pH-value in skincare

pH

pH-scale ranges from 0 to 14 and expresses the acidity or alkalinity of a substance (usually an aqueous solution). A solution with a pH value of 7.0 is considered neutral, while a solution with a lower pH value is acidic, and a solution with a higher pH value is basic or alkaline. The lower the pH value, the more acidic the substance, and the higher the pH value, the more basic/alkaline it is.
pH is a very important term in many contexts because the acidity of a solution has a significant impact on the chemical and biochemical processes that can occur in the solution. Therefore, the regulation of a solution's pH value is also crucial in many situations. An important term in this context is "buffer capacity": The buffer capacity is a measure of a solution's potential to maintain its pH value when a base or acid is added.

pH value and buffer capacity are essential in many industries (almost all involving water) and also for humans themselves. The human body is completely dependent on being able to regulate and maintain pH in tissues in order for the correct biochemical processes to take place. For example, the blood's pH is tightly regulated to be within 7.35-7.45 under normal conditions. Blood has a reasonably high buffer capacity due to the substances it contains, and additionally, the lungs and kidneys contribute to maintaining the blood's pH value by regulating the excretion of certain substances from the blood. Conditions such as diabetes, as well as lung and kidney diseases can result in an excess of acid in the blood, causing pH to drop below 7.35 - this is called "acidosis," while hyperventilation, for example, can increase the blood's pH - this is called "alkalosis." The other tissues in the body also rely on an appropriate pH, which can be affected by certain diseases - for instance, healthy lungs have a pH around neutral, but in cystic fibrosis, the pH in the lungs decreases. The pH on the surface of the skin is skin is normally in the acidic range but varies significantly depending on factors such as the body area and the degree of inflammation in the skin. Generally, a higher pH is observed in areas with inflammation and where the skin is more enclosed - such as in the armpits. Like in other tissues, the skin's biochemical processes and microbiota are also dependent on the pH value in the area.

Therefore, pH is an interesting topic concerning cosmetics and something that PUCA PURE & CARE pays significant attention to in the development of its products.

pH - a Danish invention

The concept of pH was originally invented by the Danish chemistry professor Søren P. L. Sørensen in 1909 while he was leading the chemical department of the Carlsberg Laboratory. Søren P. L. Sørensen used the notation ”pH·” which was later changed to "pH" in 1924 during a minor revision of the concept.
Before 1909, scientists primarily used vague terms to describe the acidity of a solution, which was not precise enough for Søren P. L. Sørensen in his work with beer brewing, where he focused on enzymatic processes. He needed a precise tool to standardize beer production. At that time, it was well known that the concentration of H⁺ions (hydrogen ions) in a solution determined its acidity, but expressing the concentration in decimal numbers was not practical, as these are very small numbers.

The solution was the pH scale, which is the negative decimal logarithm of the concentration of hydrogen ions, which can be written simply as: pH = -log([H⁺]). In the next section, the concept and definition will be discussed more thoroughly. The abbreviation "pH" has been a subject of speculation. There is no doubt that "H" stands for Hydrogen (ion), but "p" is more debated since it can represent different words (with similar meanings) in French, Danish, German, the languages in which Søren P. L. Sørensen wrote his articles, and Latin, which is also widely used in scientific literature. In Danish, "p" could stand for "potens" or "potentiale," while in German, it could be "potenz," and in French, it could be "puissance" and finally, in Latin, "pH" could stand for "pondus hydrogenii" (quantity of hydrogen) or "potentia hydrogenii" (power/potential of hydrogen). However, if you look at the original article from 1909, it appears that "p" was simply the letter Søren P. L. Sørensen assigned to his hydrogen electrode arrangement while he used "q" for his reference electrode arrangement.
Today, the lowercase "p" is used in chemistry to denote "the negative logarithm of..." and is also used in expressions like "pK_a," which will be presented in the next section.

The use of the pH scale quickly became widespread and was commonly used in scientific articles just ten years after its invention. Today, pH is used everywhere where acidity is relevant, which includes a wide range of applications.
For instance, in the production of food, medicine, cosmetics, paper, textiles, and in agriculture, wastewater management and in general in many scientific studies.
The body's own biochemical processes generally rely on fairly specific pH values. Therefore, body fluids such as blood, cerebrospinal fluid, urine, and the fluids inside each cell's organelles¹ are tightly regulated and equipped with buffer capacity to maintain pH despite external influences.

Examples of different solutions' pH values include: Stomach acid: 1.5-3.5, lemon juice: 2.4; vagina: 3.8-4.5, skin: 4.1-5.8 (undamaged, healthy, and unoccluded skin), milk: 6.5, pure water: 7.0 (neutral pH at 25°C), blood: 7.35-7.45, urine: 7.5-8.0 (morning urine is usually more acidic: 6.5-7.0), seawater: 7.5-8.4, classic solid soap: 9.0-10.0, and a 0.1 molar (approx. 4%) aqueous solution of Sodium Hydroxide: 13.0.

¹Organelles are the term used for the cell's internal structures ("organs") surrounded by a membrane and performing various functions. A couple of examples of organelles are the cell nucleus containing DNA (pH inside is 7.1-7.3) and mitochondria, which produce most of the cell's energy (ATP) (pH in human mitochondria is 7.8-8.0 in the matrix and 7.0-7.4 in the intermembrane space).

pH - a brief overview of the silence and technology behind it

As mentioned earlier, the pH value of a solution is a measure of its degree of acidity or alkalinity – more specifically the negative decimal logarithm (base 10) of the concentration, or more precisely, the activity (a)² , of hydrogen ions (H⁺). This is expressed as follows:

pH = -log(a_H) ≈ -log([H⁺])

The term H⁺ (hydrogen ion) is most often used, but in reality, free, H⁺ions are not present in an aqueous solution, as H⁺ions will react with water (H₂O) to form H₃O⁺, which is called the hydronium or oxonium ion.

The pH scale ranges from 0 to 14³ , and since pH is logarithmic (base 10 logarithm), it has no unit, and each pH value represents a 10-fold difference in H⁺ion concentration. Thus, a solution with a pH of 5.0 will have ten times higher H⁺ion concentration compared to a solution with a pH of 6.0.
Corresponding to H⁺ for acidic levels, one has OH^- (hydroxide-ion) for basic levels⁴ - the balance between these two ions is decisive for the aqueous solution's pH. A low pH indicates a relatively high concentration of H⁺ and a low concentration of OH^-. As pH increases, the concentration of H⁺ decreases, and the concentration of OH^- increases. At pH 7.0, the concentration of these ions will be equal (this is the situation with pure water), and at pH above 7.0, the concentration of OH^- will exceed the concentration of H⁺.

