Comparison of sebum secretion, skin type, ph in humans with and without acne

Transcription

1 DOI /s ORIGINAL PAPER Comparison of sebum secretion, skin type, ph in humans with and without acne Myo-Kyoung Kim Æ Sun-Young Choi Æ Hee-Jin Byun Æ Chang-Hun Huh Æ Kyoung-Chan Park Æ Rajul A. Patel Æ Annie H. Shinn Æ Sang-Woong Youn Received: 7 September 2005 / Revised: 1 March 2006 / Accepted: 19 April 2006 Ó Springer-Verlag 2006 Abstract Differences of skin type and ph between subjects with and without acne have not been investigated. In addition, the relationship between sebum secretion and ph in these populations has not been determined. This study assessed the differences in objective and subjective skin types between these two groups. Secondly, this study evaluated the difference in ph on five facial areas (forehead, nose, chin, right and left cheek) between the two populations. Lastly, the relationship between ph and sebum secretion was analyzed in each population. Sebum casual levels (CL) of the five facial areas in 36 Koreans with acne and 47 Koreans without acne were measured by using a Sebumeter SM 815 Ò and subjects were classified into objective skin types by CL. Subjects reported the type of skin they believed they had, which determined the subjective skin type. The ph levels of the five facial areas were measured by the Skin-pH-Meter PH 905 Ò. Data was assessed with adequate statistical tests depending on data type and distribution. Among the five areas, the nose of the subjects with acne showed a significantly higher CL, compared to the subjects without acne. This difference in CL on the nose resulted in the difference in CL on the T-zone and mean facial sebum excretions (MFSE). Although CL differed, objective skin types did not differ between the two groups (P > 0.05), but the subjective skin types differed significantly (P = 0.001). In addition, the objective skin types were significantly different than the subjective skin types in subjects with acne (P = 0.001), whereas the two skin types did not differ in subjects without acne. Subjects with acne actually overestimated their skin types and stated their skin types were oilier than they were. In respect to ph, none of the five areas differed significantly between the two groups. Among the five sites in subjects with acne, CL showed a significant negative correlation with ph on the left (r 2 = 0.12) and right (r 2 = 0.15) cheeks, which resulted in a significant negative correlation on the U-zone (r 2 = 0.14). In contrast, in subjects without acne, there was a significant negative correlation between CL and ph on the forehead (r 2 = 0.10) and chin (r 2 = 0.16), which led to a significant negative correlation on the T-zone (r 2 = 0.14). Keywords Acne Æ Skin type Æ ph Æ Sebumeter Ò Æ Skin-pH-Meter Ò M.-K. Kim Æ R. A. Patel Æ A. H. Shinn Pharmacy Practice, University of the Pacific, Thomas J. Long School of Pharmacy, 751 Brookside Road, Stockton, CA 95211, USA S.-Y. Choi Æ H.-J. Byun Æ C.-H. Huh Æ K.-C. Park Æ S.-W. Youn (&) Department of Dermatology, Seoul National University, College of Medicine and Seoul National University Bundang Hospital, 300 Gumi-dong, Bundang-Gu, Seongnam, Kyounggi-do , South Korea swyoun@snu.ac.kr Introduction Acne is a common skin disorder, especially during adolescence. The four most contributable causes in the development of acne are excess sebum, inflammation, follicular epidermal hyperproliferation, and the presence and activity of pathogens such as Propionibacterium acnes [4]. Specifically, hyperseborrhoea in subjects with acne compared to subjects without acne

2 has been well established by several researchers [5, 12, 13]. However, the difference in skin types between these two groups has not been well described, due in part to the lack of standard methods to objectively determine skin type. Recently, Youn et al. [16] proposed a method to objectively classify facial skin types depending on sebum casual levels (CL) measured by the Sebumeter Ò, which is a well acceptable photometric method in measuring sebum casual levels by the European Expert Group on Efficacy Measurement of Cosmetics and Other Topical Products (EEMCO) guidance for the in vivo assessment of skin greasiness [14]. In the succeeding study, Youn et al. [17] also implemented their classification to demonstrate regional and seasonal variations of skin types in their subjects. Skin surface ph is another noteworthy skin biophysical parameter to associate with acne because the acid mantle (acidic skin surface) is thought to have at least four major important effects on (1) the