The impact of cleansing products on the skin surface pH. by Sven Munke e altri

The impact of cleansing products on the skin surface pH. by Sven Munke e altri

 

Sul numero di gennaio marzo del IFSCC Magazine, la rivista che a volte viene inviata ai chimici cosmetologi iscritti alla SICC, è stato pubblicato un articolo particolarmente interessante per me che nel blog ho più volte parlato della detersione e di come pur preferendo la cosmesi a pH tendenzialmente acido ( 4,5 ) non considero una tragedia per la pelle normale la detersione con acqua e sapone alkalino, visto che l’omeostasi di una pelle sana in poco tempo riporta in equilibrio i vari fattori chimico-fisici.

La ricerca compara le variazioni di pH della pelle in funzione del pH del detergente e mette in risalto come non basti un pH acido del detergente a ridurre i tempi perché si ristabilisca il normale pH cutaneo.

Interessante come molti miti cosmetici (compresi quelli che hanno sostenuto il mercato dei saponi neutri o dei detergenti a pH fisiologico ) vengano rimessi in discussione con una semplice verifica.
Parrebbe che :” Il solo pH acido non garantisce la protezione ( ri-equilibrio ) del pH superficiale. Un pH acido , come 4,5, dovrebbe essere associato a tensioattivi idonei per minimizzare l’effetto della detersione sul pH cutaneo.”

 

The impact of cleansing products on the skin surface pH

Sven Munke, Uwe Aßmus, Bernhard Banowski, Jürgen Blaak, Michael Brock, Jessica Erasmy, Andreas Fitzner, Uta Kortemeier, Silke Langer, Hartmut Schmidt-Lewerkühne, Dörte Segger, Gunja Springmann, Claudia Wood

Introduction

The physiological acidic skin surface pH (SS-pH) was first described by Heuss [1], and so far many variables values have been reported. Recent multicenter studies determined the normal skin surface pH on the forearm to be below 5 [2-3]. In the stratum corneum the pH ranges from below 5 in the outer layers to approximately 7 at the interface to the viable epidermis [4]. The stratum corneum pH regulates at least three epidermal functions: the antimicrobial barrier. permeability barrier homeostasis and barrier integrity/cohesion [reviewed in 5]. Alterations of the physiological stratum corneum pH lead to abnormal epidermal barrier function. The relevance of an acidic skin surface pH as an antimicrobial barrier has been demonstrated frequently. The growth of pathogenic bacteria is enhanced under neutral pH conditions, while the resident flora is negatively affected by an increased skin surface pH [6-9]. Moreover, depletion of the resident bacteria from skin is increased under alkaline conditions [3]. Permeability barrier homeostasis depends on the pH gradient throughout the stratum corneum and on the activity of two lipid-processing enzymes exhibiting a pH optimum between pH 4 and 5 [10-11]. Barrier recovery is delayed after application of ‘super bases’ [12] or neutral pH buffers [13] to experimental perturbed skin sites. Further, stratum corneum integrity/ cohesion (the converse of desquamation) depends on the activity of two proteolytic enzymes which exhibit neutral pH optima [14-15]. The acidic stratum corneum pH regulates desquamation by reducing, but not completely inhibiting, these enzymes [16]. Under neutral to alkaline pH conditions (pH 6-8) the activity of these enzymes is enhanced, which leads to impaired barrier integrity/cohesion [12. 17].

Factors influencing the stratum corneum pH and skin surface pH can be categorized into three classes: (1) endogenous factors unrelated to pathological situations; (2) endogenous factors related to pathological features and (3) exogenous factors [18]. Many exogenous factors such as topical irritants, occlusive dressings, skin care products and cleansing products are known to affect or disturb the physiological skin surface pH [reviewed in 19. 20].