To understand where H⁺ and OH^- come from, one must understand how acids and bases function. In brief, an acid is a substance that can release one or more hydrogen ions, while a base is a substance that can accept one or more hydrogen ions.
The ease with which an acid releases its hydrogen is an expression of its strength – the easier it releases hydrogen, the stronger the acid. The same principle applies to bases – the easier a base accepts a hydrogen, the stronger the base.

The reaction in which an acid releases hydrogen (H⁺) is called a dissociation reaction and is expressed as follows:

HA ⇌ H⁺ + A^-

Here, HA represents the acid, H⁺ is the released hydrogen ion, A^- represents the so-called conjugate or corresponding base (the remaining part of the acid), and the ⇌ sign indicates that it is a reversible reaction.
This forms an acid-base pair. For such acid-base pairs, it holds that a strong acid's corresponding base is relatively weak, and likewise, a strong base's corresponding acid is relatively weak. Similarly, a weak acid's corresponding base is also relatively weak, and a weak base's corresponding acid is likewise relatively weak.
Therefore, a solution of an acid (or base) is a balance between being in the acid-form (HA) and dissociated into H⁺ and the corresponding base, A⁵.
This balance ratio between the concentration of the dissociated form (H+ and A-) and the acid form (HA) is a dimensionless value called the acid's dissociation constant, denoted as K_a⁶. K_a increases in parallel with the strength of the acid.

²The concentration of hydrogen ions [H⁺] is usually the term used when describing pH, but more accurately, it is the activity of H⁺ ions. In practice, they are almost the same.

³In special cases with a high concentration of very strong acids or strong bases, the pH can be respectively below 0 and above 14.

⁴And just as we have pH, there is also the less commonly used measure pOH, which similarly represents the negative logarithm of the OH- ion concentration, expressing the solution's base/alkali level.

⁵This is a temperature-dependent reaction and, in itself, constitutes a weak buffer system (with low capacity).

⁶Other terms for the dissociation constant include equilibrium constant and acid strength constant. The small "a" in K_a stands for "acid"._s, so that the pH does not change significantly.

The strenght of the acid

Like pH, this value is usually converted to the more "manageable" and equally dimensionless pK_a, which is defined as the negative decimal logarithm of the acid dissociation constant:

pK_a = -log(K_a)

With the pK_avalue, acids can be classified as strong, moderately strong, weak, and very weak acids as follows:

pK_a ≤ 0: Strong acid
0 < pK_a ≤ 4: Moderately strong acid
4 < pK_a ≤ 10: Weak acid
pK_a > 10: Very weak acid

In an aqueous solution of a weak acid, most of the molecules will be in the acid-form (HA). On the other hand, a solution with a strong acid will primarily contain the dissociated form (H⁺ and A^-, resulting in a high concentration of H⁺ in the solution, and thus, the pH will be low.

is 7.0. The acid-base reaction for water is as follows, where two water molecules react and either donate or accept an H_a is 7.0. The acid-base reaction for water is as follows, where two water molecules react and either donate or accept an H⁺ion – and the reaction can also proceed in the reverse direction (reversible reaction):

2 H₂0 ⇌ H₃O⁺ + OH^-

As water is both a weak acid and a weak base, only a very small portion of water molecules will dissociate into H₃O⁺ and OH^-. In completely pure water, about 10^-7 (equal to 0.0000001 or one ten millionth) of the H₂O molecules will be dissociated, resulting in a neutral pH of 7:

pH = -log (10^-7) = 7

The relationship between pH and pK_a is expressed by the Henderson-Hasselbalch equation, also known as the buffer equation. This equation is an approximation and contains some assumptions. It is not very accurate for strong acids and bases and does not account for water's own acid-base properties. The Henderson-Hasselbalch equation is as follows:

pH = pK_a + log [A^-]/[HA]

Buffer systems

From this equation, it can be seen that the pH of a solution containing an acid (or base) will be equal to the acid's pK_a added to the logarithm of the concentration of the corresponding base divided by the concentration of the acid.

If the concentration of the corresponding base and the concentration of the acid are equal, the pH of the solution will be equal to the acid's pK_a. The equation is also called the buffer equation because it is mainly used to calculate buffer systems. For example, the equation can be used to estimate the pH of a buffer system and to calculate the concentration of the acid and the corresponding base if we know the pH and the acid's pK_a.

A buffer system consists of a (usually relatively weak) acid and its corresponding base (or the relatively weak base and its corresponding acid) and is used to maintain the pH within a relatively narrow range despite the addition of acid or base to the system – thus, a buffer system acts as a pH buffer with a certain capacity. The capacity is a measure of how much acid or base can be added to the system without the pH changing significantly, and it primarily depends on the concentration of the acid and the corresponding base and the pH of the solution. The capacity is highest when the concentration of the acid and the corresponding base is close to being equal and when the pH of the solution is close to the pK_avalue of the acid – generally, the capacity is highest in the pH range of pK_a ± 1.

Buffer systems work by the weak acid and the corresponding base reacting with the added acid (H⁺) and/or base (OH^-), thus "neutralizing" the added H⁺ or OH^-When the capacity is exceeded, for example, by adding so much acid that all the corresponding base in the solution is used up to react with the added acid, the pH will decrease relatively steeply – and vice versa, if more base is added than the buffer system's capacity can handle (because the acid in the buffer system is used up), the pH will increase relatively steeply.

In practice, a buffer system is usually created by adding an acid (or base) with an appropriate pK_a in relation to the desired pH and adding a corresponding amount of the corresponding base in the form of the salt of the acid (or if a base is used, adding the corresponding amount of the corresponding acid).

Buffer systems in use

An example of a buffer system (an acid-base pair) is Acetic acid and Sodium acetate. Acetic acid has a pK_a of 4.7, and thus its buffer capacity is highest in the pH range of 3.7-5.7.
In cosmetics, for example, Citric acid is often used, which is a trivalent acid (can release three H-atoms), and thus it has three pK_avalues: 3.1, 4.8, and 6.4, and can thereby cover a relatively broad and cosmetically relevant pH range.

⁷pH can also be measured in non-aqueous substances, but the process is slightly different.

⁸You can read more about these technical topics in articles such as: Buck, R. et. al. Measurement of pH. Definition, standards, and procedures (IUPAC Recommendations 2002). Pure and Applied Chemistry, 2002, 74(11), 2169-2200 and Zulkarnay, Z et. al. An Overview on pH Measurement Technique and Application in Biomedical and Industrial Process. 2015, 2nd International Conference on Biomedical Engineering (ICoBE), Penang, Malaysia, March 2015, pp. 1-6.

pH measurement of aqueous solutions⁷ can be performed using various methods, and these methods may yield slightly different values. Therefore, one should be mindful of the measurement method when comparing pH values. The more technical details of how pH is measured, the uncertainties to consider with different measurement methods, and the mathematical formulas behind them will not be thoroughly discussed here⁸.

