of the common cold. Due to the increased stability, the transmission of these viruses is particularly favored. Studies have already been conducted to examine the effects of prolonged exposure to low humidity on perceived indoor air quality, sensory irritation symptoms in the eyes and respiratory tract, work performance, sleep quality, virus survival, and voice disorders. Results showed that an improvement in indoor humidity can have a positive effect on perceived indoor air quality, eye symptoms, and possibly work performance in the office environment [5,10,18,19]. However, effects on increased diseases are not only attributed to the higher stability of the viruses depending on the humidity, but are also caused by the influence on the host. Thus, due to low humidity, the host defense changes as well as tissue repair is reduced as Kudo et al. [20] showed in their study on mice. Lowen et al. [21] summarize as a result of their study with guinea pigs as model host the mechanisms of influenza virus transmission as a function of humidity at three levels: Level of host concerning the mucociliary clearance and the associated defense potential, level of particle concerning the stability of the influenza virions, and level of vehicle in the form of respiratory droplets. They state that there is a possibility of reducing influenza virus spread by "maintaining room air at warm temperature (>20 ◦C) and either intermediate (50%) or high (80%) RHs" [21]. These studies on animals will aid in understanding the ways and types of transmission between human populations [21].

However, it is important that the relative humidity does not reach too high values, as this allows selected viruses to survive, as well as the growth of mold spores and fungi. The relative humidity must therefore be within a certain defined range in order to achieve a positive health effect. This optimal range where overall health risks may be minimized regarding relevant biological and chemical interactions has already been defined in 1985 by Sterling et al. [22], with a relative humidity between 40–60%. This optimal comfortable range between 40–60% is also pointed out by Arundel et al. [23] as a result of their study. In this study, different studies from schools, offices, and barracks were summarized, which deal with the "indirect health effects of relative humidity in indoor environments" with the clear statement that absenteeism or respiratory infections were found to be lower among people working or living in environments with mid-range versus low or high relative humidity [23]. Other studies also came to the result of an optimal range of relative air humidity concerning the viability of bacteria and the viability of viruses [24], the virus stability and transmission rates [25], and the reduction of human stress levels in comparison to drier conditions [26].

Furthermore, temperature is also attributed an important role in the spread and the toll of influenza. Shaman et al. [27] therefore investigated the relationship between absolute humidity and influenza survival and transmission, with the result that this relationship has even stronger significance than when considering the dependence of relative humidity. The consideration of hygrothermal comfort as a function of not only humidity but also air temperature is therefore crucial. This connection has also been pointed out by Wolkoff [28] in his review article concerning indoor air humidity and air quality and their influence on health. He gives an overview of numerous studies conducted in schools, offices, hospitals, and factories investigating the influence of air humidity on ocular surface, sleep quality, and the airways, but also its influence on the survival of influenza virus with the conclusion that not only relative air humidity plays a decisive role, but everything that is connected to

it such as air pollutants. Due to the complexity of this, more attention should be paid to the term of absolute humidity, as already done by [27].

These effects of low humidity in office rooms in the winter period inevitably manifest themselves in higher absences due to illness. Employees in Austria spent an average of 13.1 days on sick leave in 2018, compared to 12.5 days in 2017. Short absences due to illness (1–3 days) are very common and accounted for about 40% of all recorded sick leave cases in 2018. However, they are not recorded, which means that the actual sickness rate is higher. The most frequent causes of sickness are mainly diseases of the musculoskeletal and respiratory systems [29]. Together, these illnesses cause about 50% of all sick leave cases and 43% of all sick leave days. The overall economic costs of sick leave and accidents are made up of several components that can be measured with varying degrees of accuracy. While the direct payments made by companies and social insurance agencies in the form of continued pay and sick pay can be estimated relatively accurately. However, there is little evidence of the indirect economic costs or the medical treatment costs incurred in the health care system. In 2017, continued salary payments in Austria accounted for 2.9 billion euros, and a further 725 million euros were spent on sick pay. The directly attributable sick leave costs thus amount to 1% of Austria's GDP. Sickness-related absences from the workplace also lead to losses in added value and possibly to other operational costs (productivity losses, costs for replacement employees, follow-up costs of accidents at work, etc.) that exceed the direct costs of continued remuneration of the sick employee. These costs are difficult to quantify, as they vary greatly depending on the economic cycle, the industry, and the size of the company. Under highly simplified assumptions, it can be estimated that, in addition to the cost of salary replacement, sickness-related absenteeism generates indirect business and economic costs of 0.8% to 1.7% of GDP. In addition to these direct and indirect sick leave costs, there are also costs to the health care system in the form of medical care, hospitals, medication, etc. The above-mentioned cost factors are directly related to sickness absence; a decrease in sickness-related absenteeism has a correspondingly positive effect on these factors [30]. Not least because of the high costs involved, companies worldwide are striving to reduce absenteeism. Since the subject matter and the reasons for absences are very different and complex, different approaches to their reduction are also pursued. These include organizational measures related to the scope of duties, but also the upgrading of the workplace and the creation of a positive working environment in the offices with the aim of health promotion [31].

