Dry Skin

The most important function of our skin is to prevent valuable substances (e.g., body fluids and electrolytes) from being lost from the inside to the outside, and to prevent harmful substances, such as irritants or toxins, from penetrating from the outside in. To fulfill these functions, the skin uses its own building blocks to build a barrier that is permeable to some substances (e.g., body fluids that cool us on the skin's surface as sweat) and impermeable to others. Such a barrier is called semipermeable. With its barrier, the skin ensures the cohesion of our body and thus safeguards our integrity.

Since our body is predominantly made up of water and the environment predominantly consists of more or less dry air, the skin, which borders or seals our body to the outside, has to protect against this sudden jump in moisture from 'moist inside' to 'dry outside' so that we do not literally 'leak'. To do this, the skin builds up several cell layers that are initially relatively moist, i.e. have a high water content, and then become increasingly drier towards the outside as the water content decreases. At the very outermost or top of our skin lies the last layer, the stratum corneum, which consists of dead skin cells of the epidermis. These gradually peel off and flake off. As a result, the skin loses many millions of cells every day, each at a different time, i.e. asynchronously and without us noticing (desquamatio insensibilis). If the upper skin cells were to shed all at the same time, i.e. synchronously, this would be noticed by a shedding of skin - as happens in snakes, for example (desquamatio sensibilis).

To ensure that the described barrier, composed of layers of skin cells at different stages of development, is solid yet not rigid, but can function as the mobile, flexible, and finely regulated skin we know today, it is constructed to be both solid and mobile, which is one of the most wonderful achievements of evolutionary biology. To achieve this, the barrier must smoothly manage the moisture transition between moist (inside) and dry (outside).

The skin must be dry in this outermost layer, but not too dry, otherwise it would lose mobility and crack, and cracks would damage the barrier
(1) destroy and
(2) are painful because they represent small wounds that also heal poorly.

In addition, inflammations, so-called drying eczema (exsiccation eczema), often develop on skin that is too dry.
(1) tend to itch (itching is a phenomenon of dry skin), which
(2) feel unpleasant and cause discomfort (dysesthesia), and the
(3) are often colonized by germs, as eczema is often wet and thus forms a good breeding ground for germs.

Dry skin is caused by:
(1) genetic predispositions (e.g. in neurodermatitis (atopic eczema))
(2) low sebaceous gland activity
(3) increasing skin aging
(4) incorrect care
(5) lots of water contact
(6) frequent use of cleaning products (containing so-called surfactants)
(7) dry indoor air (favoured by poorly ventilated and overheated rooms)
(8) dry outside air (favoured e.g. by cold)
(9) Sun exposure
(10) Low fluid intake (too little drinking)
(11) Inflammations (e.g. eczema)

Healthy skin protects itself from dehydration by a densification of dead cells in the outermost layer, which adhere relatively tightly together and are more or less compact (the stratum corneum). We have calluses on every part of our skin, even on our eyelids. The calluses are particularly thick in certain mechanically stressed areas, such as the palms of the hands and the soles of the feet. This callus, which can be imagined as a stone wall (the stones are the dead skin cells (corneocytes), and the mortar between the stones represents all the substances and molecules that hold the dead cells together for a while). The stratum corneum represents the actual barrier. Together with the correct pH value (slightly acidic), a mixture of lipids (fats (ceramides, free fatty acids and cholesterol)) and many small water-binding molecules (natural moisturizing factor (NMF)), the skin builds up a mixture of water and lipids (fats) in its outermost layer, the stratum corneum, i.e. a hydro-lipid emulsion. Water and fats can normally only bond with each other in the presence of emulsifiers. Here, too, evolutionary biology has accomplished something miraculous: it has enabled the skin's fats to bind water. This means that the skin's fats themselves have a certain emulsifying effect. This explains why oily skin is usually moist and why skin with a low oil content is usually dry.

If you suffer from dry skin, for whatever reason (see above), you should supply both lipids (fats) and water. This is best achieved with water-in-oil emulsions, i.e., relatively fatty creams that have an ointment-like quality, meaning they contain a lot of fat and little water (a true ointment in the pharmaceutical sense is anhydrous). So-called oil-in-water emulsions (creams with a low fat but high water content) are less suitable for truly dry skin, but are still better than no care at all.

Dry skin should be cared for regularly with emulsions. There are considerable differences in opinion as to how high the fat content of an emulsion should be. Some people insist on very rich skincare, while others prefer something lighter or less "heavy" - i.e., one that contains less fat and more water. The important thing is to care for it. Even less-than-ideal care is still better than no care at all. Dry skin should be moisturized twice a day, although once is better than never. Ideally, the skin should be moisturized after bathing or showering. The calloused skin is then fully saturated with water from contact with the water, and this hydration promotes the absorption of a more or less fatty emulsion, which can then be absorbed more easily.

Skin dryness can be determined using a device called corneometry, also known as a corneometer.

Dry skin is caused by complex interrelationships, which have been outlined here. Since dry skin often causes secondary problems (e.g., itching, tightness, irritation, and possibly infections), and dehydration processes also promote premature aging, dry skin should always be treated.