One of the less precise methods, which many might be familiar with from school, is pH measurement with acid-base indicators. This can be done using indicator solutions or indicator paper (pH strips) impregnated with indicator solution. These indicators will change color depending on the pH of the liquid they come into contact with, allowing pH to be visually read by comparing it to a color scale or more accurately through colorimetry (quantitative color measurement).

Today, pH meters are often used, which are electronic instruments equipped with an ion-selective glass electrode and a reference electrode. These electrodes are dipped into the solution to be measured. Upon contact with an aqueous solution, an electric potential is formed over the ion-sensitive electrode, which is dependent on the hydrogen ion concentration and thus the pH. The potential of the reference electrode remains constant and is set during the pH meter calibration process. Hence, the instrument can quantitatively compare the electric potentials between the two electrodes and calculate the pH.
It is important to calibrate a pH meter regularly to ensure accurate measurements. Not much water is needed for pH measurement, allowing pH to be measured on surfaces like the skin using a pH meter with a flat electrode (and possibly a small amount of pure water). However, since such electrodes are relatively large, pH is measured over a certain area, and it is not possible to measure pH at the cellular or subcellular level. For that purpose, other more complex methods such as Fluorescence Lifetime Imaging Microscopy (FLIM) are required.

pH and the skin - the structure and components of the skin

Before delving into the pH of the skin, it is useful to have a good understanding of how the skin is structured. Naturally, this is a complex subject that can be approached from various angles. Here, the focus will first be on the basic structure of the skin layers, primarily the epidermis, and then on the components believed to be significant for the skin's pH.

The skin consists of three main layers⁹: At the bottom is the subcutis (subcutaneous layer) - also known as the hypodermis, which primarily consists of fat and connective tissue.
In the middle is the dermis (dermal layer), which mainly consists of connective tissue and contains e.g. nerve endings, blood vessels, hair follicles, sebaceous glands, and sweat glands.
The outermost layer is the epidermis (epidermal layer), which is composed of several layers. At the bottom (just above dermis) is the stratum basale, which is a single layer of cells including melanocytes, undifferentiated keratinocytes, and stem cells that continuously produce new keratinocytes (cells).

These keratinocytes migrate outward and eventually/gradually form the other epidermal layers, which are: stratum spinosum, stratum granulosum, stratum lucidum, and finally, the outermost layer, stratum corneum (SC), which is about 10-30 µm thick.
The stratum basale, stratum spinosum, and stratum granulosum contain living cells and are referred to as the viable part of the epidermis, while the stratum lucidum¹⁰ and stratum corneum consist of dead cells and are called the non-viable part of the epidermis - but there are still many different chemical processes occurring in these layers.
It is important to know that the skin contains many different communication pathways and interactions between keratinocytes, immune cells, and microorganisms on the skin, which can affect different functions of the skin, such as maintaining the skin barriers.

Regarding the skin barrier and the skin's pH, the outermost layer of the epidermis, the stratum corneum, is particularly interesting.

The stratum corneum contains several layers (usually 15-25) of primarily dead, flat keratinocytes called corneocytes - these are embedded in an intercellular lipid-rich matrix with specially organized lipids, which are a crucial element in the skin barrier. This structure of the stratum corneum is often described as a brick-and-mortar arrangement, with corneocytes being the "bricks" and the intercellular lipid structure being the "mortar." The outermost surface of the skin is constantly shedding, renewing the skin in a process called "desquamation," which is normally well-regulated.

⁹An illustration of the skin can be found in the description of Glycerin on this website.

¹⁰Stratum Lucidum is normally found only in skin with a thick epidermis (such as the palms of hands and soles of feet) and consists of 2-5 layers of flat primarily dead keratinocytes containing the substance eleidin, which is a precursor to keratin.

¹¹Also called extracellular lipids.

Additionally, the skin's outer layer is "populated" by various microorganisms - the skin's microbiota - which in many ways have been shown to be very interesting in relation to the functions and qualities of the skin.
The corneocytes in the stratum corneum are flat cells that primarily contain keratin filaments, various enzymes, and water. Surrounding the corneocytes is a special cell envelope called the cornified cell envelope, which mainly consists of cross-linked proteins such as filaggrin, loricrin, and involucrin, forming a highly insoluble and stable structure. Bound to these proteins is a single layer of lipids consisting primarily of long-chain ceramides - this layer is called the lipid envelope.
This lipid layer forms important interactions with the intercellular lipid layer between corneocytes.

Two other important structures between corneocytes are corneodesmosomes, consisting of various proteins that hold the cells in the stratum corneum together, and the so-called tight junctions, also made of proteins, which contribute to the barrier function.

The intercellular lipids¹¹ make up about 15% of the weight of the stratum corneum and are primarily composed of ceramides (about 50%), cholesterol (25-30%), free fatty acids (10-15%), cholesterol esters (about 10%), cholesterol sulfate (2-5%), and very low concentration of phospholipids. This is in contrast to the other layers of the epidermis and dermis, where phospholipids make up a considerable part of the lipids.
The primary source of these intercellular lipids is the lamellar bodies, which are oval secretory organelles located intracellularly in viable keratinocytes, primarily in the stratum granulosum. These lamellar bodies can secrete lipids such as phospholipids, glucosylceramides, and cholesterol, which contribute to the well-organized intercellular lipid-rich matrix.
Additionally, these lamellar bodies secrete certain enzymes such as lipid hydrolases, lipases, proteases, and certain enzyme-inhibiting proteins and antimicrobial peptides such as beta-defensins and cathelicidins. Therefore, these organelles are extremely important for both the skin's permeability barrier and microbial barrier.

Therefore, these organelles are extremely important for both the skin's permeability barrier and microbial barrier.

pH and the skin – the low surface pH of the skin

Over the approximately 10-30 µm thick layers of the stratum corneum, the pH changes from the 7.0-7.4 that the rest of the skin has to be significantly lower on the surface, which varies between different areas of the body; but most often, the pH is around 5.0 on the skin's surface. This represents a substantial pH difference of 2 units (approximately 100 times higher H⁺ concentration on the surface compared to just 10-30 µm deeper in the skin). This gradient has e.g. been studied on normal healthy skin and compared to two different types of ichthyosis¹² (fish scale skin).
The approach used was to measure the pH on the surface using a pH meter and gradually remove the stratum corneum until reaching the stratum granulosum using tape (this technique with tape is well-known and called tape stripping). Between every tenth tape strip, the pH was measured, allowing the creation of a curve showing how the pH changes as one goes deeper through the stratum corneum. In healthy skin, the curve showed that the pH went from around 4.5 on the surface to approximately 7.1 at the stratum granulosum, with the pH being about 5.4 halfway through the SC. Thus, the change in pH was more abrupt in the deeper half of the stratum corneum, where the structure is also more compact. This is the area where many pH-dependent enzymes function.

