Deciphering the skin barrier

It is only the interaction of individual functions of the epidermis that together form the concept of the skin barrier. 

Functions

The epidermis, whose functional structure is described in detail in numerous textbooks¹, serves to protect against:

1. solids, liquids and gases,
2. microorganisms,
3. radiation and
4. mechanical and temperature-related influences.

In summary, we no longer talk about environmental influences, but rather about the exposome, which is prevented by the skin barrier from causing damage to the body.
It is well known that the epidermal protection against the exposome is not 100% effective. Accordingly, attempts have always been made to help nature along with skin protection and skin care. In modern times, however, ideas about "how" this should be done have changed. But even today, there are still different concepts circulating that result from studies, practical experience, beliefs or marketing considerations.

Skin protection

In general, only substances with a molecular mass of up to approximately 500 daltons can penetrate the epidermis from the outside and, if necessary, enter the bloodstream (permeation)². Lipophilic substances penetrate better than hydrophilic ones, and uncharged substances better than ionic (salt-like) ones.
The atmospheric gases oxygen, nitrogen and carbon dioxide, with their low masses, can penetrate well, but are only present in small quantities in the upper layers of the epidermis.
Oxygen behaves inertly in this environment as long as it is not activated by external radiation and generates radicals. At low levels of exposure, little happens at first, as the NMF (Natural Moisturising Factor), provided it is not damaged, safely intercepts the radicals.³ This applies equally to smog peroxides and to radical atmospheric nitrogen oxides (NOx), which are neutralised by the amino acids of the NMF, among other things, through the formation of alpha hydroxy acids.
Increased exposure to radiation can lead to inflammation such as sunburn, in addition to the formation of radicals and peroxides. In this case, it is advisable to use skin protection products containing UV filters that are able to convert radiation into heat before the formation of radicals is induced.
However, the problem for the skin barrier today is less the external natural substances than the culturally driven surfactants found in shampoo, liquid soap, kitchen cleaners, etc., which, in addition to dirt, remove substances such as fatty acids, ceramides and cholesterol from the lamellar barrier layers (bilayer) of the skin and leach out the NMF. They open the way for skin-damaging substances from the environment. 
It is a misconception that NMF can be imitated by applying only hygroscopic substances, i.e. substances that strongly bind water, such as body lotion, after a cleansing process. The substances must also be able to destroy radicals in the same way as NMF amino acids. 
It goes without saying that solvents, corrosive and other hazardous substances must be kept away from the skin surface by measures such as wearing gloves; at the same time, however, the natural exchange of substances (TEWL, sweat, etc.)⁴ must not be impeded.
The issue of nanoparticles, i.e. solid particles smaller than 100 nm, has been regulated in skin care by bans and mandatory declarations by the legislator (Cosmetics Regulation).⁵ Other areas such as titanium dioxide⁶ in emulsion paints followed, although penetration into the bloodstream has not yet been conclusively proven.
In occupational skin protection, the aspect of regeneration during non-working hours is now becoming more important than 100% occlusive protection during working hours.

Microorganisms

The Dalton rule also applies to external microorganisms, of course. This means that their size prevents them from penetrating the skin. In principle, they can only "eat their way through", but they are prevented from doing so by the skin's own microbiome, unless this is weakened by surfactants, for example. This means that the microbiome should be kept as intact as possible, similar to the lamellar layers and the NMF. 
Not all ingredients in skin care products are good for the microbiome. Counterproductive preservatives, high doses of antioxidants and chelating agents have often been reported in this regard.
Many higher molecular weight skin care substances are, so to speak, "pre-digested" by the microbiome. Prominent high molecular weight ingredients include hyaluronic acid, collagen and triglycerides from natural oils, for example. Their fragments are then small enough to be absorbed by the epidermis and, if necessary, produce specific effects.
The strategy for maintaining this sensitive society is to avoid disrupting its balance and thus prevent facultative pathogens from multiplying at the expense of the benign ones. This reduces the risk of infections involving external populations. 
Problems also arise when the epidermal antimicrobial peptides (AMP), which are, so to speak, on the front line of the microbiome, are disrupted in terms of their formation or degradation. This leads to inflammatory symptoms. The rapid degradation of AMP by endogenous proteases results in rosacea and perioral dermatitis (POD), for example. 

Radiation

As already indicated, the skin barrier has protective mechanisms in place to destroy radicals that arise from external radiation. In addition, there are UV-absorbing substances such as the skin's own urocanic acid, which is structurally very similar to the cinnamic acid derivatives of older organic filters. The most effective broad-spectrum UV filter is melanin, but this is not immediately available when modern humans are first exposed to intense sunlight on holiday. 
In earlier times, people were outside when the first rays of sunshine appeared in spring and were able to gradually build up their melanin protective shield day by day. At the same time, they could rely on their vitamin D balance being correct and did not need any dietary supplements.
So, when it comes to protecting yourself, optimal conditions are important:

• The sun protection factor (SPF) should not be higher than is actually necessary.
• Melanin synthesis is then further stimulated by the residual radiation and
• vitamin D synthesis is not interrupted.

This is, of course, a balancing act that does not appeal to the overly cautious among us. But physiologically speaking, it is the most sensible solution, which even allows you to gradually reduce external protection during a longer holiday and ultimately save money.

Forces & temperature

Physical influences such as mechanical stress or low and high temperatures are only cushioned by the skin surface to a certain extent. If this is not sufficient, there is a risk of injury from shear forces, frostbite or burns. 
In all cases, protective clothing can be used as a precaution. The alternative is to literally develop a "thick skin", i.e. a layer of calloused skin. As with radiation, this happens almost automatically when the skin slowly becomes accustomed to it. Some extremists walk around barefoot and without outer clothing all year round. Swimming in open water all year round has a similar effect.

Corneotherapy

Last but not least, perhaps one more tip. When it comes to medical skin conditions, the associated problems can often be managed without topical medication. With a little patience, corneotherapy can have a long-term effect without side effects. However, one should not expect the rapid effect that is characteristic of cortisone. The approach is very simple and is called "outside-in", i.e. if the barrier is brought into a natural order, then exposome-dependent problems in deeper skin layers also come to a halt.⁷
Like the keyword "skin barrier", "corneotherapy" is not yet to be found in Wikipedia, even though knowledge about the mechanisms in the skin barrier has increased enormously since the first publication by A. M. Kligman.⁸

Literature

1. https://de.wikipedia.org/wiki/Epidermis_ (vertebrates), accessed on 14 October 2025
2. J. D. Bos, M. M. Meinardi, The 500 Dalton rule for the skin penetration of chemical compounds and drugs, 2000; 9 (3):165-9
3. W, Dröge, Free Radicals in the Physiological Control of Cell Function, Physiol Rev 2002 (82), 47-95
4. H. Lautenschläger, Natürliche Hautpflege: Von Sebum bis Schweiß, Medical 2025 (4), 20-23
5. https://eur-lex.europa.eu/legal-content/DE/TXT/HTML/?uri=CELEX:52021DC0403, accessed on 14 October 2025
6. H. Lautenschläger, Titandioxid – der Weißmacher, Beauty Forum medical 2022 (3), 16-17
7. Lübbe J, Evidence-Based Corneotherapy, Dermatology 2000 (200), 285-286
8. Lundborg M, Narangifard A, Wennberg CL, Lindahl E, Daneholt B, Norlén L, Human skin barrier structure and function analysed by cryo-EM and molecular dynamics simulation, J Struct Biol 2018 (203), 149–161

Dr Hans Lautenschläger

published in
Beauty Forum
2025 (12), 76-78