growth rate of P. acnes, (2) the production rate of exoenzymes and antigens, (3) the stability of exoenzymes, and (4) enzyme activity [1 3, 6, 7]. As suggested by Greenman [1], some ph change is expected because the production of propionic acid, acetic acid, and free fatty acid produced by P. acnes and their enzyme may lower ph, whereas the production of ammonium ions may increase ph. In addition, damage to the follicular wall may result in the loss of phosphate, bicarbonate, and other buffers from the tissue. However, Greenman did not conclude the overall effects. Furthermore, a change in ph in subjects with acne compared to subjects without acne has not been documented to the best of our knowledge. In addition, a correlation between sebum secretion and ph has not been researched in these populations. This study examined the differences in objective and subjective skin types between subjects with and without acne by using the Sebumeter Ò and classification suggested by Youn et al. [16, 17]. This study also evaluated the differences in ph on the five facial sites between the two populations. Lastly, the relationship between ph and sebum secretion was analyzed in each population. Subjects and methods Subjects Korean subjects who visited Seoul National University Bundang Hospital (Kyounggi-do, Korea) were asked to participate in the study. The eligibility criteria included the minimum age of 18 and no physical and dermatologic illness, excluding acne. Thirty-six subjects with acne (mean ± SD age 27.0 ± 4.39 years, range years; 23 females, 13 males) and 47 subjects without acne (24.7 ± 1.50 years, years; 23 females, 24 males) participated in this study. Although the ratio of two genders in each group was not intentionally matched, it did not differ significantly between two groups when the statistical difference was assessed by a Chi-square test. Investigators followed the Declaration of Helsinki principles during all procedures. Objective skin type The method in determining objective skin type has been previously published elsewhere [16, 17]. Briefly, facial sebum casual levels (CL) were assessed by a Sebumeter SM 815 Ò (Courage & Khazaka, Köln or Cologne, Germany) on five different sites of the face: forehead (mid-glabella), nose tip, chin (mental prominence), right and left cheeks (the most prominent area of both zygoma). Participants were asked not to wear any cosmetics and to not wash their faces within 4 h prior to the measurements. All procedures were performed by the same investigator in a room at a constant temperature (20 C) and relative humidity (52%) from October in 2004 until January in 2005 to minimize variations caused by different investigators, temperature, humidity, and season. The Sebumeter SM 815 Ò was calibrated to zero with an unused tape section prior to each measurement. Sebum was collected from each site on a plastic strip using a constant pressure of 10 N for 30 s. Measurements of the five facial sites were recorded and classified as follows: high sebum secreting zone (T-zone: forehead, nose, and chin) and low sebum secreting zone (U-zone: both cheeks). Reference values for the CL of the T-zone, U-zone and mean facial sebum excretions (MFSE) were calculated by using the following equations: T zone ¼ðthe sum of CL of forehead; nose; and chinþ=3 U zone ¼ðthe sum of CL of right and left cheeksþ=2 MFSE ¼ðthe sum of CL of forehead; nose; chin; right and left cheeksþ=5: The objective facial skin types were determined as suggested by Youn et al. [16, 17], who adopted the

3 Table 1 Classification of the objective facial skin type by reference values of sebum casual levels (CL) measured with the SebumeterÒ (lg cm 2 ) Skin type MFSE a T-zone b U-zone c Dry < 88 < 100 < 70 Combination < 88 < dry 100 < 70 Normal Combination < 70 or > 180 normal < 100 or > <100 or > 220 <70 or > 180 Oily > 204 > 220 > 180 Combination > > 180 oily > a MFSE = (the sum of CL of forehead, nose, chin, right and left cheeks)/5 b T-zone = (the sum of CL of forehead, nose, and chin)/3 c U-zone = (the sum of CL of right and left cheeks)/2 breakpoints of each skin type from the Sebumeter SM 815 Ò manual. This is summarized in Table 1. Subjective skin type Before measuring their sebum casual levels, a questionnaire was given to each of the subjects for documenting their subjective skin type. Subjects were asked, What do you think your skin type is? and they chose an answer from four choices (dry, normal, oily, and combination). While the questionnaire was being answered, no explanations regarding skin types were provided in order to exclude any