Some other similar studies have shown that the pH in the outermost layers of the stratum corneum is slightly lower than on the surface and then gradually increases as one goes through the layers, reaching a pH of 7.0-7.4 when reaching the stratum granulosum.

'Acid mantel'

This phenomenon, where the surface of the skin is significantly more acidic than the rest of the skin, is called the acid mantle. This term was coined in 1928 by two researchers and has been used ever since, despite "mantle" probably not being the most accurate description. At that time, it was believed that the low pH was for protecting against microbial infection, but it has since been found to have much greater significance.

The acid mantle can be described as a natural mixture of various substances such as free fatty acids, amino acids, and other small acids, which ensure that the surface and the outermost layers of the stratum corneum have a relatively low pH. This will be described in more detail in the next section. Generally, the pH on the skin surface is between 4.0 and 6.0, with a few exceptions that have higher pH levels.

In the literature, several different pH values are reported as being the normal pH, but when comparing, one should be aware of where on the body the measurement was taken, and which measurement method was used.

An interesting study from 2006 showed that if nothing was applied to the skin on the inner forearm for 24 hours, the pH on average would decrease from 5.12±0.56 to 4.93±0.45. It was estimated that the "natural" pH of the skin in this area would average around 4.7.
The study also demonstrated that generally, skin with a pH value below 5.0 was in better condition than skin with a pH over 5.0; this was shown through measurements of various biophysical parameters such as barrier function, moisture level, flaking level, and resistance to induced irritation (e.g., using the skin-irritating substance Sodium lauryl sulfate). It was also observed that the "normal" skin microbiota adhered better to skin with a relatively low pH.

The relatively low pH of the skin's surface and the pH gradient through the stratum corneum have been found to have many important and often interconnected functions for the skin, which will be described here:

Enzyme Activity: Many enzymes' activity is dependent on pH. This applies to certain enzymes involved in building skin barriers and enzymes involved in breaking down corneodesmosomes, thus promoting desquamation (which should be in balance).
Two key enzymes involved in the formation of ceramides crucial for the skin barrier are pH-dependent: ß-glucocerebrosidase has a pH optimum¹³ at 5.6, and acid sphingomyelinase has a pH optimum at 4.5. If the pH goes significantly above or below these values, enzyme activities are reduced, leading to fewer ceramides being formed.
Other pH-dependent enzymes include phosphatases, phospholipases, and the enzyme group serine proteases, which includes kallikrein enzymes.
Serine proteases are enzymes that break down peptide bonds in proteins, including the proteins that make up corneodesmosomes, which bind corneocytes together, and thus, these proteases can inhibit the skin's integrity and cohesion.
Other serine proteases can inactivate lipid-processing enzymes, inhibit secretion from lamellar bodies, and stimulate epidermal hyperproliferation (a factor in conditions like acne). These serine proteases have a pH optimum that is slightly higher (for many of them around pH 7).
So, if the skin's pH increases, these enzymes become more active, while the two key enzymes involved in ceramide formation become less active.

An elevated pH can thus inhibit skin barrier functions, which is the next topic/theme where the skin's acid mantle is important.

Maintenance of the akin barrier: The extremely important barrier functions of the skin can be divided into different interconnected systems: the physical barrier, the chemical barrier, the microbial barrier, and the immune barrier.
Together, they provide physical, chemical, and biological protection to the body against external factors, and the physical-chemical barrier also prevents excessive water loss.

The physical permeability barrier, which is also vital for chemical and biological protection, primarily consists of the components of the stratum corneum, such as the hydrophilic corneocytes, the structures that hold them together, and the organized intercellular lipophilic matrix.
As mentioned earlier, the skin's pH is critical for several aspects of barrier functions: secretion from lamellar bodies of enzymes, lipids, and antimicrobial substances; as well as the activity level of enzymes responsible for the metabolism required to form intercellular lipids.

It is also believed that pH plays a crucial role in the organization of intercellular lipids.
Stratum corneum integrity – the desquamation balance: There is an important dynamic balance between the intercellular cohesion via corneodesmosomes and tight junctions and the natural and necessary breakdown of the same and thereby desquamation.
Here too, pH plays an essential role, especially due to the activity of pH-dependent enzymes, as described above.
Cytokine activation and inflammation: Corneocytes in the stratum corneum contain a reservoir of pro-inflammatory cytokine precursors (Pro-IL1α and pro-IL-1β).
If the skin barrier is disrupted, the pH normally increases, which enhances the activity of serine proteases such as kallikrein enzymes. This activation contributes to the release and activation of the cytokines IL-1α and IL-1β, which then initiate a cascade of reactions that help restore the barrier.

Thus, a temporary increase in pH aids in barrier restoration, but if the pH remains elevated for an extended period, it may lead to inflammation mediated by cytokines. Conversely, a reduction in pH is believed to reduce inflammation.
Skin microbiota and microbial barrier: The skin has a mutual symbiotic relationship (where both parties benefit from the relationship) with the skin microbiota: the skin provides the right environment for certain microorganisms, while these, in turn, contribute to the skin's microbial defense by e.g. inhibiting the colonization of other (e.g., pathogenic) microorganisms and also helps the skin in other ways.
The low pH on the skin's surface contributes to the skin being a suitable medium for the "normal" resident microbiota and has been shown to inhibit the growth of certain pathogenic microorganisms.
In addition to the skin's microbial barrier, as mentioned before, pH affects the release of certain antimicrobial substances such as antimicrobial peptides from lamellar bodies. The activity of these substances, such as antimicrobial peptides like cathelicidin and dermicidin, as well as cationic substances and nitrates found in sweat, is also pH-dependent and optimal at pH 5.5.

The conversion of nitrite, which is produced by bacteria in the microbiota from nitrate in sweat, into nitric oxide also occurs only at a weakly acidic pH. Nitric oxide performs several essential functions, not only in terms of maintaining the microbial balance on the skin but also as an intra- and extracellular signaling molecule, playing a significant role in wound healing.

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¹²Ichthyosis (fish scale skin) is a collective term for a number of different forms of the disease, which manifest as dry and scaly skin.

