Hand hygiene and hand rubbing are unequivocally the first line of defense in patient safety and even social safety in a pandemic. Since the ground-breaking discovery of Ignaz Semmelweis, hand hygiene protocols have been created, reassessed, reviewed, and rewritten . In 2009 the World Health Organisation (WHO) initiated the “SAVE LIVES: Clean Your Hands” program, marking hand hygiene as the cornerstone of infection transmission prevention. Unfortunately, even today, hospital-acquired infections (HAIs) are dominantly transmitted by hands . Current research is primarily focused on synthesizing more effective disinfectant agents and investigating healthcare worker compliance factors , however, some recent studies have raised questions about whether we are neglecting crucial factors involved in hand rubbing, and their implications on hand hygiene [4, 5]. A decade after the WHO Hand Hygiene Guideline, we now possess the technological resources  required to re-examine and reassess factors, which had been neglected, either due to their complexity to be measured, or were deemed insignificant and negligible.
In the worldwide-followed WHO guidelines [7, 8], the instructions regarding the application of ABHR are clear and explicit: “Apply a palmful of the product in a cupped hand, follow the 6-step protocol for 20–30 s, and cover all hand surfaces.” Initially, it may seem inconsequential, but the absence of an exact volume regarding the applied ABHR led to several issues regarding the clinical application of the guideline. The disinfectant volume does not only determine hand coverage, but also application time (drying time). To attain the desired microbial reduction on all hand surfaces, the disinfectant volume and application time are crucial.
Goroncy-Bermes et al. , demonstrated that an ABHR volume of 3 ml is required to obtain a sufficient microbiological reduction. Surveying the literature, it is now evident that hand size is an overlooked parameter regarding optimised hand coverage during hand hygiene [4, 10]. Zingg et al. , although having some limitations, concluded that for larger hands, even an ABHR volume of 3 ml might not entirely cover the entire hand surface. As written in the WHO guidelines, the term “palmful” can only be considered to be a relative form of quantification, as Healthcare Workers (HCWs) cannot objectively quantify the applied volume. At first glance, the issue seems insignificant, as logic dictates that a person with larger hands would apply more disinfectant before initiating the hand rubbing. However, a recent large-scale study  summarizing more than 28 million recorded hand hygiene events demonstrated how 86% of the hand rubbing events only use one push of the disinfectant applying apparatus (pump), even if this one push resulted in only 0.75 ml. Furthermore, according to Bansaghi et al. a clear decrease in volume per push is observed for numerous disinfectant dispensers  further indicating the presence of a disinfectant application volume issue. Variance among institutions and departments is almost certainly present, as some studies document higher volumes (3.3─3.4 ml) per hand hygiene event  while others lower (1.4 ml) ones .
Consequently, a large population of health care workers is:
consciously not applying enough disinfectant as they are unable to reliably assess the applied volume;
unconsciously not applying as much disinfectant as they think they are;
not applying enough disinfectant for their particular hand size.
Hand rubbing time or application time is another focus point concerning hand rubbing quality . While the WHO guidelines predicate a 20–30 s application time, some studies demonstrated that the 20–30 s application time is not enough for a 3 ml applied volume to dry on the hands [15,16,17], suggesting a 2 ml applied dosage. This however, may pose drawbacks as a smaller volume could potentially result in the decrease of the total disinfected hand surface . In other words, for a part of the HCW population, the WHO proposed application time is only feasible when the applied volume is not sufficient to provide proper microbial reduction. Interestingly, other studies documented that an application time of 15 s is more than enough, and has no significant difference in efficacy [18,19,20].
Combing in the results of the aforementioned studies confirms that important open issues still exist in present hand hygiene protocols, which should be addressed and ultimately solved, as they may have serious implications, especially during a pandemic [21,22,23]. Proper hand hygiene is paramount to reduce coronavirus transmission and HAI rates alike .
Strictly speaking, a microbial reduction can only be considered sufficient once a total hand coverage is achieved, as non-disinfected areas can still transmit pathogens, or re-contaminate already disinfected hand areas. In other words, a proper microbial reduction as seen in laboratory conditions (typically on inanimate surfaces) may differ from the clinical setting. Typically, improperly disinfected or missed areas include the fingertips, dorsum of the hands and wrists [24, 25]. Unfortunately, the information found in the literature regarding infection transmission models is limited and no information was found regarding the significance of the size of non-disinfected hand areas. Nevertheless, increasing the disinfected hand area can decrease the transient flora, and therefore the infection transmission risks. While theoretically simple, defining an exact application time (for a real-life clinical setting) is rather complicated. As a HCW starts the hand rubbing and the disinfectant is being spread, the disinfectant-volume/area ratio (µl/cm2) is not constant. The ratio changes during the hand rubbing process as the ABHR is simultaneously being spread and absorbed while also evaporating. To further complicate the equation, the evaporation rate is influenced by volume, and chemical composition (e.g., alcohol concentration)  therefore, this dynamic relationship can only be estimated. In vitro environment application time (time required for the disinfectant to take effect and reach the standardized microbial reduction) cannot be identical with the practical in vivo application time (contact time). In practical terms, and real-life clinical scenarios, application time (time until hand rubbing results in dry hands) depends on the applied disinfectant volume and specific chemical composition of the handrub, which dominantly dictates the evaporation rate.
The primary objective of this research is the comprehensive and accurate evaluation of the ABHR volume—coverage area relationship. To our knowledge, no study of a similar scale exists. In addition, individual drying times, disinfectant spills and the subject’s ability to assess volume were also investigated. Ultimately, an optimized ABHR volume in addition to a proper rubbing technique would ensure total hand coverage, and consequently sufficient microbial reduction. Furthermore, this would also decrease the long-term over-application of disinfectants, which can lead to dermatological issues (e.g., skin irritation, contact dermatitis) [26, 27] for the HCWs and increased costs to the hospitals and institutions.