The influence of skin cleansing on the skin surface pH has been evaluated extensively. The well-known effect on the skin surface pH is related to the pH of the applied cleansing product [21]. Publications in this field report various methods, conditions, panels and effects on the skin surface pH and refer to the change in pH and the time of pH restoration/normalization. Depending on the study design (e.g. used products, washing procedure), the change in pH varies from ±0 [22] up to +3 [23] units and the recovery time ranges from 45 minutes [22] to 8 hours [24]. Further, in a repeated washing model Grunewald et al. [25] determined significant skin pH alterations 12 hours after the last washing procedure. In addition to the mentioned effects on skin surface pH, cleansing products lead to skin irritation [25], dryness [26], stratum corneum delipidation [27], stratum corneum swelling [28] and barrier dysfunction [29]. These effects also seem to be related to the pH of skin cleansers as well as to their composition.

To obtain a comprehensive overview, a literature survey was conducted on the impact of skin cleansing products on skin pH. Additionally, a controlled and randomized multicenter study was carried out to determine the effect of different cleansing products on the skin surface pH.

 

Literature Survey

Materials and methods

Medline, Kosmet and Bioscience Databases were searched for the following topic: “Influence of cleansing products on skin pH”. The search was limited to publications in the German or English language appearing since 1970. The publications were analyzed with the focus on design, test products and effects on the skin surface pH.

 

Results

The results of the search are not claimed to be complete but give a fair overview of the topic of »skin pH« and »skin cleansing «. Ten out of 69 evaluated publications relevant to our topic were analyzed extensively (Table 1). These publications included information on methods, test products and, particularly, effects on skin pH (pH shift after application). Due to the different targets and test conditions of the studies, inconsistent results were reported. Therefore, no clear correlation between the pH of the cleansing product and the influence on skin pH was evident. In addition to the skin surface pH, many publications include further biophysical measurements like stratum corneum hydration, transepidermal water loss, skin surface lipids, skin color and microcirculation. Some studies include investigations on skin flora because of the well known correlation between the skin surface pH and skin microbiota.

The skin surface pH was determined in vivo using noninvasive skin pH meters with flat glass electrodes but obtained from different manufacturers. Changes in the skin surface pH after skin cleansing varied from ±0 to +3.0 units. Bechor et al. [22] found no pH shift after a single use for 3 of 41 cleansing products (Cetaphil Lotion, Hawaii and Minon) . Mücke et al. [23] showed a mean pH change of +3.0 units after washing with alkaline soaps but provided no further details on the product formulation and pH.

Table 1: Major results found in a literature survey on the pH shift after skin cleansing

 

 

Experimental Study

Materials and methods

The controlled and randomized multicenter study was performed with 63 healthy male and female subjects aged from 40 to 65 by three companies (SIT Skin Investigation and Technology, Hamburg; proDERM Institute for Applied Dermatological Research, Hamburg; Sara Lee H&BC, The Hague). The subjects were informed about the importance and meaning of the study. Written informed consent was obtained from all subjects prior to entry into the study. The study was carried out during the winter season (January).

The subjects were instructed not to use any skincare products on the test areas for three days before the test started. Further, no water contact with the test areas was allowed for 24 hours prior to measurements. Each company used one of the following instruments: SkinpH-Meter 900® (Courage & Khazaka, Cologne, Germany), Skin pH Electrode InLab® 426 (Mettler-Toledo AG, Urdorf, Switzerland) or Orion AquaPro pH Flat Surface (Thermo Electron Corporation, Woburn, USA). The measurements were taken under standardized climatic conditions at 21°C (±1) and 50% (±2) rel. humidity after acclimatization for at least 20 minutes. Two test sites were marked on both volar forearms. One of these four test sites served as the control and remained untreated in two test centers and was treated only with tap water in the third test center. Before applying the test products or tap water the baseline skin surface pH was measured in these areas (t0). At each measuring time, three consecutive measurements were taken on each test site and the mean value was calculated.

The investigation was performed with three products based on two basic formulations. One of the basic formulations was adjusted to two different pH levels (pH 4.5 and pH 7.0) to evaluate the effect of only changing the pH of the formulation. The other basic formulation consisted of a different surfactant system at pH 4.5. The sodium chloride level was adapted to maintain the application properties (viscosity) of the formulation (Table 2).