¹³pH-optimum is the pH value at which the enzyme has the highest activity.

pH and the skin – endogenous mechanisms and factors for skin pH

The endogenous mechanisms and substances that maintain the low pH on the skin's surface and the pH gradient in the stratum corneum are a complex subject. Different scientific articles place more or less emphasis on various mechanisms and substances, and it is not entirely clear which ones are most significant. Most likely, these different mechanisms interact with each other, and both the mechanisms and substances likely have varying importance in different layers of the stratum corneum.
Regarding the substances that constitute the acid mantle and thus control the pH in and on the stratum corneum, it is believed that Alpha hydroxy acid (AHA¹⁴) such as Lactic acid e.g. found in sweat and fatty acids from sebum, along with Urocanic acid (UCA), Pyroglutamic acid (PCA), and certain amino acids, are the primary sources of stratum corneum pH.
Amino acids resulting from the breakdown of the protein filaggrin, and cholesterol sulfate are thought to have some influence on the pH in the deeper layers of the stratum corneum.

Another essential component in the acidification of the deeper layers of the stratum corneum is the plasma membrane protein, NHE1, located in the cell membrane of keratinocytes. NHE1 is a Na⁺/H⁺ antiporter that can pump a hydrogen ion (H⁺) out of the cell while simultaneously transporting a sodium ion (Na⁺) into the cell, thereby regulating the pH inside the cell and contributing to reducing the pH in the intercellular (extracellular) space. More specifically, NHE1 is believed to form extracellular microdomains¹⁵ with a relatively low pH in the deeper part of the stratum corneum, close to the stratum granulosum, which generally has a pH around 7.0-7.4.

These microdomains with a relatively low pH are thought to be crucial for activating the pH-dependent enzymes that process the lipids secreted from lamellar bodies and contribute to the formation of the barrier-creating organized intercellular lipid matrix.
NHE1 is also believed to be important for cell differentiation of e.g. keratinocytes, and is a factor in wound healing by regulating pH at the wound surface.

The acidifying lipids, such as cholesterol sulfate and free fatty acids, are also believed to be contributing factors to the pH gradient in the stratum corneum. Free fatty acids can be released from phospholipids secreted by lamellar bodies, a process catalyzed by the enzyme group PLA2, which is pH-dependent group of phospholipases that has an optimum in the slightly acidic range of the pH scale.

Certain amino acids and amino acid-derived substances also play a role in the pH of the stratum corneum. There are, for example, amino acids in sweat and a very important source is the breakdown of the filaggrin protein.
The breakdown of filaggrin results in amino acids such as Glutamic acid, which can be converted to Pyroglutamic acid (PCA), and the amino acid Histidine, which can be converted to Urocanic acid (UCA).
Both PCA and UCA help reduce pH and are also moisturizing agents as they are part of the natural moisturizing factors (NMF)¹⁶.

Studies in animal experiments, particularly focusing on the filaggrin-histidine-urocanic acid pathway, suggest that the breakdown to UCA is not essential for pH regulation, as compensatory mechanisms can take over and regulate pH.
Small acids, such as Lactic acid and Butyric acid, are also believed to lower the pH in the stratum corneum. These acids are found, for instance, in sweat from the eccrine glands, which are located all over the body. The sweat from these glands¹⁷ is directly secreted to the skin's surface, has a pH of 4.0-6.8, and primarily consists of water, along with low concentrations of small electrolytes, small acids such as Lactic acid, Citric acid, Ascorbic acid (vitamin C), Urea, amino acids, and fatty acids.

Melanin from melanosomes in the stratum granulosum is also believed to contribute to pH reduction and partially explain why more pigmented skin generally has a lower pH (approximately 0.5 pH lower).

Finally, the skin's microbiota can also contribute to the relatively low pH on the surface of the stratum corneum.

More factors for the pH-value of the skin

There are many endogenous (internal) factors that affect the skin's pH. The primary ones include the anatomical skin area, skin moisture (both elevated and low moisture levels are associated with increased pH), pigmentation level (darker skin generally has a lower pH), sebum level, sweat level, skin diseases, genetics, age, and gender - the latter is still debated. Most of these factors will be discussed in the following sections:

The pH of the skin varies significantly on different areas of the body. When considering the overall body, a 95% interval for pH falls between 4.1 to 5.8, with an average of 4.9. The primary areas that lie outside of this interval are semi-occluded and typically relatively moist regions such as the armpits, groin, near the genitals, between the toes, and in skin folds, where the pH is generally higher, around 6.1 to 7.4¹⁸. Here are some examples of typical pH intervals for various skin areas found in healthy adults: forehead and eyelids: 4.7-5.1, cheeks and inner forearm: 5.1-5.5, chin: 5.4-5.7, armpits: 5.8-6.8, and groin: 6.2-7.1.

The meaning of age

Age significantly influences the skin's pH. Very young and older skin generally have a relatively higher pH – and lower buffer capacity. Individuals between 18 and 60 years of age usually have a relatively stable skin surface pH. Newborn infants (non-premature) have a pH around 6.0-7.0, and this pH is fairly uniform across all skin areas. The skin's pH drops quite sharply during the first days after birth and more gradually in the following months. After 4-6 months, the skin of infants typically reaches the "normal" range similar to that of adults, with different pH levels in different skin areas. Especially for babies wearing diapers, the pH in the diaper area is relatively high, especially due to the occlusive and moist environment created by the diaper. This makes the skin more vulnerable.

Elderly individuals around 60-70 years old generally exhibit an increasing skin surface pH and a reduced buffer capacity. Some of the reasons for the higher pH in older individuals are thought to be a lower expression of NHE1 and reduced conversion of phospholipids into free fatty acids, as well as a reduced rate of degradation of filaggrin into NMF, including UCA and PCA. Additionally, the production of sebum and sweat decreases, further reducing the skin's buffer capacity and the supply of acids found therein. The higher pH is also associated with lower production of epidermal lipids such as ceramides, cholesterol, and fatty acids, as well as changes in the skin's microbiota, collectively resulting in a weaker skin barrier.

Skin pigmentation and pH

As mentioned, the skin's pigmentation level also influences its pH. Compared to lighter skin, skin with higher pigmentation level and, therefore, lower pH has shown increased lipid production and lamellar body density, as well as better stratum corneum integrity, barrier function, and faster barrier restoration after tape stripping or other forms of superficial skin damage.

It is known that after a disruption of the permeability barrier, there is a rapid increase in lamellar body secretion from the stratum granulosum, which aims to replace the lost components, and new lamellar bodies are formed quickly. In a study comparing individuals with light skin (Fitzpatrick scale type I-II¹⁹) to individuals with darker skin (Fitzpatrick scale type IV-V), it was observed, among other things, that if the surface pH of the light skin was reduced to match the pH of the dark skin by applying a vehicle containing Lactobionic acid or Gluconolactone (PHA substances²⁰) immediately after tape stripping, the rate of skin barrier regeneration was significantly increased after 1, 6, and 24 hours compared to light skin treated with the vehicle alone (the same product but without Lactobionic acid and Gluconolactone) or the same vehicle with neutralized Lactobionic acid or Gluconolactone.