influence from the investigators. Facial ph The facial skin ph was measured using the Skin-pH- Meter PH 905 Ò (Courage & Khazaka). The ph electrode was calibrated prior to each measurement using two standard buffers (Mettler-Toledo, Greifensee, Switzerland) at ph 4.0 and 7.0. The electrode was washed with distilled water before each measurement. Immediately after measuring the sebum casual levels of a subject, the ph measurement was conducted on the surface of the skin on the same five facial areas for measurement of sebum casual levels. A site immediately adjacent to that used for testing sebum casual levels was chosen in each area to avoid any interference potentially caused by the previous procedure. The measurements were performed three times and the average of the three measurements was used for the results. The ph levels of the T-zone, U-zone, and mean facial ph (MFPH) were calculated by using the following equations: T zone ¼ðthe sum of ph of forehead; nose; and chinþ=3 U zone ¼ðthe sum of ph of right and left cheeksþ=2 MFPH ¼ðthe sum of ph of forehead; nose; chin; right and left cheeksþ=5: Statistical analyses The differences of sebum casual levels between subjects with and without acne at the five facial sites, T-Zone, U-zone, and MFSE were tested by an independent t-test after evaluating normality. The same test was used to evaluate the differences of ph between the two groups at the five facial sites, T-Zone, U-zone, and MFPH. The differences of sebum casual levels among the five facial sites in each population (subjects with acne and subjects without acne) were tested using the ANOVA and Tukey s HSD tests (post-hoc tests), after testing for normality. These tests were used to assess ph differences among the five facial sites in each population. The differences in both sebum casual levels and ph between the T-zone and U-zone in each population were evaluated by using the paired t-test. The Mann Whitney U test was used to test gender differences in both objective skin types and subjective skin types. The differences between objective and subjective skin types for each gender were tested using the Wilcoxon Signed Rank test. Before performing this test, combination dry, combination normal, and combination oily objective skin types were combined to combination skin type to match with subjective skin types. Relationships between sebum casual levels and ph on the five facial sites, T-zone, U-zone, and MFSE MFPH in each population were tested using the Pearson Correlation Coefficient test. The data was considered significant if P < 0.05, unless otherwise noted. Results Differences of sebum casual levels between subjects with and without acne Sebum casual levels of the five facial sites in subjects with and without acne and their reference values of T-Zone, U-zone, and MFSE were compared in Fig. 1. Among the five facial areas, the nose of the subjects with acne showed significantly higher sebum casual levels, compared to the control population. This

4 among the five sites differed significantly in both subjects with and without acne (P < 0.001). However, post-hoc tests showed slight dissimilarity between the two groups. In subjects with acne, the sebum casual level of the chin was significantly different than that of the nose tip. However, this difference was not observed in subjects without acne. The mean and standard deviation of sebum casual levels of the five studied sites in the two groups as well as the P-values are listed in Table 2. When the sebum casual levels on the T-zone and U-zone were compared, sebum casual levels on the T-zone were significantly higher than those of the U-zone (P < 0.001) in both populations (P < 0.001). Differences in objective and subjective skin types between subjects with and without acne The numbers of each skin type are listed in Table 3 Subjects with acne had significantly different subjective skin types than subjects without acne (P = 0.001); whereas the objective skin types did not differ between the two groups (P > 0.05). In subjects with acne, the objective skin type was significantly different than subjective skin type (P = 0.001). However, in subjects without acne, no difference between objective and subjective skin types were seen (P > 0.05). Fig. 1 Difference in sebum casual levels between subjects with and without acne a on the right and left cheeks, chin, forehead, and nose tip, and b on the T-zone, U-zone, and MFSE. Black bar subjects with acne; grey bar subjects without acne; straight line standard deviation; *P < 0.05 difference resulted in statistically higher sebum casual