JOURNAL OF COSMETIC SCIENCE 82 global range observed simply illustrates the differently sized fi ngernails (e.g., pinky or index vs. thumb). However, in spite of their low impacts, statistical differences were observed between ages and ethnicities. For example, interage or interethnicity comparisons on same fi ngers show that the thickness of the youngest age group is signifi cantly (p 0.05) lower than that of the oldest age group and is slightly but signifi cantly higher (p 0.05) in Asian and African–American subjects than that of Caucasian and Hispanic subjects. THICKNESS OF THE NAIL Since thickness is an important factor that may impact hardness, resistance, or fragility of fi ngernails, a statistical analysis (principal component analysis, PCA) integrates all factors associated to thickness (age, ethnicity, dimensions of morphologic criteria, expert scor- ings, and self-assessments). It allows to defi ne four large clusters: Cluster 1: strong and healthy nails, Cluster 2: medium, Cluster 3: dull and aging nails, and Cluster 4: weak and soft nails. This PCA analysis is illustrated in Figure 3. Table I Ranges of Values (in mm) of Some Recorded Dimensions of the 10 Fingernails of All People (2800 Nails) According to Age and Ethnicity Parameter Global range Range of average values/age Range of average values/ethnicity Thickness 0.074–0.967 0.341–0.381 0.341(HI)–0.381(AS) Length 9.8–20.3 13.06–13.76 13.15 (CA)–14.05 (AA) width 8.4–12.4 9.84–10.3 9.93 (AS)–10.46 (AA) Depth/height 2–4.5 3.19–3.34 3.06 (AS)–3.44 (AA) Curvature radius 4.4–8.5 5.46–5.91 5.6 (HI)–5.82 (CA) AA: African–American AS: Asian CA: Caucasian HI = Hispanic. Figure 3. PCA of most factors associated to the thickness of the fi ngernails.

TYPOLOGY AND ATLASES OF HUMAN FINGERNAILS 83 PRESENCE OF VERTICAL RIDGES Experts graded these longitudinal striations according to their densities and visual depth (see Figure 4). The expert assessed scores indicated that the grades of ridges signifi cantly increase with age, irrespective of ethnicity and where, overall, 67% of all subjects de- clared presence of vertical ridges. EXPERT AND SUBJECT PERCEPTION OF “UNHEALTHY” FINGERNAILS Although the very term “unhealthy” is of a complex defi nition, it likely integrates aspects linked to surface irregularities (bumps, irregular growth of the terminal edge, lack of shine, vertical and horizontal striations, yellowing/discoloration, etc.). Figure 5 shows examples of the various severity of unhealthy appearance according to expert assessment. Increased grades were found clearly associated to age (see Figure 6). These age-related trends were found in the four ethnic groups studied. This parameter, together with the presence or severity of ridges, again suggests that aging has a much larger impact on aspects of fi ngernails than ethnic origin. DISCUSSION The work presented here attempts to draw some major characteristics of the fi ngernails of adult women of different age groups and ethnicities. With regard to the rather low num- ber of studied women vis à vis the world population, it cannot obviously represent a de- fi nitive data base. It nevertheless allows observation of low amplitudes of variations in morphometric features of the fi ngernails and impacts of age or ethnicity. The 10 fi nger- nails of women, for the most part, have larger dimensional differences interdigit (e.g., thumbs vs. pinky) than interindividual, interage, or interethnicity. The respective narrow ranges of geometric fi gures (width, arc width, thickness, etc.) by fi ngers are similar on both hands (e.g. same data obtained in left/right indexes or thumbs). In some epidemio- logical studies for skin and hair, large interindividual variations among a same ethnic group are observed (1, 2). In contrast to some highly variable skin or hair criteria with age or ethnicity (thickness, color, microrelief, shape, etc.) fi ngernails appear a rather constant skin appendage in dimensional aspects (3). However, apart from low affected geometric dimensions, aging induces noticeable changes in the structure/relief and thickness properties of fi ngernails in all ethnic groups Figure 4. Examples of the various severity in the vertical ridges of fi ngernails. Figure 5. Examples of the “unhealthy” appearance of fi ngernails arranged from left to right in the order of severity based on expert assessment.