Gender and pH - is there a correlation?

Some studies suggest that skin pH has a weak correlation with gender, but there is no clear consensus, as some studies suggest that women have the lowest pH and other studies suggest that men have the lowest pH.
Overall, some studies indicate that men tend to have slightly lower pH compared to women, but not significantly lower.

Studies comparing the skin of both genders suggest that the skin's barrier function (measured in terms of TEWL²¹) in men under 50 years of age is better than in women of the same age, regardless of the skin area being measured. This difference in skin barrier reduces with age.
Stratum corneum moisture level appears to remain stable or slightly increase with age in women, while it decreases from around the age of 40 in men.
The skin's pH also follows a circadian rhythm, with the lowest pH observed at night and the highest in the afternoon. It also exhibits an annual rhythm, with pH generally being slightly lower in winter compared to summer (whether these fluctuations are endogenous or exogenous factors can be debated).

¹⁴You can read more about AHA in the description of AHA, BHA, and PHA on this website.

¹⁵Such microdomains cannot be measured with a pH meter with a glass electrode, and therefore, the pH in the deeper part of the stratum corneum is often measured to be around neutral.

¹⁶NMF is briefly described in the description of Glycerin on this website.

¹⁷There are also apocrine sweat glands, which are connected to a hair follicle, and thus, this sweat is normally mixed with sebum from sebaceous glands as they also have an outlet in the hair follicle. Apocrine sweat glands are primarily found in the armpits and genital areas, and their secretion has a pH of 6.0-7.5. It consists of water, proteins, carbohydrates, some of the body's waste products, lipids, and sterols. It is a relatively viscous fluid, which is itself odorless, but some of these substances are metabolized by the microorganisms on the skin, and their metabolites contribute to body odor.

¹⁸Due to the relatively high pH in places like the armpits, the microbiota there is different, which contributes to the body odor formed from their metabolism of the secretion from the apocrine glands.

¹⁹The Fitzpatrick scale is a scale from I to VI, which indicates how pigmented the skin is and how it reacts to UV exposure.

²⁰PHA is the abbreviation for Poly Hydroxy Acid. You can read more about PHA in the description of AHA, BHA, and PHA on this website.

²¹TEWL is teh abbreviation for Trans Epidermal Water Loss

pH and the skin – exogenous factors affecting skin pH

The exogenous (external) factors are, for example, whether the skin is occluded (e.g., with gloves), the skin's microbiota (which can also be influenced by endogenous factors), climatic factors, and the substances and products the skin is exposed to.

Occlusion of the skin increases the skin's pH - this has been known since the 1970s - and this has several consequences.
It has been shown that after five days of occlusion of the skin on the forearm of healthy individuals, the pH increased from 4.38 to 7.05, the skin's microbiota changed significantly, and the skin's transepidermal water loss (TEWL) increased threefold (impaired permeability barrier). Even three days of occlusion significantly raised the pH and returned to normal only after a day.
The skin's microbiota both affects - through the formation of metabolites - and is affected by the skin's pH. There is, therefore, a relatively complex interaction between the skin and its microbiota, which is the subject of ongoing research.

Products applied to the skin can of course also affect the skin's surface pH and, in some cases, the skin's buffering capacity. For example, classic soap with high pH can increase the skin's pH, and similarly, leave-on products that remain on the skin can also affect the skin's pH and buffering capacity - in both directions.
Tap water can also influence the skin's pH. In Europe, there is a considerable variation in the pH of water - for example, in Denmark, it is around 6.5-8.0, while the pH of groundwater generally becomes lower the further north you go and higher the further south you go in Europe - but still within the range of 5.5-8.5.
Studies have shown that washing the skin with tap water alone can increase the skin's surface pH for about four hours.

pH and the skin – the skin’s buffering capacity

As mentioned earlier, the buffering capacity of a system refers to its ability to withstand significant pH fluctuations despite external influences.

Studies have found that the skin has a reasonably good buffer capacity between pH 4 and 8. It is believed that this buffer capacity comes from various buffer systems in the skin, and there is ongoing debate about which systems contribute the most.
Studies suggest that components in sweat contribute to the buffer capacity, and some studies point to the Lactic acid / Lactate and Carbonic acid / Bicarbonate buffer systems as possible contributors.

Lactic acid, which is found in sweat, has a pK_a of 3.8, which is on the lower end compared to the pH of the stratum corneum, and recent studies suggest that the Lactic acid / Lactate buffer system is not the primary buffering system in the stratum corneum. The Carbonic acid / Bicarbonate buffer system also does not seem to play a significant role in the skin's buffering system.

Some older studies have worked with the hypothesis that sebum contributes to the buffer capacity by protecting the epidermis from external acids and bases - simply by inhibiting the penetration of substances from the outside and thus reducing their impact on the skin. This part is believed to be correct.

It was also thought that the fatty acids in sebum contributed to the buffer capacity, but nowadays, it is considered negligible. Keratin has also been considered as a possible component of the buffering capacity, but there is still no evidence for this.

Recent studies suggest that it is primarily amino acids that provide the skin's buffering capacity.
However, the exact amino acids that contribute are not yet known, but it could be amino acids in sweat from the eccrine sweat glands. This sweat contains about 0.05% amino acids. It could also be amino acids from the breakdown of proteins in the skin, such as proteins in desmosomes and filaggrin, as well as from hair follicles.

There is no doubt that normal healthy skin has a reasonably good buffer capacity within the skin's normal pH range, but more studies are needed to determine which buffer systems contribute the most to this buffer capacity.

pH and the skin – skin problems and diseases

Several skin problems, skin diseases, and wound healing are associated with an elevated pH value in the skin's stratum corneum. For the skin diseases that have been studied, it is not clear whether it is the skin condition that results in the pH increase or if the pH increase contributes to the development of the disease.

Many skin problems are linked to inflammation in the skin, and generally, inflammation in the skin is associated with an increase in pH. Dry and sensitive skin is also often associated with a slight increase in pH on the skin's surface.
In general, a compromised skin barrier is often associated with skin problems, and as it appears, pH plays a significant role in the skin's barriers and their maintenance. Several studies have investigated whether reducing the skin's surface pH can alleviate and improve the skin's condition. The following will present several relevant skin problems and associated studies.

Ichthyosis

Ichthyosis (fish scale skin) is a group of diseases characterized by dry and scaly skin with an increased pH on the surface. It is associated with reduced functional filaggrin (due to mutations in the filaggrin gene), which is a crucial component in the structure of the stratum corneum, the organization of the intercellular lipid matrix, and, not least, maintaining the skin's important moisture levels by contributing to several components of the Natural Moisturizing Factors (NMF). The increased pH contributes to the desquamation process not functioning as it should.

Psoriasis

Psoriasis, like ichthyosis, is a hereditary skin disease characterized by well-defined, scaly rashes and is often associated with a slight increase in pH on the skin's surface. While this disease has been extensively studied, the significance of pH in relation to psoriasis has not been extensively explored. Changes in skin cell differentiation, skin barrier, and inflammation are known to play crucial roles in the pathogenesis of psoriasis, and pH is believed to be a factor in this process.

Candida Intertrigo

Candida Intertrigo is a fungal infection of the skin - typically occurring in areas where skin touches skin - characterized by shiny, red, and itchy skin. Some studies have shown a correlation with higher pH on the skin's surface. This skin disease is also associated with diabetes and patients undergoing dialysis.
In an experiment with healthy individuals, a solution of the fungus Candida albicans in a buffer solution with pH 6.0 or 4.5 was applied to healthy skin under occlusion, which showed that this fungus did not thrive as well in an acidic environment. This indicates that higher pH on the skin increases the risk of this fungal infection.

Acne

Acne is associated with inflammation in the skin, increased growth of certain strains of Cutibacterium acnes (formerly known as Propionibacterium acnes), and increased pH on the skin's surface.
In a study involving 200 acne patients and 200 individuals without acne (equally distributed between men and women aged 15-30), pH was measured on the forehead, nose, cheeks, and chin, showing a significant difference: the average pH for individuals without acne was 5.09±0.39, while the average for acne patients was 6.35±1.3. The higher pH is believed to be advantageous for the growth of the bacterium Cutibacterium acnes.

Wounds

Wounds are also associated with increased pH. Open wounds have a pH around 6.5-8.5, while problematic chronic wounds have a pH around 7.2-8.9. Wound healing is a complex process, and the surface pH will change during the healing process.
During the healing process, pH needs to decrease for several crucial processes to occur, such as fibroblast proliferation, collagen formation, macrophage activity, and keratinocyte differentiation.

In a study, it was investigated whether pH could be a tool in diagnosing the wound healing process and thereby help assess what kind of treatment a wound should have, such as antibiotics (if there is a bacterial infection in the wound). However, more research is needed. Some studies have shown that certain strains of bacteria relevant to wound infections have a greater tendency to form biofilm²² at higher pH levels. It has also been found that in certain situations, treating wounds with a topical²³ product with low pH can have a positive effect on wound healing, probably by e.g. increasing the antimicrobial activity of certain substances on the skin's surface and regulating the activity of certain enzymes.

Eczema

Eczema, such as atopic dermatitis, contact dermatitis, and diaper dermatitis, is associated with inflammation in the skin and increased pH.
In atopic dermatitis, a reduction in active filaggrin is often seen, which, as mentioned earlier, is crucial for the skin barrier and moisture levels. In the diaper area, the skin typically has elevated pH, which can contribute to the activation of protease and lipase enzymes, among others, thereby impairing the skin barrier and contributing to the development of eczema.
Regarding atopic dermatitis, mouse models of the disease have been extensively studied. For example, studies have examined whether treatment with topical products with a relatively low pH (e.g. containing Lactobionic acid - a PHA) can alleviate symptoms, and the results suggest that they can.

It is generally known that maintaining a normal skin pH with suitable topical products can improve the skin's condition. A study with this mouse model even suggests that maintaining a slightly acidic stratum corneum can inhibit the development of atopic dermatitis. Similar experiments have been performed on newborn infants and the elderly and on rats, which showed that topical use of products with a relatively low pH containing, for example, PHA or AHA can normalize the skin's pH and barrier function.

In a study involving people with mildly dry skin, a product with a pH of 3.7-4.0 containing 4% Lactic acid (an AHA) was applied twice a day for 4 weeks. This resulted in a significant improvement in stratum corneum ceramide concentration, barrier function, and reduced sensitivity to Sodium lauryl sulfate-induced irritation. Regarding how long a product with low pH can act on the skin, it depends on the product's composition and its buffering capacity and the skin's condition.

In a vehicle-controlled study with humans, researchers tried to investigate this using a cream with Acetic acid or Hydrochloric acid at pH 3.5. They observed that immediately after application, the pH dropped and then slowly rose again after 15 minutes, but a relatively low pH was maintained for up to 6 hours after application.

²²Biofilm is a thin coating or film of bacteria embedded in a special matrix that the bacteria themselves produce. Biofilms can be found in many places and can cause problems when, for example, they are present in a wound, as it makes the bacteria more resistant to various interventions such as high or low pH and antibiotics.

²³Topical use refers to using a product by applying it to the body's surfaces; thus, all cosmetics are used via topical administration.

pH and topical products

The products we use on our skin can of course influence the skin's pH to a greater or lesser extent. As stated above, it is often a too high pH on the skin that is associated with skin problems, and therefore many studies have looked into whether topical leave-on products can reduce the skin's pH and thus improve the skin barrier function - and many studies show that it is indeed possible.

Similarly, many studies have focused on how different types of products affect the skin's pH, with particular attention to cleansing products, as some of them can increase the skin's pH. In this connection it is important to remember that the skin normally has a reasonably good buffering capacity, and thus it will normalize the skin's pH after some time. Moreover, the impact of products on the skin can be complex and depends on the precise composition of the product, how the product is used, and, not at least the condition of the skin. Therefore, it is not only the pH of the product that determines its effect on the skin's pH.
An important factor is also the product's buffer capacity - which is rarely known and is only rarely investigated in the studies that specifically examine how products can affect the skin's pH. The studies that have been conducted are seldom easy to compare due to many variations - such as experimental methods, test subjects' skin, the time after application when pH is measured, etc. Also, please note that the pH on the skin after the application of a product is the result of both the product's and the skin's pH as well as the product's and the skin's buffer capacity.

Cleansing products

Cleansing products are a large group of very different types of products that can affect the skin in several ways - most of them will increase the skin's pH - for many of them because they are washed off the skin with water (rinse-off products), and as mentioned, water itself can increase the skin's pH, but normally not for a long time, mainly because water has almost no buffering capacity.

Classic soap, which has been used for over a thousand years, contains naturally derived surfactants (surface-active substances, also often called detergents or tensides), which are produced by the saponification reaction of fats. Such soaps are usually alkaline with a pH between 8.0 and 11.0, and therefore it is not surprising that these can increase the skin's pH.
Studies with such classic soaps have shown that the skin's pH typically increases by about 2 units and does not return to the normal pH level within 6 hours.
Compared to using only water to wash the skin, the classic soap prolongs the time it takes for the skin to return to its normal pH level, and in addition, like many other cleansing products, classic soap can remove a lot of lipids from the skin - which is believed to make the skin more vulnerable and susceptible to irritation.

