# In Vivo Assessment of Water Content, Trans-Epidermial Water Loss and Thickness in Human Facial Skin
## 5 Discussion  
The basic working aim of this study was to determine whether the six facial areas considered were statistically different in terms of the parameters investigated. Likewise, we wanted to find potential clusters of facial zones that are statistically undistinguishable from one another. Our results confirm that water content and TEWL can vary significantly between different facial areas--with the former showing more prominent differences than the latter. The statistical analysis indicated that 13 pairs of facial sites were significantly different in terms of TEWL--i.e., more than twice as many than were in terms of water content (five). Conversely, cluster analysis showed that there were larger clusters of statistically indistinguishable areas in terms of water content than were in terms of TEWL. Specifically, the results show that, as regards water content, there were three prominent clusters: (1) cheeks, chin and neck; (2) chin, forehead, neck and nose; and (3) forehead, lips and nose. The presence of three relatively large clusters is consistent with the water content being relatively uniform among the six areas investigated (see also Figure 5a). Earlier works also demonstrated variations of the water content in different facial areas, although the results available in the literature are not always consistent. Water content in the cheeks and forehead was, for instance, evaluated in [23, 27, 28], but despite the same type of corneometer being used in the three studies, [23] reported higher values in the cheeks than the forehead (which is in agreement with our results), and [27, 28] the opposite. Regarding TEWL, we found prominently more inter-area variation in this case than in terms of water content. This clearly reflects in the presence of only one and relatively small cluster in this case--i.e., chin and nose. By contrast, the lips emerged as a clear outlier with values well above those of all the other areas (Figure 5b). Notably, a previous study by [PERSON] and [PERSON] [25] determined that chin and nose had significantly higher values than cheeks, which is in accordance with our findings. [PERSON] and [PERSON] [26] reported that the nasolabial area had higher values than most other areas, which is again consistent with the results of our experiments. We also found that TEWL in the forehead was significantly higher than in the cheeks, which agrees with [17] but contrasts with [22, 23]. These difference could be possibly a consequence of the diverse ethnic composition of the  
Figure 9: Cluster analysis on the distribution of the skin thickness in the stratum corneum, epidermis and dermis. In each subfigure the clusters are shown from left to right in descending order of average thickness. study populations, as other authors have suggested [27; 29]. Among them, [PERSON] et al. [27], for instance, reported a reversed TEWL difference between the cheeks and forehead when considering populations of Vietnamese and Singaporean female subjects. It is known that the dermis accounts for most of the skin's thickness, as our results confirm. Additionally, differences in the skin thickness between the facial areas have been documented in a number of papers [30; 31; 32; 44]. In terms of absolute values of the whole thickness, our results are in line with those reported in [32]. As for the dermis, our findings indicate the presence of three clearly identifiable clusters, which, in descending order of thickness, were: (1) chin and nose, (2) cheeks and forehead and (3) lips and neck. This finding is in agreement with the results that appeared in [31], where the authors determined that the chin and nose were the areas with the highest thickness. The nose also emerged as the region with the highest overall thickness in [32]; however, differently from [31], and in contrast with the results found here, in [32], the chin had a relatively low thickness compared with the other areas. In [44] [PERSON] et al.