levels in the T-zone and MFSE of the acneic population than the control population. Sebum casual levels Differences in ph between subjects with and without acne The ph levels of the five facial sites, T-Zone, U-zone, and MFPH in subjects with and without acne were compared in Fig. 2. The ph did not differ on any of the five areas between the two groups (P > 0.05). The ph levels among the five studied sites differed significantly in both subjects with acne and subjects without acne (P < 0.001); although, post-hoc tests showed a minor Table 2 Sebum casual levels (mean and SD) measured in five facial sites and P-value between two different sites Mean ± SD (lg cm 2 ) Nose tip (150 ± 59.3) Forehead (137 ± 61.5) Chin (113 ± 51.2) Left cheek (70.1 ± 48.6) Right cheek (64.5 ± 42.5) Subjects with acne Nose tip P > 0.05 P < 0.05 P < P < Forehead P > 0.05 P < P < Chin P < 0.01 P < 0.01 Left cheek P > 0.05 Subjects without acne Mean ± SD (lg cm 2 ) Nose tip (120 ± 64.1) Forehead (114 ± 66.4) Chin (90.8 ± 49.4) Left cheek (52.0 ± 42.0) Right cheek (55.7 ± 50.0) Nose tip P > 0.05 P > 0.05 P < P < Forehead P > 0.05 P < P < Chin P < 0.01 P < 0.05 Left cheek P > 0.05

5 Table 3 Number of subjects in objective and subjective skin type Classification Skin type With acne c Without acne d Objective Dry 9 20 skin type a Normal Oily 0 0 Combination dry 4 8 Combination normal 10 8 Combination oily 0 0 Subjective Dry 4 19 skin type b Normal 6 8 Oily 8 10 Combination Total a Subjects with acne do not have significantly different objective skin types than subjects without acne (P > 0.05) b Subjects with acne have significantly different subjective skin types than subjects without acne (P = 0.001) c Objective skin type is significantly different than subjective skin type in subjects with acne (P = 0.001) d Objective skin type is not significantly different than subjective skin type in subjects without acne (P > 0.05) dissimilarity between the two groups. In subjects without acne, the ph of the forehead was significantly different than the ph of the nose tip, whereas this difference was not seen in subjects with acne. The posthoc test results (P-value), as well as the mean and standard deviation of the ph for the five tested areas in both subjects with and without acne are listed in Table 4. In both groups, the ph levels of the T-zone were significantly lower than the ph of the U-zone (P < 0.001). Correlations between sebum casual levels and ph Among the five facial sites in subjects with acne, there were significant negative correlations between sebum casual levels and ph on the left (P < 0.05, r 2 = 0.12) and right (P < 0.05, r 2 = 0.15) cheeks. These significant correlations led to a significant negative correlation for the U-zone (P < 0.05, r 2 = 0.14), which resulted in a significant negative correlation for the MFSE MFPH (P < 0.05, r 2 = 0.10). In contrast, among the five sites in subjects without acne, a significant negative correlation between sebum casual levels and ph was observed on the forehead (P < 0.05, r 2 = 0.10) and chin (P < 0.01, r 2 = 0.16). These correlations resulted in a significant negative correlation for the T-zone (P < 0.05, r 2 = 0.14), which lead to a significant negative correlation for the MFSE MFPH (P < 0.05, r 2 = 0.10). Correlations between sebum casual level and ph on the T-zone and U-zone in subjects with and without acne are shown in Fig. 3. Fig. 2 Comparison of ph between subjects with and without acne a on the right and left cheeks, chin, forehead, and nose tip, and b on the T-zone, U-zone, and MFPH. Black bar subjects with acne; grey bar subjects without acne; straight line standard deviation. (None of the ph was significantly different between two groups) Discussion In this study, the sebum casual levels on the nose of the acneic population were significantly higher than those of the control population, which resulted in statistically higher sebum casual levels on the T-zone and MFSE of the acneic population, compared to the control population. These results are similar to the results of other studies, showing that the seborrhea is more intense in subjects with acne. However, there is a difference between this study and previously published studies in regards to sebum secretion parameters and facial areas, which showed a significant difference. Pierard et al. [13] demonstrated a different overall sebum excretion rate (SER: the amount of sebum excreted during a defined period of time) between subjects with and without acne when they measured sebum secretion of foreheads using Lipometre Ò, a photometric technique.