Milton et al. [32] investigated the connection between sick leave and indoor air quality among office workers in the USA. They established the link between the cost of sick leave and the currently recommended air exchanges, which, based on the length of sick leave attributable to air quality and the labor costs of an employee, can save about USD 400 per employee per year by improving indoor air quality through air exchange with the outside. In the mentioned article, ventilation is considered the main factor in improving indoor air quality. In any case, air exchange is the best way to prevent the spread of viruses and pathogens that are transmitted through the air. Moreover, the relative humidity influenced by humidifiers is included in this study with the knowledge that, in any case, too high humidity should be avoided, as this can not only lead to a higher survival rate of certain viruses, but also allows the development of mold spores and fungi. As also highlighted in Arundel et al. [23], maintaining a relative humidity between 40 and 60% should therefore be ensured. In their article, authors clearly state, supported by various epidemiological studies, that there is a significant correlation between absentee rates and relative humidity indoors. This correlation has also been investigated by Reiman et al. [33] in their study on humidity as a non-pharmaceutical intervention for influenza A in different classrooms. Comparing humified rooms to control rooms, they observed a significant reduction of the total number of influenza A virus positive samples. Taylor et al. [34] point out that there is a connection between low indoor relative humidity and reduced outdoor air ventilation and sick leave and productivity. Mendell et al. [35] suggest that health benefits for indoor workers by improving the building environments can lead

to high economic benefits. One of the other measures they advise is the influence of temperature and humidity of air.

Indoor greening has many advantages. In addition to the aesthetic enhancement of the room, it can not only contribute to a reduction of the reverberation time and thus to better speech intelligibility, but also influences the air quality in a room. This has already been proven in numerous studies and investigations [36–38].

In particular, vertical indoor greening in the form of wall greenery has a great effect, since a large area of vegetation can be created on a small floor surface. Among other things, vertical indoor greening has a positive effect on hygrothermal comfort. Particularly in winter, this is a great advantage due to the health effects of too low humidity. This has already been shown by means of measurement data from [36] and international studies such as [39–41].

Further, Reimherr and Kötter [42] examined the effects of indoor greening in offices on health, well-being, and work performance in the context of a research project. Through their surveys, they found out that with about 55%, the psychological and psychosomatic effects have the greatest health-promoting effect, followed by the advantages of air humidification (30%). Furthermore, the reduction of dust and noise as well as the reduction of pollutants are also cited. Similar results were obtained by Fjeld et al. [43] through a survey addressing neuropsychological symptoms, mucous membrane symptoms, and skin symptoms through indoor air conditions among office workers. The situation with and without plants in the office was compared, and it was found that complaints regarding cough and fatigue were reduced by 37% and 30% through plants present. They though clearly suggest that foliage plants in offices can lead to an improvement in health and a reduction in symptoms of discomfort. Studies by Smith and Pitt [44] also show that plants can be a low maintenance tool to improve indoor air quality. Their in situ measurements show that plants can not only influence the humidity in offices, but can also influence other air pollutants such as VOCs.

Vertical indoor greening also has the advantage that very little to no floor space is lost in the room, and yet plants can be available in large numbers in the room. In comparison to individual plants in pots or troughs, however, wall plantings are associated with higher costs for installation as well as for the upkeep and maintenance of the technical system.

When making decisions about investments in buildings, costs and benefits are always weighed against each other. Cost–benefit analyses are therefore used to compare the monetary advantages and disadvantages. In a cost–benefit analysis, the value of a project is thus quantified in monetary terms with the aim of the support of social decision making on a rational basis. A plan is worthy of realization if, compared to doing nothing, the sum of its advantages is greater than the sum of its disadvantages [45], or as defined by Cambridge Dictionary, "the process of comparing the costs involved in doing something to the advantage or profit that it may bring" [46]. However, such cost–benefit evaluations are very complex for indoor and outdoor greening of buildings. This is not least due to the fact that the positive effects of the living, nevertheless technical, system of the vertical green are varied and not only the investor profits, e.g., in the form of energy saving, but also substantial positive effects on the health as well as also on the cityscape, which are so far difficult to quantify and/or in a further step to monetarize, as already explained in detail in [47]. A classical cost–benefit analysis is therefore not the correct instrument to illustrate the effects holistically for building greenery. Alternative assessment and evaluation concepts are therefore necessary.

In this article, the costs of an investment and operation of vertical indoor greening are to be examined and analyzed in relation to the benefits in the form of reduced sicknessrelated downtime in office buildings due to improved humidity thanks to the vertical indoor green. These comparisons and the conclusions drawn from them are based on the following context: Particularly in winter, interiors often have too low humidity. This has health effects for the people who stay in these rooms—this also applies to offices and the people who work in them and who are on sick leave because of these health consequences.

Vertical greenery improves the hygrothermal comfort in indoor spaces and, especially in winter, can contribute to an increase in air humidity to a comfortable level and thus also influence the associated health consequences. Sick leave due to health consequences and the associated absence cause costs for the company, which are reduced accordingly when sick days are reduced. Indoor greenery can contribute to this reduction, but it also causes costs for installation and maintenance. These costs for greenery and possible savings by reduced sick days are compared, and a method of quantifying and monetizing the effects of vertical greening is shown.
