## 4 Discussion
The hand alcohol and hand washing lotion comply with European standards EN 1500 and EN 14885, providing evidence of virus and bacteria reduction on sufficiently treated surfaces that in the assessment are made visible with the UV dye [53]. A correlation between UV dye visibility and reduction of contamination of treated surface areas has been demonstrated by [PERSON] et al. [54] for hand washing assessments and has been validated by [PERSON] et al. [47] in the context of hand disinfection using a UV dye. However, despite the established link between the UV dye covered surface and the reduction of contamination, a quantitative determination of the reduction factor according to the Szilagyi criteria is yet to be established [49]. Additionally, interpretation by different observers and further investigation of the effect of hand hygiene of personnel on the safety and stability of produced medicines may provide insightful revelations [55]. Nevertheless, when the average hand surface area of women (392 cm\({}^{2}\)) and men (448 cm\({}^{2}\)) is considered, even with the two small errors allowed according to the Szilagyi criteria [49], 99.70% and 99.74% of the hand surface area were covered by the applied product [56], respectively. Also, when considering alternatives, such as calculating the surface percentage, the Szilagyi criteria [49] remain the most favorable option for qualification assessments. Other UV-dye-based assessment methods typically require the use of advanced photo analysis software and are highly reliant on the camera resolution utilized [41; 47; 49]. An interesting observation was made during hand washing. For one cleanroom operator, the UV dye was still visible on the skin folds after more than 2.5 min of hand washing and performing the set of hand washing motions twice. Unlike the other cleanroom operators, this person had a very dry and aged skin. This is an important finding, since cleanroom operators are likely to encounter dry hands frequently because of the intense hand hygiene practices to which they are continuously exposed. The visibility of UV dye after multiple washing attempts suggests that UV coloring may accumulate on the skin surface in the case of dry or aged skin. Although the risk of systemic absorption of substances is generally lower when applied to a dry skin compared to a hydrated skin, deeper folds can trap a substance on the skin surface [57]. This may yield false positive results when using the UV dye method as a way of qualification. To rule out such false positives, the fold area can be considered to lie within the error margin, as a skin fold has an area lower than 0.6 cm\({}^{2}\). Alternatively, the skin can be pretreated with a moisturizer such as petroleum jelly up to half an hour before the training starts to assess the outcome based on a hydrated skin. The average procedure duration in our study exceeded the WHO recommended time of 1 min for hand washing and 20-30 s for hand disinfection [37]. No clear correlation was observed between the UV-dye-based assessment outcome and the measured time to complete the procedure. Regarding hand washing, a time-effectiveness relationship of 5 s to 30 s has been established by [PERSON] et al. [58]. However, regarding disinfection, it is more complex: [PERSON] et al. concluded that hand rubbing motions are more relevant for the quality of the procedure than time, although disinfection shorter than 15 s was not found to be sufficient [59]. The measured times in our study were considerably longer than those reported in the literature, possibly because the cleanroom operators were very experienced with the protocol up front or due to a detection bias. Another factor influencing the effectiveness of hand disinfection is the volume of disinfectant solution applied. It was found in our study that, after the 4 th step (see Figure 1) of the disinfection process, the majority of the operators required a second portion of 1-3 mL hand alcohol as the prescribed actions could not be completed within the drying time.