The test products were applied according to the following procedure:

Approximately 5 μl per cm2 skin of the test product were applied with a small amount of water. The washing procedure was performed in a standardized manner for 30 seconds; the foam remained on the skin for an additional 30 seconds and was then carefully rinsed off with 500 ml of tap water (38° C). Then the skin was pad dried without rubbing. The skin surface pH was determined again after a 10-minute drying period (t1).

Table 2: Composition and pH of the test products and tap water

 

 

Statistical analysis

A two-sided significance level of 0.05% was chosen for statistical analysis. No adjustments for multiplicity were made. The following analyses were performed:

  • ANOVA on the pH with the factors »test center« and the within-subject factors »time point« and »treatment«
  • ANOVA on the pH shift with the factors »test centre« and the within-subject factor »treatment«
  • Comparisons of the pH before and after application (t0 vs. t1) by treatment and pooled over test centers and with the paired t-test
  • Comparisons of products with respect to the pH shift (t1 – t0) pooled over test centers with the paired t-test

The pH is presented as the arithmetic mean [30]. The statistical dispersion is described by the minimum, maximum, 25th percentile, and 75th percentile. Outliers are presented via box plots. The top and bottom edges of the box represent the 75th and 25th percentiles. The median is presented by the central horizontal line. The central vertical lines (whiskers) extend from the box as far as the data extend to a distance of at most 1.5 interquartile ranges (Figure 1).

Figure 1: Legend describing presented box plots

 

 

Results

In the present multicenter study no statistical differences were found between the three test centers (based on ANOVA, data not shown). Moreover, no significant differences (p>0.05) were observed among the test areas before the washing procedure. Table 3 shows the baseline means, which ranges from pH 5.12 to 5.34. A broad dispersion of the baseline data from pH 4.03 to 6.63 was notable (Fig. 2).

Figure 2: Skin surface pH. 25th and 75th percentiles: (A) Skin surface pH after (Assessment: t1) a standardized washing procedure. (B) Skin surface pH shift (Assessment: t1-t0) caused by a standardized washing procedure

 

 

The single washing procedure increases the skin surface pH in all treated test areas significantly (p<0.001), between approximately 1.0 and 1.3 units (Table 3). The smallest shift was observed after washing with test product C and the highest after treatment with test product A. In all treated test areas skin surface pH was enhanced to a minimum of 6.18. Treatment with test product A raised the skin surface pH up to 6.51, which corresponds to 1.23 pH units relative to baseline. The maximum pH shift in all areas treated with the cleansing products (A, B, C) was ≥ 2.00 units.

Ranking of the pH shifts in the different test areas gave the following sequences:

  • untreated < product C; water; product B; product A
  • product C < product B; product A
  • water < product A

Table 3: Skin surface pH before and after a standardized washing procedure with different test products and water (statistical analysis based on the paired t-test)

 

 

Comparison of the test areas after treatment showed that the differences between the treated sites and the untreated site were significant (p<0.001) (Table 4). The pH shift induced by test product C is significantly smaller than that with test product A (p<0.001) and B (p=0.014). Moreover, the differences between test product A and B (p=0.115) and between C and water (p=0.379) were not significant. The pH shift relative to water was significant only for test product A (p=0.002) but not for B (p=0.098) or C (p=0.379).

Table 4: Differences between test areas after a standardized washing procedure (statistical analysis based on ANOVA (p-values))
Discussion

The 69 publications found in the literature search (reviews and original papers) demonstrate the impact of rinse-off products on very different parameters: skin surface pH, stratum corneum hydration, transepidermal water loss, skin surface lipids, skin color, skin microcirculation and skin flora.

The results of the present literature survey are not complete because the search was restricted to publications in the German and English language appearing since 1970. Analysis of the 10 extracted relevant publications (Table 1) revealed inconsistent results. Due to the different experimental conditions, such as the biophysical devices used, measuring times, test areas, applied product amounts and statistical calculations, no conclusions could be made on the effect of cleansing products on the skin surface pH.

