3.5.4. Initial Situation: Reduction of Sick Days in Percent

Another approach that has been followed is based on the correlation that statistically speaking, when humidity improves into a comfortable area, there is a certain percentage decrease in sickness absence due to respiratory diseases. As already explained in Section 1, this correlation has already been scientifically proven with regard to the transmission and survival of viruses at different levels of humidity. In addition, in an experiment described under [56], 30–40% fewer symptoms related to symptoms of the mucous membranes were detected. Based on their research, Fjeld et al. [43] found that plants reduce dry throat symptoms by 25% and coughing symptoms by as much as 37% due to the increased humidity in the room.

The reduction of these complaints results in reduced sick leave due to these symptoms. Here, an assumption of a reduction of 25% is made.

Based on the average number of sick days in Austria due to respiratory diseases, which were explained in Section 1, this results in a reduction of sick days by about 2 days.

This results in a profitability of the installed greening systems after between 3.3 and 12.7 years, depending on the greening system and area under consideration. Moreover, in this case, the large spread of values is due to the large differences in the size of the rooms and the installed greening systems.

## 3.5.5. Generalization of Assumptions Based on Dimensioning Factors

The calculations carried out so far are based on data obtained in the course of research projects in implemented projects. Within the framework of the research project, a formula for the dimensioning of vertical indoor greenery to achieve the optimum level of comfort in relation to humidity (relative humidity 45%) in the interior could also be developed. The dimensioning differentiates between technical and manual ventilation and between good and bad user behavior with regard to ventilation or the circulation of the ventilation system, i.e., the existing air exchange. Using these dimensioning factors, the necessary green area is calculated in m2 depending on the floor area of the room (technical ventilation, circulation 1x per hour: 0.2; technical ventilation, circulation <1x per hour: 0.1; manual ventilation, good user behavior 0.08) [48,50]. Figure 5 shows the surface of an office depending on workplaces and the resulting surface of greenery for the different options of ventilation.

**Figure 5.** Surface of office (*Aoffice*) depending on workplaces (y) and resulting surface of greenery (*Agreen*) with technical and manual ventilation according to dimensioning factor (g).

If the sizes of the greenery installed in the three buildings in Vienna are checked on the basis of these dimensioning factors, it is noticeable that they are significantly larger than those calculated after the dimensioning. However, this does not represent a contradiction, but the developed formula is in a way based on these research results. If the systems are too large in relation to the room size, there is a risk that the humidity in the room is too high due to the greenery. However, this fear could be excluded on the basis of the long-term measurement data. In other words: These effects can also be achieved with smaller surfaces only in relation to air humidity. However, if other effects are also considered, such as the influence on the room acoustics or aesthetic aspects, other greening areas may well prove to be useful. However, it is always necessary to pay attention to the increase in humidity and to select an optimum of effects. Last but not at least, these effects also depend on the choice of plants.

Furthermore, this generalization now includes costs for installation as well as green care and technical maintenance based on current manufacturer information. These costs amount to 800 euros per m<sup>2</sup> for the installation and 150 euros per m<sup>2</sup> per year for the maintenance of the green care and technical maintenance.

Based on the calculations carried out for the model buildings, the following Equation (3) applies for a general dimensioning, whereby the following conditions apply ((4) to (7)). For an office space, the dimensioning factor thus results in a calculation according to Equation (8) for the number of years after which the installation of greenery based on reduced sick leave has paid off. Figure 6 visualizes these Equations for better understanding.

$$A\_{\text{green}} \cdot \mathbb{C}\_{\text{green},\text{annual}} = \mathbb{C}\_{\text{sick},\text{office},\text{annual}} \tag{3}$$

$$A\_{\text{green}} = A\_{\text{offic}} \cdot \text{g} \tag{4}$$

$$A\_{offic,min} = 8 + 5\ (y - 1)\tag{5}$$

$$\mathcal{C}\_{\text{green, annual}} = \left(\frac{\mathcal{C}\_0}{\mathbf{x}} + \mathcal{C}\_{\text{core}}\right) \tag{6}$$

$$\mathbb{C}\_{\text{sick},} \rho\_{fric,annual} = \mathbb{C}\_{\text{sick}} \cdot y \cdot d \tag{7}$$

$$\mathbf{x} = \frac{\mathbf{C}\_0}{\frac{\mathbf{C}\_{xick} \cdot \mathbf{y} \cdot d}{A\_{sffic} \cdot \mathbf{y}} - \mathbf{C}\_{care}} \tag{8}$$

**Figure 6.** Overview of application of Equations (3)–(8) to calculate number of years to profitability of greenery through reduced sick leave.

*Agreen*—surface greenery; *Cgreen*,*annual*—total annual costs for greening per m2; *Csick*,*o f fice*, *annual*—annual costs for sick leave per office space; *Ao f fice*—surface of office; *y*—number of work places; *g*—dimensioning factor for greenery; *Ao f fice*,*min*—minimum size of an office for x employees; *C*0—installation costs for greenery per m2; *x*—number of years; *Ccare*—costs for technical maintenance and green care per m2 and year; *Csick*—costs per sick day per person; *d*—number of reduced sick days.

For an office space with 6 workplaces and the minimum size of 33 m<sup>2</sup> specified by AStV and assuming technical ventilation and a circulation of 1x per hour (dimensioning factor 0.2) as well as the costs for the greenery according to the above-mentioned manufacturer's specifications, it follows that, assuming a reduction in sick leave by 2 days per person, the greenery will be profitable after about 4.5 years. For the same space with manual ventilation, the greening will be profitable already after about one year, since the size of the greened area of about 3 m<sup>2</sup> is significantly smaller than in the first example with 6.6 m2. Figure 7 illustrates the number of years to profitability comparing the costs for greenery and the according benefits for reduced sick leave for different cases such as different sizes of offices (3 or 6 persons) and different reduction of sick leave (2 or 3 days) as well as different types of ventilation (manual and technical). Profitability is given as soon as the line of costs intersects with the line of benefits.

**Figure 7.** Years to profitability of costs for greenery through benefits of reduced sick leave for different cases.
