Stuart R. Gallant, MD, PhD

Our skin does so much for us. Skin protects us against infection and dehydration.  Skin allows us to be in contact with the world via its many nerve endings.  Skin allows us to grip objects securely by its many tiny ridges, but it also allows us to let things slide through our hands—for example when we are paying out line during a boat docking procedure.

Today’s post looks at three key measurements of skin:  pH, water content, and evaporative water loss, and what they say about skin health.

Skin Anatomy

Let’s start with a quick review of the anatomy of the human epidermis:

• The epidermis sits on top of the dermis and the subcutaneous fat.
• The dermis is perfused by capillaries which supply nutrients and remove waste products.  Blood plasma leaks from the capillaries supplying interstitial fluid to the dermis.  The pH of the dermis is 7.4—same as plasma pH.

The human epidermis is a self-renewing protective coat of progressively differentiating cells (at its lower limit composed primarily of dividing keratinocytes and at its upper surface composed of dead keratinocytes which are shed over time due to abrasion):

• At the deepest level of the epidermis (the stratum basale) reside the least differentiated keratinocytes.  This layer of cells divides, replenishing the layers above.
• Keratinocytes move upward into the stratum spinosum, developing a thickened protein coat and secreting glycolipid molecules (in aggregate called lamellar bodies) which are important to sealing the dermis (like the grout between shower tiles).  As these cells transition into cells of the stratum granulosum, they secrete keratohyalin (keratin, profilaggrin, loricrin, and trichohyalin proteins) further enhancing the durability of the epidermis.
• Because the cells of the stratum granulosum and the overlying stratum corneum are dead, they cannot repair themselves—they have a limited lifetime and are worn off by abrasion to be replaced by cells from below.
• The thickness of the epidermis varies by region of the body—the epidermal thickness of the eyelid is about 50 microns while that of the soles of the foot is about 1500 microns.
• In addition to providing mechanical protection against abrasion, the epidermis protects against moisture loss through the sealing effect of the glycolipids in lamellar bodies.

Measuring Skin Properties

Three measurements commonly used to characterize skin health are:

• pH:  Originally, skin pH was measured by placing a small sampling sheet in contact with the skin surface and subsequently measuring the pH of the sheet.  Today, dedicated skin pH meters are available which directly measure the skin surface pH.  The epidermal surface is buffered by constituents of the keratinocytes, including weak organic acids, fatty acids, urocanic acid, lactic acid, carbonic acid, amino acids, peptide, and proteins, as well as some bases [1].  In practice, this means that although washing with soap or even rinsing with tap water can briefly raise the measured epidermal pH, healthy skin may subsequently return to its normal range of pH 4 to pH 6 due to buffering [2].  Some disease states cause the pH of the skin surface to increase, and they will be discussed below.

• Capacitance:  Capacitance is a non-invasive measurement of the electrical properties of the skin.  Because the dielectric constant of skin changes with water content, capacitance is an indirect measure of the amount of water within the top 10 to 20 microns of the skin surface (i.e., within the upper layer of the epidermis).  The Corneometer provides measurements on a 0 to 100 scale with normal skin above 40 units and very dry skin below 30 units [3].

• Transepidermal water loss (TEWL):  The epidermis acts as a protective layer to prevent moisture loss (like a layer of Saran wrap prevents an extra slice of birthday cake from drying out in the refrigerator).  Though the epidermis is resistant to water passage, it is not completely impermeable to water.  The rate of water loss from the epidermis can be measured with laboratory instruments.  Typical value range from 2.3 g/m²/h (for skin of the breast) to 44 g/m²/h (for skin of the armpit) [4].  For older folks (> 65 yr), TEWL drops off, compared to younger people.

Since these measures vary widely (based on age, sex, and location on the body), the best method for understanding the significance of a particular skin property measurement may be to use a given patient as a control for themselves.  For example, make a measurement in a diseased area of skin and compared the measured value of pH, capacitance, or TEWL versus the same value measured at a non-diseased section of skin close to the diseased patch.

Skin Properties By Location, During Washing, and During Disease

Normal pH by Location

Skin surface pH varies somewhat with location on the body [5].  Areas of higher pH are somewhat more prone to infection since lower pH is somewhat antimicrobial.

Location	pH
Chin	pH 5.6
Inner Forearm	pH 5.3
Bridge of Nose and Neck	pH 5.2
Upper Eyelid	pH 4.6
Forehead	pH 4.4
Arm Pits, Groin, Toe Interdigit, Anus	pH 6.1 to pH 7.4

Response of Skin to Washing

A three-week study of washing compared rinsing skin with water only versus washing with a solution containing micelle formers (e.g., sodium laureth sulfate) and ingredients designed to maintain the skin surface (e.g., fatty acids (cocamidopropyl betaine), amino acids and sugars) [6].  This composition is common in the formulation of solutions and bars designed to wash human skin—washing removes water insoluble oils by the action of micelles and water-soluble materials, but it can leave skin dry and cracked if some replacement (particularly of oils) does not occur.  An interesting aspect of the study is that the washing solution was significantly lower in pH (pH 5.5) than many hand and body soaps.  Findings included:

Property	Result of Three-Week Study
Forearm pH	Initial pH of 4.85.  Measurements for both water alone and water combined with the washing solution are the same—no change in pH for first two weeks, then in 3rd week, pH increases to 5.1.  It would have been interesting to see the effect of a higher pH soap—presumably skin pH would have increased to a greater degree.
Forearm Skin Capacitance	Initial capacitance of 48 (indicating good moisture content).  Measurements for both water alone and water combined with the washing solution are the same—no change in first week, then linear decline to 43 in 3rd week.  This still qualifies as “normal” skin, but it shows that some water loss has occurred due to the washing procedure.
Transepidermal Water Loss	Initial TEWL of 8.  The water alone did not seem to increase TEWL, but use of the washing solution with water increased TEWL to 9, indicating a mild decline in barrier function.

Presumably, the person using the washing solution would feel clean and fresh after a shower, compared to the person who had only rinsed with water.  However, there is a slight cost to pay in the form of mild drying and increase of skin pH as a result of washing with soap.  Many people follow a shower by moisturizing to counter that effect.  This study suggests that lower pH soaps could be a good idea.

Also, it is worth recalling that some areas of the skin are less well perfused by blood flow than others (e.g., heels and toes of feet).  These areas are particularly vulnerable to drying and cracking—and therefore benefit more from moisturization.

Skin Properties and Skin Disease

Low skin pH is mildly protective against skin disease [7]; however, this effect should not be overblown.  A study of the rate of growth of the common dermatophytes Microsporum canis, Trichophyton rubrum, and Trichophyton mentagropyhtes (causative organisms for ringworm, some toenail infections, and other skin infections) showed a mild decrease in the size of colonies grown in media at pH 4.5, compared to the optimal pH of 6.0; however, colonies grew at all pHs tested [8].

A study of atopic dermatitis patients showed a correlation of disease severity with TEWL and capacitance, but not pH [9].  Presumably, the higher evaporative losses and the drying are indicative of reduced barrier function of the epidermis, permitting or aggravating the dermatitis.

One particularly effective method of preserving the skin barrier is the wet-moisturization method advised by the American Academy of Dermatology for treatment of venous stasis dermatitis [10].  In venous stasis disease, the skin is repeatedly stressed as swelling occurs in the legs during the day when the patient is vertical and resolves at night when the patient’s feet are elevated during sleep.  In the AAD’s moisturization method, the skin is washed gently and rinsed to remove soap residue.  The skin is not dried, instead a thin layer of petroleum jelly is applied, preventing the skin from drying out.  This method can be employed even in the absence of skin disease, particularly to areas likely to dry and crack (e.g., toes, heals, knees, elbows).  Towel drying following moisturization eliminates any greasy feeling.

Conclusions

In the film Ronin, Robert De Niro’s character reacts to a comment by Sean Bean’s character:

Spence : You worried about saving your own skin?

Sam : Yeah, I am. It covers my body.

I feel somewhat the same.  I want to do everything I can to preserve my skin for at least as long as I need it.  Hopefully, this post has provided some helpful insights into how to measure the properties of skin and how to protect it.

[1] Proksch, E.  “Buffering Capacity,” pH of the Skin: Issues and Challenges, Karger (2018).

[2] Kuo, S.H., et al.  “Role of pH Value in Clinically Relevant Diagnosis,” Diagnostics 10, 107 (2020).

[3] Heinrich, U., et al.  “Multicentre comparison of skin hydration in terms of physical-, physiological-, and product-dependent parameters by the capacitance method (Corneometer CM 825),” International Journal of Cosmetic Science 25, 45-53 (2003).

[4] Kottner, J., et al.  “Transepidermal water loss in young and aged healthy humans:  a systematic review and meta-analysis,” Review Arch Dermatol Res 305, 315-23 (2013).

[5] Proksch, E.  “pH in nature, humans and skin,” J Dermatol 45 1044-1052 (2018).

[6] Bornkessel, A., et al.  “Functional assessment of a washing emulsion for sensitive skin: mild impairment of stratum corneum hydration, pH, barrier function, lipid content, integrity and cohesion in

a controlled washing test,” Skin Research and Technology 11, 53–60 (2005).

[7] Lambers, H., et al.  “Natural skin surface pH is on average below 5, which is beneficial for its resident flora,” International Journal of Cosmetic Science 28, 359–370 (2006).

[8] Hasan, A.M. and Al-Jubori, M.H.  “Determination of Optimal Temperature and pH for Radial Growth of Some Dermatophyte Species Isolated from Leukemia Patients,” Iraqi Journal of Science 56, 95-99 (2015).

[9] Kim, D.W., et al.  “Correlation of clinical features and skin barrier function in adolescent and adult patients with atopic dermatitis,” International Journal of Dermatology 45, 698 –701 (2006).

[10] American Academy of Dermatology.  “12 healthy habits that help prevent worsening stasis dermatitis,” www.aad.org/public/diseases/eczema/types/stasis-dermatitis/self-care

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