6 Table 4 The ph (mean and SD) measured in five facial sites and p-value between two different sites Mean ± SD (lg cm 2 ) Nose tip (5.05 ± 0.452) Chin (5.12 ± 0.399) Forehead (5.28 ± 0.429) Left cheek (5.36 ± 0.423) Right cheek (5.48 ± 0.411) Subjects with acne Nose tip P > 0.05 P > 0.05 P < 0.05 P < Chin P > 0.05 P > 0.05 P < 0.01 Forehead P > 0.05 P > 0.05 Left cheek P > 0.05 Subjects without acne Mean ± SD (lg cm 2 ) Nose tip (4.93 ± 0.424) Chin (5.04 ± 0.403) Forehead (5.18 ± 0.500) Left cheek (5.23 ± 0.376) Right cheek (5.36 ± 0.361) Nose tip P > 0.05 P < 0.05 P < 0.01 P < Chin P > 0.05 P > 0.05 P < 0.01 Forehead P > 0.05 P > 0.05 Left cheek P > 0.05 However, the follicular sebum secretion rate (FER: delivery rate of sebum from the follicular reservoirs), which was measured using Sebutape Ò (a technique implementing lipid-sensitive tape), was almost identical Fig. 3 Correlations between sebum casual level and ph on the a T-zone and b U-zone in subjects with acne and without acne. Filled square subjects with acne; open circle subjects without acne; straight line linear fit of subjects with acne; dashed line linear fit of subjects without acne between the two groups [13]. Pierard-Franchimont et al. [12] also demonstrated significant difference of the total SER on the forehead between subjects with moderate acne and subjects without acne, whereas it did not differ between subjects with mild acne and subjects without acne. Finally, in the study by Harris et al. [5], subjects with inflammatory acne had a higher sustainable rate of sebum excretion (SRSE: constant rate presumed to be equal to the rate at which the glands secrete sebum) than control subjects by using disks of fine Dacron mesh, embedded in fresh clay. Despite the significantly different sebum casual levels between the two groups in this study, objective skin types did not differ. This study is consistent with a recently published study by Youn et al. [18] who demonstrated that there was no significant difference in total lesions of acne between persons with dry and normal skin types. They also reported that although increased levels of facial sebum secretion were seen in acneic groups, increased sebum levels did not directly cause the development of acne lesions when they evaluated correlations between facial sebum secretion and acne lesion counts in five independent facial sites. Interestingly, our study showed that the objective skin types were significantly different than subjective skin types in subjects with acne (P = 0.001), whereas the two skin types did not differ (P > 0.05) in subjects without acne. In other words, subjects with acne overestimated their skin types and considered their skin oilier, compared to subjects without acne. This explains that subjective skin types differed significantly (P = 0.001) between the two groups. Therefore, it may aid the consumer by measuring the objective skin types when skin care products are selected in subjects with acne, rather than solely depending on the subjective feelings of the consumer. In addition, the suppression of facial sebum secretion may not directly prevent or

7 reduce acne. Leyden et al. [9] demonstrated that the suppression of facial sebum secretion by a potent, selective inhibitor of type I 5 a-reductase did not improve acne lesions in a multicenter, placebo-controlled clinical trial. In respect to ph, none of the five areas differed significantly between the two groups (P > 0.05). This may be due to the production of propionic acid, acetic acid, and free fatty acid produced by P. acnes counteracting the lower ph caused by the production of ammonium ions [1]. Although there are several studies [2, 3, 6, 7] showing the negative effects of elevated ph on the skin, there are no published studies to evaluate the ph difference between subjects with and without acne, to the best of our knowledge. However, there were several studies that investigated the difference in ph in other populations. In the study of Wendling et al. [15], age did not correlate with skin ph. In addition, Zlotogorski [19] showed no difference in ph between two genders. Considering our results and results of other studies, significant differences of ph in special populations may not be easy to detect due in part to its natural buffering system. Therefore, our results suggest that ph may not have a significant impact on development or maintenance of acne. Interestingly, ph distribution among the five studied sites differed significantly in both subjects with acne and subjects without acne (P < 0.001). The difference in sebum secretion among different facial areas has been well documented [8, 10, 11, 16]. In fact, there is a general consensus in distinguishing facial areas into the T-zone and U-zone depending on sebum secretion. This study also showed that sebum casual levels of the T-zone were significantly higher than those of the U-zone in each population (P < 0.001). However, this study showed, for the first time to our knowledge, that the ph levels of the T-zone were significantly lower than the ph of the U-zone (P < 0.001). This may be associated with the difference in sebum secretion due in part to significant negative correlations between sebum casual levels and ph, which