Studies have shown that adding lipids to cleansing products can reduce the interaction between the surfactants in the cleansing product and the lipids on the skin, thus reducing this effect.

Surfactants

Around 1950, a new type of surfactants called "syndet" was invented, which is short for synthetic detergent, and since then, many more and milder - compared to the syndet surfactants - more natural surfactants, have been invented.

With these surfactants, it is possible to produce cleansing products with a pH value similar to the pH of the skin. They are often marketed as being much milder for the skin (which many of them are) compared to the classic soap. But here, it is important to look at the entire chemical composition of the products and what they can do to the skin besides changing the pH.

In a study from 2014, an attempt was made to compare the skin on the inside of the forearm of two groups of healthy people who had used either classic high-pH solid soap or a cleansing product with a pH close to the skin's pH for over 5 years. It was found that the use of classic high-pH solid soap did not affect the skin's pH-regulating ability - buffering mechanisms. It was also found that the skin's pH increased almost equally with the use of both products, and for both groups, the skin's pH returned to normal after about 6 hours.

Does the pH-value of a product determine if it is mild to the skin?

Another interesting study has looked more closely at the claim that cleansing products with a pH close to the skin's pH would be better for the skin.

In this study, a series of measurements were performed on a group of healthy people's skin on the inside of the forearm after using a range of different cleansing products in the form of syndet bars and a few liquid soaps with a known composition (some of them only qualitatively) and known pH - all based primarily on anionic²⁴ surfactants, which are used in most cleansing products today.

Among other things, they examined skin dryness and barrier (TEWL measurement). This study showed that the use of a cleansing product based primarily on anionic (negatively charged) surfactants with a pH close to the skin's pH increased skin dryness and irritation level more than the same formulation adjusted to pH 7.0. The possible explanation for this is related to increased electrostatic interaction (negatively charged ions will interact with positively charged ions) between anionic surfactants in the cleansing product and the stratum corneum at low pH compared to neutral pH.
The more in-depth and technical explanation is as follows: The isoelectric point of the stratum corneum is around pH 4.0. At the isoelectric point, the surface of the stratum corneum will have almost equal numbers of positive and negative charged ions and thus have a net charge of approximately 0. At pH above the isoelectric point, the surface of the stratum corneum will have an excess of negative charged ions, and at pH below the isoelectric point, there will be an excess of positive charged ions.
So, if the skin is exposed to a solution (cleansing product) with a pH above 4.0 (e.g., neutral pH), there will be relatively fewer positive ions on the surface of the stratum corneum for the solution's anions (negatively charged substances) to interact with. But if the solution's pH is lower and closer to or below the isoelectric point of the stratum corneum, the number of positive charged ions on the stratum corneum will be relatively high, and thus the number of anionic substances in the solution (surfactants in the cleansing product) will have more ions on the stratum corneum to bind to.
When more surfactants bind to the skin, it can be expected that they are more difficult to wash off, and thus the surfactants can stay on the skin for a longer time, causing irritation, dry skin, and affecting the skin barrier.

The conclusion was that pH alone does not determine whether a product is mild or not - one must look at the entire composition of the product and the interaction between the substances used in the product and the stratum corneum at the specific pH.

Sweat odor, deodorants and the pH-value of the skin

The fact that products can affect the pH of the skin is interesting in relation to deodorants and the often-unwanted body odor from the armpits. The higher pH in the armpits is one of the reasons why certain microorganisms thrive there. The metabolism of these microorganisms is the cause of body odor. Therefore, there have been investigations into whether deodorants that can lower the pH in the armpit can also reduce the odor.

Results from a study showed that with daily use of specific deodorants with a pH of 5.0, the pH of the skin in the armpit could be reduced for a minimum of 2-4 hours and the body odor as well. The pH returned to its original level two days after the last application.

Beneficial for the skin

Reduction of the skin's pH and the general use of leave-on products with a pH close to the skin's pH are also interesting in other contexts, particularly for older skin and skin with atopic dermatitis. Some studies show that older skin, which typically has a slightly higher pH and a compromised skin barrier, benefits from the application of products that do not have a pH that is too high.

Studies suggest that the effect takes time and becomes apparent after longer periods of daily use. One of these studies involved 20 older individuals in their 60s who used either a specific emulsion adjusted to a pH of 4.0 or pH 5.8. After 4 weeks, the skin's pH was significantly reduced when the product with pH 4.0 was used, while the results for skin moisture levels and trans-epidermal water loss (TEWL) were not significantly different for the two emulsions.
Both emulsions increased the skin's overall lipid content, with the product at pH 4.0 being slightly more effective in this regard. After the 4 weeks, the skin was challenged with the irritating substance Sodium lauryl sulfate – this experiment showed that the skin treated with the emulsion at pH 4.0 was more resistant.

A meta-study on relieving dry skin, itching, and improving the overall skin barrier showed that leave-on products with a pH of 4.0 can generally enhance the skin barrier in older skin. Similarly, treating atopic dermatitis with relatively low pH leave-on products appears to be beneficial for the condition. Overall, it is observed that normalizing the skin's pH through topical products in certain cases can help establish a more balanced skin microbiota, improve the skin barrier, induce epidermal differentiation, and reduce inflammation in the skin.

²⁴An anionic molecule has a negative charge, a cationic molecule has a positive charge, an amphoteric molecule has both a positive and a negative charge, and a non-ionic molecule has no charge.

pH-variation in Batch

Different types of cosmetic products are often formulated within specific pH ranges. There are no specific rules for the pH of a cosmetic product or how much its pH may vary from batch to batch. Manufacturers of products for particularly sensitive areas such as the eye area and vagina usually choose a pH close to the area's normal pH. Cosmetic products must, of course, be safe to use.

Therefore, very low and very high pH values are usually reserved for special product types (for example, some hair straightening products have a very high pH). There can be several reasons why the pH of a product may vary slightly from batch to batch. For instance, there may be pH variations in the raw materials used, and the production method can make it challenging to readjust the pH (for example, if a product needs to be filled into packaging while in a hot state). Not all manufacturers readjust the pH (after all other raw materials have been mixed together), when, for example, it comes to products that are not particularly sensitive to pH variation in terms of effectiveness and stability, and/or the manufacturer has found that the formulation's pH does not typically vary much from batch to batch. Generally, cosmetic manufacturers ensure that the pH of a given product is within a relatively narrow range – typically within 0.5-1 pH unit.

PUCA PURE & CARE also adheres to this practice, and in most products, the Citric acid and Sodium hydroxide are used to adjust the pH of water-containing products.

Vitamin A Retinol Eye Cream | PUCA - PURE & CARE

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