Several publications included ≤ 10 volunteers in the study, which is not sufficient from a statistical point of view. To achieve significance, the sample size has to be at least N = 20, and the level of significance should be set to 5% (á = 0.05). Moreover, the test sites differ probably due to the product category (e.g., face cleanser > cheek). The subject’s forearm is the test area most often chosen.

In Table 1 it can be seen that several studies lacked a description of the product, which means no information on the ingredients, formulations or the product pH was available. Such information, i.e., the pH and ingredients, is necessary to interpret and discuss the results.

It is commonly reported [18-20] and also shown by the publications analyzed (Table 1) that the effect of selected formulations on the skin surface pH is related to the pH of the applied cleansing product. Moreover, alkaline cleansing products like soaps lead to a stronger increase in the skin surface pH than pH-adjusted acidic cleansing products.

Specifically, a single treatment with soaps leads to a pH increase of approximately 1.5 to 3.0 units. In contrast, a single use of skin pHbalanced products like syndets elevates the skin surface pH by approximately 0.5 to 1.5 units. This is broadly defined and based on the publications shown in Table 1 but demonstrates that not only (alkaline) soaps but also syndets with a given pH of 5.0 to 7.0 may have a negative impact on the skin surface pH and therefore on epidermal barrier function [12] and the skin microflora [3]. It is demonstrated that already short-term elevation of the stratum corneum pH of about 0.5 units produces functional abnormalities such as decreased epidermal barrier cohesion [17]. Furthermore, a pH shift is described for single and repeated use but with no appreciable differences.

In addition to the literature search, a controlled and randomized multicenter study was performed to determine the effect of three different basic formulations on the skin surface pH relative to water or no treatment. As previously measured [2], the experimental part confirmed that the baseline skin surface pH varies from pH 4.03 to pH 6.63.

The standardized washing procedure led to an increased skin surface pH on all treated areas. After treatment the skin surface pH ranged between pH 6.18 and 6.51. Treatment with water alone raised the skin surface pH up to pH 6.25, which corresponds to an increase of 1.07 units. This had been previously described but with smaller pH changes, i.e. ~0.2 units [31] and ~0.6 units [3], respectively. In the untreated area the skin surface pH median changed from 5.34 to 5.43. The skin surface pH in this area was significantly (p<0.001) lower than in all the washed test areas. There fore all the pH changes observed in the treated areas were probably induced by the test products.

The increases in the skin surface pH differed for the cleansing products. Ranking the pH shift of the treated sites resulted in the following sequences:

  • untreated < product C; water; product B; product A
  • product C < product B; product A
  • water < product A

The smallest pH shift was found for cleansing product C (AK 124/5) relative to the other two formulations.

It can be seen in Table 2 that the composition of product C differs from that of product A (AK 94/1) and B (AK 94/2). Cleansing formulations A and B are based on sodium laureth sulfate and cocamidopropyl betaine. However, product C was formulated with cocamidopropyl betaine and lauryl glucoside.

Therefore, the present study confirms that not only the given acidic pH of a cleansing product, here pH 4.5, but also the composition of the surfactants is important to maintain the physiological skin surface pH and thus epidermal barrier function [32]. Moreover, it can be postulated that the impact of product pH on the solution chemistry of charged head groups influences the skin surface pH and thereby the mildness of cleansing products. Sodium laureth sulfate is classified as a surfactant with no pH dependence in the range of 4.0 to 10.0 [28]. Presumably, this explains the similar effect on the skin surface pH of product A (given pH of 7.0) and product B (given pH of 4.5), which are both based on sodium laureth sulfate. An acidic pH alone is no guarantee for protection of the physiological skin surface pH. This could be proven by the present experimental study. An acidic product pH, such as 4.5, has to be combined with the right surfactant system to minimize the effect of cleansing products on the skin pH.

This is important for people with an endogenous increased stratum corneum pH [18-20]. An elevated skin surface pH has been described for newborns [33], the elderly [34] and people with atopic dermatitis [35] or sensitive skin [36]. For these skin conditions maintaining or perhaps even normalization of the skin surface pH seems to be very important. As recently shown in the elderly population [37], topical normalization of the increased skin surface pH is one way to improve epidermal barrier function.