were also seen in this study. However, in spite of the statistically significant correlations, the correlations (r 2 ) were not strong enough solely to explain the difference in ph by the difference of sebum secretion since the highest r 2 value in our study was only Therefore, at least 84% of other factors (such as the presence of P. acnes) play important roles in explaining the difference in ph among facial sites. Another interesting result of our study was that the correlations were significant in only the U-zones in subjects with acne and only in the T-zones in subjects without acne. However, these different correlations between the two groups are difficult to explain with the results of this study. This difference should be further examined in future studies. In conclusion, the sebum casual levels of the nose in the acneic population were significantly higher than those of the control population, which resulted in statistically higher sebum casual levels in the T-zone and MFSE of the acneic population, compared to the control population. On the other hand, the ph of the five studied areas did not differ between two groups. In addition, the objective skin types were significantly different than the subjective skin types in subjects with acne (P = 0.001); whereas, the two skin types did not differ (P > 0.05) in subjects without acne. Finally, the ph distribution among the five studied sites differed significantly in both subjects with acne and subjects without acne. The negative correlation between sebum casual levels and ph was statistically significant in only the U-zones of subjects with acne and in the T-zones of subjects without acne. However, the correlations (r ) were not strong enough solely to explain the difference of ph by the difference of sebum secretion. References 1. Greenman J (1981) Follicular ph and the development of acne. Int J Dermatol 20: Greenman J, Holland KT, Cunliffe WJ (1983) Effects of ph on biomass, maximum specific growth rate and extracellular enzyme production by three species of cutaneous propionibacteria grown in continuous culture. J Gen Microbiol 129: Hachem JP, Crumrine D, Fluhr J, Brown BE, Feingold KR, Elias PM (2003) ph directly regulates epidermal permeability barrier homeostasis, and stratum corneum integrity/ cohesion. J Invest Dermatol 121: Harper JC, Thiboutot DM (2003) Pathogenesis of acne: recent research advances. Adv Dermatol 19: Harris HH, Downing DT, Stewart ME, Strauss JS (1983) Sustainable rates of sebum secretion in acne patients and matched normal control subjects. J Am Acad Dermatol 8: Holland KT, Greenman J, Cunliffe WJ (1979) Growth of cutaneous propionibacteria on synthetic medium; growth yields and exoenzyme production. J Appl Bacteriol 47: Korting HC, Kerscher M, Schafer-Korting M, Berchtenbreiter U (1993) Influence of topical erythromycin preparations for acne vulgaris on skin surface ph. Clin Investig 71: Le Fur I, Lopez S, Morizot F, Guinott C, Tschachler E (1999) Comparison of cheek and forehead regions by bioengineering methods in women with different self-reported cosmetic skin types. Skin Res Technol 5: Leyden J, Bergfeld W, Drake L, Dunlap F, Goldman MP, Gottlieb AB, Heffernan MP, Hickman JG, Hordinsky M, Jarrett M, Kang S, Lucky A, Peck G, Phillips T, Rapaport M, Roberts J, Savin R, Sawaya ME, Shalita A, Shavin J, Shaw JC, Stein L, Stewart D, Strauss J, Swinehart J, Swinyer L, Thiboutot D, Washenik K, Weinstein G, Whiting D, Pappas

8 F, Sanchez M, Terranella L, Waldstreicher J (2004) A systemic type I 5 alpha-reductase inhibitor is ineffective in the treatment of acne vulgaris. J Am Acad Dermatol 50: Lopez S, Le Fur I, Morizot F, Heuvin G, Guinot C, Tschachler E (2000) Transepidermal water loss, temperature and sebum levels on women s facial skin follow characteristic patterns. Skin Res Technol 6: Park SG, Kim YD, Kim JJ, Kang SH (1999) Two possible classifications of facial skin type by two parameters in Korean women: sebum excretion rate (SER) and skin surface relief (SSR). Skin Res Technol 5: Pierard-Franchimont C, Pierard GE, Saint-Leger D, Leveque JL, Kligman AM (1991) Comparison of the kinetics of sebum secretion in young women with and without acne. Dermatologica 183: Pierard GE, Pierard-Franchimont C, Le T (1987) Seborrhoea in acne-prone and acne-free patients. Dermatologica 175: Pierard GE, Pierard-Franchimont C, Marks R, Paye M, Rogiers V (2000) EEMCO guidance for the in vivo assessment of skin greasiness. The EEMCO Group. Skin Pharmacol Appl Skin Physiol 13: Wendling PA, Dell Acqua G (2003) Skin biophysical properties of a population living in Valais, Switzerland. Skin Res Technol 9: Youn SW, Kim SJ, Hwang IA, Park KC (2002) Evaluation of facial skin type by sebum secretion: discrepancies between subjective descriptions and sebum secretion. Skin Res Technol 8: Youn SW, Na JI, Choi SY, Huh CH, Park KC (2005) Regional and seasonal variations in facial sebum secretions: a proposal for the definition of combination skin type. Skin Res Technol 11: Youn SW, Park ES, Lee DH, Huh CH, Park KC (2005) Does facial sebum excretion really affect the development of acne? Br J Dermatol 153: Zlotogorski A (1987) Distribution of skin surface ph on the forehead and cheek of adults. Arch Dermatol Res 279: