96 TESTING
Skin pH assessment for sensitive skin claims
Christiane Uhl - Courage + Khazaka Lorette van’t Hoff - Absolu Beauté
Specific amounts of water and lipids on the skin surface determine the composition of the hydrolipidic film of the skin. The various functions of sebum and moisture on the skin surface to keep it supple, flexible and healthy have been investigated from the beginning in the cosmetic industry. The slightly acidic pH-value of the
hydrolipidic film is a major protective factor for the skin, buffering acids and alkaline products that get in contact, as well as providing an environment favourable to our natural microbiome, at the same time restricting the growth of pathogenic microbes. The term ‘acid mantle’ describing this
aspect was first used in 1928 by Heinrich Schade and Alfred Marcchioni.1
Today, due
to further technical developments and continuous research in dermatology and skin physiology, biochemistry, immunology and genetics, we know that the acidic skin pH value is involved in many more functions. The pH value of the skin is influenced by
different endogenic factors such as genetic disposition, age, gender, health, melanin content and sebum secretion. It is easily impacted by external influences, especially cleansing and care products, temperature (sweat) and relative humidity (moisture). Newborn babies show a more or less neutral
skin surface pH. In the first four weeks after birth, this value will gradually decrease,2-4
reaching
an acidic level that will only change again much later in life. Then, in elderly skin, after 60-70 years of age, an increased pH value can be measured again. At the same time, researchers have observed a decreasing buffering capacity of the skin for these elderly patients.5,6 The use of alkaline cleansing products and
treatments or even water (with a pH-value of approximately 7.0) will lead to immediate changes of the skin surface pH. Experiments measuring the skin pH value before and after washing with an alkaline cleanser have shown that the skin takes more than three to four hours to recover from this disruption.7-9 Frequent and repetitive washing will lead to an accumulation of these disturbances. Combined with the removal of lipids in the
skin by more aggressive cleansers, over time, it will result in a weakening of the acid mantle and skin barrier, resulting in a permanent skin damage. Products claiming a ‘skin-neutral pH value’ like 5.5 are less disruptive towards the acid mantle but even they are not absolutely
PERSONAL CARE April; 2023
identical to physiological skin pH-values at the limbs and upper torso. They can still cause minor changes in skin surface pH,10
and even,
depending on their composition, damage the skin.
The skin’s microbiome and pH value go hand in hand Not only physiological processes will be affected by an altered pH level, but organisms living in the skin microbiome, dependent on specific pH environments, may also be influenced by elevated pH ranges. Since microorganisms produce substances to inhibit potential invaders and trigger immune responses to ward off dangerous microbes, changes in their associations may contribute to a weakening of the skin barrier. To protect the skin barrier, different physical and chemical mechanisms work together and reinforce each other to ensure that dangerous substances cannot penetrate into the deeper skin layers and blood vessels.
along the body but depends very much on the body site.13
The skin surface pH value is not constant Generally, it ranges from pH 4 to
6. It is especially high at the soles of the feet, in the axilla, the gluteal crease and under the chest, most of these areas being well-known for irritation and itching, or in case of the axilla also for the colonization of odour-producing resident bacteria.14 Since particularly fungi are able to sense
favourable pH conditions that enable them to adhere, invade and thrive, diabetics with an increased pH value in intertriginous areas are often suffering from candidiasis at these locations.15 Other consequences of altered pH-
conditions of the skin are microbial infections such as diaper or incontinence dermatitis. Here, the increase in pH and the impairment of the skin barrier is triggered by the permanent contact with urine and faeces, producing ammonia and thereby generating an alkaline environment.15
Skin care with slightly acidic
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