Johann W. Wiechers [42] recommended, based on the current definition of the skin surface pH [2-3], formulating cosmetics in the range of pH 4 to 5 due to the positive effects on epidermal barrier function and the skin microbiota. Moreover, based on the present results one future challenge for manufacturers of skin cleansing products will be to combine a low pH, such as 4.5, with the optimal composition and right ratio of surfactants.

 

Acknowledgements

This study was supported by the German Society for Scientific and Applied Cosmetics (DGK). The literature survey was sponsored and carried out by Henkel AG & Co. KGaA

 

References

  1. Heuss, E., Die Reaktion des Schweißes beim gesunden Menschen, Monatsh Prakt. Dermatol., 14 (1992) 343-59.
  2. Segger, D., Aßmus, U., Brock, M., Erasmy, J., Finkel, P., and Fitzner, A., Heuss, H., Kortemeier, U., Munke, S., Rheinländer, T., Schmidt-Lewerkühne, H., Schneider, W., and Weser, G., Multicenter Study on Measurement of the Natural pH of the Skin Surface, IFSCC Magazine, 10(2) (2007) 107-10.
  3. Lambers, H., Piessens, S., Bloem, A., Pronk, H,. and Finkel, P., Natural skin surface pH is on average below 5, which is beneficial for its resident flora, Int. J. Cosmet Sci., 28(5) (2006) 359-70.
  4. Öhman, H., and Vahlquist, A., In vivo studies concerning a pH gradient in human stratum corneum and upper epidermis, Acta Derm. Venereol., 74 (1994) 375-9.
  5. Elias, P.M., Stratum corneum defensive functions: an integrated view, J. Invest, Dermatol., 125(2) 2005 183-200.
  6. Holland, K.T., Cunliffe, W.J., and Roberts, C.D., The role of bacteria in acne vulgaris: A new approach, Clin. Exp. Dermatol., 3 (1978) 253-7.
  7. Korting, H.C., Das Säuremantelkonzept von Marchioni und die Beeinflussung der Resident-Flora der Haut durch Waschungen in Abhängigkeit vom pH-Wert, In: Braun-Falco, O., and Korting, H.C. (Eds.), Hautreinigung mit Syndets, Springer Verlag, Berlin, Heidelberg, New York, Paris, Tokyo, Hong Kong, 1990, pp. 93-103.
  8. Kurabayashi, H., Tamura, K., Machida, I., and Kubota K., Inhibiting bacteria and skin pH in hemiplegia: effects of washing hands with acidic mineral water, Am. J. Phys. Med. Rehabil., 81 (2002) 40-6.
  9. Korting, H.C., Lukacs, A., Vogt, N., Urban, J., Ehret W., and Ruckdeschel G., Influence of the pH-value on the growth of Staphylococcus epidermidis. Staphylococcus aureus and Propionibacterium acnes in continuous culture, Zentralbl. Hyg, Umweltmed., 193(1) (1992) 78-90.
  10. Takagi,Y., Kriehuber, E., Imokawa, G., Elias, P.M., and Holleran, W.M., Beta-glucocerebrosidase activity in mammalian stratum corneum, J. Lipid Res., 40(5) (1999) 861-9.
  11. Schmuth, M., Man, M.Q., Weber, F., Gao, W., Feingold, K.R., Fritsch, P., Elias, P.M., and Holleran, W.M., Permeability barrier disorder in Niemann-Pick disease: sphingomyelinceramide processing required for normal barrier homeostasis, J. Invest. Dermatol., 115(3) (2000) 459-66.
  12. Hachem, J.P., Cumrine, D., Fluhr, J., Brown, B.E., Feingold, K.R., and Elias, P.M., pH directly regulates epidermal permeability homeostasis and stratum corneum integrity / cohesion, J. Invest. Dermatol., 121 (2003) 345-53.
