Skin pH – Its Role in Maintaining Our ‘Acid Mantle’ For Optimum Skin Health
Introduction
The skin’s location at the body’s periphery facilitates its important role as a sensor – akin to our ‘brain on the outside’. Over the last few years researchers have gained many new insights into how the skin interacts with the environment, including with allergens, environmental pollutants and bacteria, in addition to the stresses associated with modern life.
There is much more to skin than meets the eye - this organ incorporates several major auxiliary systems including blood, muscle, innervation, immuno-competency, psycho-emotional, ultraviolet radiation sensing, and hormone etc. All these help maintain our body’s homeostasis or ‘balance’.
The skin is also a vast physical barrier at our interface with the external environment and so is designed to protect us against desiccation, and mechanical, chemical and microbial insults. However, it is not an inert defensive barrier; we increasingly learn that even its outer hardened layers are extremely active biochemically. The skin’s supply of sweat and sebum and other entities help maintain a health skin surface, not least by regulating its pH.
A Guide To Our Outermost Skin Layers
The Epidermis is that part of intact skin which is visible to the naked eye. It represents a stratified avascular layer of 0.075 to 0.150 mm (palms/soles can be up to 0.6 mm thick). Underneath the epidermis is the shock-absorbing Dermis, which houses blood vessels and multiple accessory mini-organs (e.g., Sebaceous, Sweat glands, and Hair follicles). Together these help to regulate our temperature and importantly our acid vs. base balance via their pores which open onto the skin surface. The upper layer of the epidermis (so-called stratum corneum) is essentially impervious to aqueous (water-based) solutions. Its lipid-rich and so water-repelling structure is however interrupted by aqueous pores - potential routes for delivery of active ingredients into the skin.
It is the fate of every keratin-producing cell in our skin to mature to form a tough external skin surface layer to cope with the continual onslaught of noxious external agents. For example, the topical application of solvents (e.g., alcohol) to the skin will remove lipids from the skin surface and lead to a striking increase in water loss from the skin.
The quality of the skin barrier differs between people, between body sites and during different phases of our life. Throughout, skin depends principally on the presence of 3 major constituents; ceremides, cholesterol and free fatty acids. Changes in the concentrations of any of these can adversely affect the barrier’s quality.
• For example, skin aging and photo-aging are associated with a cholesterol-predominant barrier,
• while atopic dermatitis is associated with ceremide-predominance
• a predominance of free fatty acids is associated with psoriasis.
The production of these 3 lipid species can be further modified by external stimuli that induce enzymes that generate these three critical lipids. Thus, inhibiting key enzymes involved in the synthesis of these lipids can disturb barrier permeability. The optimal pH for many of these enzymes is in the acidic part of the pH range. Therefore, despite the negative view people have when they hear the word ‘acid’, acidic pH is actually required for maintenance an adequate skin permeability barrier.
Origin Of Skin Surface Acidity
It is reported that the acidic nature of the skin’s surface is unique to humans, and that at least for specific body site the pH of the skin surface in males is similar to that in females. Although the nature of the skin’s ‘acid mantle’ was described as early as 1928, there are still significant gaps in our knowledge of where this acidity originates from.
pH is most commonly measured on the skin surface and there can be up to 3 pH units difference between the more neutral pH environment (pH 7.4) of the living cell layers of the skin [dermis] compared to the hardened barrier surface [epidermis] (i.e., between pH 4 and 5). A large number of sources contribute to the acidity of the skin surface including; lactic acid (an alpha hydroxyl acid) and free amino acids from sweat, free fatty acids from sebum, and free fatty acids from epidermal phospholipids. In addition, amino acids and other acids derived from cell maturation events are also involved; the latter include natural moisturising molecules. Sweat and sebum contain vital components for a healthy skin.
Sweat
As warm-blooded animals we sport 3-4 million sweat glands in our skin, each producing a watery perspiration that serves to cool the body (maintain our core temperature at 37.5 oC) as well as contribute to maintenance of our skin’s acid mantle. At maximal output the sweat glands of an adult human can excrete as much as 3 litres per hour!
• Sweat is a clear, odour-free, colourless, slightly acidic fluid (pH 5-6) that is almost fully water (99.0 - 99.5%) with the remainder consisting of the electrolytes and some inorganic compounds (e.g., lactate, urea, ammonia, calcium, heavy metals).
Sebum
The products of sebaceous glands also contribute to the maintenance of our skin’s vital acid coat. Up to 800 sebaceous glands/cm2 can be found on the scalp and face, less elsewhere.
• Sebum is a yellowish viscous fluid containing triglycerides, free sterols and sterol esters, squalenes, wax and free fatty acids – though some of the latter may be converted from triglycerides by bacterial enzymes on the skin surface.
• While there are significant inter-individual (and ethnic) differences in sebum production, the normal rate is 0.3mg sebum/10 cm3/hour.
• Other important roles for the sebaceous glands include:
o contribution to the skin barrier,
o interaction with neuropeptides,
o potential production of both anti- and pro-inflammatory compounds
o synthesis of anti-microbial lipids.
Variation Of Skin pH With Age & Body Site
The mean forehead pH for males has been reported at 4.7 while that for women was very similar at 4.8. However, the pH of the cheek was a little higher at 5.1 in males and 5.2 in females. Thus, even two closely located regions of the face can have pH varying by as much as 0.4 units. After the infant months pH levels stay relatively stable during adulthood, and only increase to more alkaline at ages above 80 years.
Acid pH As A Protection Against Microbial Overgrowth
The importance of an acid pH skin surface can be seen in the context of regulating unfriendly bacteria. Indeed, a significant anti-microbial defence can be afforded by the acidic pH of free fatty acids of the stratum corneum. The metabolites of skin microflora can further contribute to the acidification of the skin surface. It is comforting to note that ‘friendly’ skin microbes (e.g., micrococci) tend to like the acidic pH while ‘enemy’ microbes (e.g., staphylococci) prefer neutral/basic conditions. The ‘normal’ bacterium associated with acne (Propiobacterium acnes) prefers a pH microenvironment closer to the neutral.
Nutrition & Skin pH
Skin disorders have been long associated with nutritional deficiencies. Dermatologic conditions linked with nutrition can range from gross malnutrition effects and more subtly protein/protein-calorie deficiencies (e.g., kwashiorkor/marasmus) to specific vitamin / trace element deficiencies and metabolic disorders. Deficiency of essential fatty acids [i.e. fatty acids that can only be derived only from diet] e.g., linoleic and linolenic acids, can cause the skin to become dry, scaly, leathery, reddened and with increased water loss. Essential fatty acids can consist of up to 30% of the skin’s supply of fatty acids, and so may have a considerable impact on skin pH.
Hygiene & Skin pH
Personal care habits may have a profound effect on skin pH. Simply rinsing the skin with water causes an immediate but transient increase in skin pH. Moreover, washing hands in standard soaps can increase skin pH by as much as three units to pH 8 – clearly in the alkaline part of the pH range.
• It can take an hour or more before the skin’s pH returns to normal after washing with standard soap.
Age & Skin pH
It is thought that the pH buffering capacity of the skin may decrease with age. This may explain the increased sensitivity to the skin to contact irritants, cleansing agents and also bacterial infections in the elderly.
Skin Region & pH
The axillary region tend to have more alkaline pH than other body sites, and this leads to a particular microflora (odour-producing bacteria) thriving in this region. Bacterial activity here can be retarded however, by using deodorants containing citrate which lower the pH of the skin.
Circadian & Seasonal Bio-rhythms & Skin pH
The skin alters its pH at different times of the day due mainly to associated changes in skin enzyme activity, especially in the stratum corneum.
• The skin is more alkaline in the afternoon than at night.
• There are also seasonal variations in skin pH (as well as in temperature, water loss and surface lipid levels).
• Skin pH can be significantly lower in July compare to January, April or October. These observations may be explained by the lowering of pH by sweat-associated acidity (highest during summer).
pH In Skin Health & Disease
Studies have shown that recovery of skin barrier function after injury (e.g., from abrasion or solvent exposure) is faster if the skin exposed to solutions of pH 5.5 rather than neutral solutions of 7.2.
Patients with atopic dermatitis have an altered skin surface pH and an altered pH buffered capacity (i.e., slower alkaline neutralization rate). Interestingly, pH values in the affected skin were higher than in the uninvolved skin of the same patients, and the latter were still higher (i.e., less acidic) than in normal control individuals. The relatively common occurrence of dermatitis in the general population has implications for skin cleansing and skin care strategies.
• Evidence shows that skin cleansing with acidic electrolyte water (pH <2.7) can reduce the severity of atopic dermatitis in children and also reduce the levels of staphylococcus aureus bacteria on their skin.
• The use of acidic topical skin care products is likely to have beneficial effects for both skin dryness and dermatitis, and reduce further sensitivity (including prophylactically) to contact irritants.
Summary
Nature has provided our skin with an acidic mantle. We would do well to protect this crucial aspect of skin health by avoiding exposure to common daily stressors (e.g. harsh alkaline soaps and solvents). Personal care strategies that add back some acid tonic may well help us maintain more optimum skin surface health.
References
Tobin DJ. Biochemistry of human skin – our brain on the outside. Chemical Society Reviews. 35(1): 52-67, 2006.
Zlotogorski A. Distribution of skin surface pH on the forehead and cheek of adults. Arch Dermatol Research 279:398-401, 1987.
Rippke F, Schreiner V, Schwanitz HJ. The acidic milieu of the horny layer: new findings on the physiology and pathophysiology of skin pH. American Journal of Clinical Dermatology 3(4):261-72, 2002.
About Professor Tobin
Professor Tobin is a lead member of the Centre for Skin Sciences at Bradford University, where he lectures in Cell Biology. He also heads the Scientific Advisory Board of Yin Yang Natural Sciences Ltd.
Copyright Yin Yang Natural Sciences Ltd
No part of this paper may be reproduced without written permission by Yin Yang Natural Sciences Ltd