  13. Mauro, T., Holleran, W.M., Grayson, S., Gao, W.N., Man, M.Q., Kriehuber, E., Behne, M., Feingold, K.R., and Elias, P.M., Barrier recovery is impeded at neutral pH. independent of ionic effects: implications for extracellular lipid processing, Arch. Dermatol. Res., 290(4) (1998) 215-22.
  14. Ekholm, E., and Egelrud, T., Expression of stratum corneum chymotryptic enzyme in relation to other markers of epidermal differentiation in a skin explant model, Exp. Dermatol., 9 (2000) 65-70.
  15. Brattsand, M., and Egelrud, T., Purification, molecular cloning, and expression of a human stratum corneum trypsin-like serine protease with possible function in desquamation, J. Biol. Chem., 274 (1999) 30033-40.
  16. Caubet, C., Jonca, N., Brattsand, M., Guerrin, M., Bernard, D., Schmidt, R., Egelrud, T., Simon, M., Serre, G., Degradation of corneodesmosome proteins by two serine proteases of the kallikrein family, SCTE/KLK5/hK5 and SCCE/KLK7/hK7, J. Invest. Dermatol., 122(5) (2004) 1235-44.
  17. Fluhr, J.W., Kao, J., Jain, M., Ahn, S.K., Feingold, K.R., and Elias, P.M., Generation of free fatty acids from phospholipids regulates stratum corneum acidification and integrity, J. Invest. Dermatol., 117(1) (2001) 44-51.
  18. Fluhr. J,W., and Elias, P.M., Stratum corneum pH: Formation and Function of the ‘Acid Mantle’, Exog. Dermatol., 1 (2002) 163-75.
  19. Rippke, R.. Schreiner, V., and Schwanitz, H.J., The acidic milieu of the horny layer. New findings on the physiology and pathophysiology of skin pH, Am. J. Clin. Dermatol., 3 (4) (2002) 261-72.
  20. Schmid-Wendtner, M.H., and Korting, H.C., The pH of the skin surface and its impact on the barrier function, Skin Pharmacol. Physiol.,19(6) (2006) 296-302.
  21. Peukert, L., Einfluß der Titrationsalkalität von Reinigungsmittlen auf den pH-Wert der menschlichen Haut, Arch. Dermatol. Syph., 181 (1941) 417-24.
  22. Bechor, R., Zlotogorski, A., and Dikstein, Effect of soaps and detergents on the pH and casual lipid levels of the skin surface, J. Appl. Cosmetol., 6(3) (1988) 123-8.
  23. Mücke, H., Mohr, K.T., Rümmler, A., and Wutzler, T., Untersuchungen über den HautpH-Wert der Hand nach Anwendung von Seife. Reinigungs- und Händedesinfektionsmitteln, Pharmazie, 48 (1993) 468-9.
  24. Post, A.. Gloor, M., and Gehring, W., Über den Einfluss der Hautwaschung auf den Haut-pH-Wert, Dermatol. Monschr., 178 (1992) 216-22.
  25. Grunewald, A.M., Gloor, M., Gehring, W., and Kleesz, P., Damage to the skin by repetitive washing. Contact Dermatitis, 32 (4) (1995) 225-32.
  26. Imokawa, G., Akasaki, S., Minematsu, Y., and Kawai, M., Importance of intercellular lipids in water-retention properties of the stratum corneum: induction and recovery study of surfactant dry skin, Arch. Dermatol. Res., 281 (1989) 45-51.
  27. Fulmer, A.W., and Kramer, G.J., Stratum corneum lipid abnormalities in surfactantinduced dry scaly skin, J. Invest. Dermatol., 86(5) (1986) 598-602.
  28. Ananthapadmanabhan, K.P., Lips, A., Vincent, C., Meyer, F., Caso, S., Johnson, A., Subramanyan, K., Vethamuthu, M., Rattinger, G., and Moore, D.J., pH-induced alterations in stratum corneum properties, Int. J. Cosmet. Sci., 25 (2003) 103-12.
  29. Moldovan, M., and Nanu, A., Influence of cleansing product type on several skin parameters after single use, Farmacia, 58(1) (2010) 29-37.
  30. [30] Kober, M., Bestimmung des Hautoberflächen-pH bei Probanden: Methodik und Ergebnisse im Rahmen klinischer Studien, In: Braun-Falco, O., Korting, H.C. (Eds.), Hautreinigung mit Syndets, Springer Verlag, Berlin, Heidelberg, New York, London, Paris, Tokyo, Hong Kong, 1990, pp. 57-66.
  31. Gfatter, R., Hackl, P., and Braun, F., Effects of soap and detergents on skin surface pH, stratum corneum hydration and fat content in infants, Dermatology, 195(3) (1997) 258-62.
  32. Baranda, L., Gonzalez-Amaro, R., Torres-Alvarez, B., Alvarez, C., and Ramirez, V., Correlation between pH and irritant effect of cleansers marketed for dry skin, Int. J. Dermatol., 41(8) (2002) 494-9.
  33. Fluhr, J.W., Behne M.J., Brown B.E., Moskowitz D.G., Selden C., Man M.Q., Mauro T.M., Elias P.M., Feingold K.R. , Stratum corneum acidification in neonatal skin: secretory phospholipase A2 and the sodium/hydrogen antiporter-1 acidify neonatal rat stratum corneum, J. Invest. Dermatol., 122(2) (2004) 320-9.
  34. Choi, E.H., Man, M.Q., Xu, P., Xin, S., Liu, Z., Crumrine, D.A., Jiang, Y.J., Fluhr, J.W., Feingold KR, Elias PM, and Mauro TM., Stratum corneum acidification is impaired in moderately aged human and murine skin, J. Invest. Dermatol., 127(12) (2007) 2847-56.
  35. Seidenari, S., and Giusti, G., Objective assessment of the skin of children affected by atopic dermatitis and in healthy children: a multicenter study, Acta Derm. Venereol., 75 (1995) 429-33.
  36. Seidenari, S., Francomano, M., and Mantovani, L., Baseline biophysical parameters in subjects with sensitive skin, Contact Dermatitis, 38(6) (1998) 311-5.
  37. Blaak, J., Hoppe, W., Baron-Ruppert, Kaup, O., Langheim, H., Wohlfart, R., John, S.M., and Schürer, N.Y., Effect of pH 4.0 skin care on epidermal barrier function and skin microbiota: A randomized, controlled and double blind long-term study in a nursing home, Exp. Dermatol., 21 (3) (2012) e50.
  38. Pösl, H., and Schirren, C.G., Beeinflussung des pH-Wertes der Hautoberfläche durch Seifen. Waschmittel und synthetischen Detergentien, Hautarzt,17 (1986) 37-40.
  39. Tamburic, S., Changing the Skin Surface – Development of Skin Surface pH after Washing with Soaps and Handwash Liquids, Parfümerie & Kosmetik, 80(11/12) (1999) 44-7.
  40. Barel, A,O., Lambrecht, R., Clarys, P,. Morrison, B.M., Jr., and Paye, M., A comparative study of the effects on the skin of a classical bar soap and a syndet cleansing bar in normal use conditions and in the soap chamber test, Skin Res. Technol., 7(2) (2001) 98-104.
  41. Gunathilake, H.M., Sirimanna, G.M., and Schurer, N.Y., The pH of commercially available rinse-off products in Sri Lanka and their effect on skin pH, Ceylon Med. J., 52(4) (2007) 125-9.
  42. Wiechers, J.W., Formulating at pH 4-5: How Lower pH Benefits the Skin and Formulations, Cosm. Toil., 123(12) (2007) 61-70.

 

Rodolfo Baraldini

pubblicato 27 dicembre 2013

Credits: l’immagine è tratta dai Nonsensical Infographic di Chad Hagen

[post_view]

I commenti, le testimonianze e le opinioni sono sempre graditi. I programmi anti-spam possono far si che il commento sia visibile nel blog solo dopo approvazione. Per inserire nei commenti un link, rimuovere http:// e www. I commenti con argomentazioni ad personam, promozionali o non pertinenti verranno cancellati. Non è questa la sede per supportare le proprie tifoserie, fedi o ideologie.