DRY SKIN
Background - Causes - Treatment

The most important task of our skin is to prevent valuable things (e.g. body fluids and electrolytes) from being lost from the inside to the outside and to prevent harmful things, e.g. irritants or toxins, from penetrating from the outside to the inside. In order to be able to fulfil these tasks, the skin builds a barrier with its own building blocks, which is permeable to some things (e.g. body fluids that cool us on the surface of the skin as sweat) and impermeable to others. Such a barrier is called semi-permeable. With its barrier, the skin ensures the cohesion of our body and thereby secures our integrity.

Since our body consists predominantly of water and the environment predominantly of more or less dry air, the skin, which limits or seals our body to the outside, must secure this sudden jump in moisture from 'moist inside' and 'dry outside' so that we do not literally 'leak'. To do this, the skin builds up several cell layers that are still relatively moist at the beginning, i.e. have a high water content, and which then become drier and drier towards the outside due to a decreasing water content. On the very outside or on top of our skin is the last layer, the cornea (stratum corneum), which consists of dead skin cells of the epidermis. These gradually detach and flake off. Through this, 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, we would notice this by moulting - as in snakes, for example (desquamatio sensibilis).

Damit die beschriebene Barriere aus Hautzellen unterschiedlicher Entwicklungsstadien aufeinander getürmten Zellschichten zwar fest, aber dennoch nicht starr ist, sondern als die uns bekannte bewegliche, flexible und fein regulierte Haut funktionieren kann, ist sie so aufgebaut, dass sie fest und zugleich beweglich ist, was eine der wunderbarsten Errungenschaften der Evolutionsbiologie ist. Um dies abbilden zu können, muss die Barriere den Feuchtigkeitssprung zwischen feucht (innen) und trocken (außen) reibungslos organisieren. 

Die Haut muss in dieser äußersten Schicht trocken, aber nicht zu trocken sein, da sie sonst an Beweglichkeit verlieren und rissig werden würde, und Risse die Barriere
(1) zerstören und
(2) schmerzhaft sind, da sie kleine Wunden darstellen, die zudem schlecht heilen.

Außerdem bilden sich auf einer zu trockenen Haut gerne Entzündungen, sogenannte Austrocknungsekzeme (Exsikkationsekzeme), die
(1) gerne jucken (Juckreiz ist ein Phänomen der trockenen Haut), die sich
(2) unangenehm anfühlen und Missempfindungen verursachen (Dysästhesien), und die
(3) are not infrequently colonised by germs, as eczema often also oozes and thus forms a good breeding ground for germs.

Trockene Haut wird begünstigt durch:
(1) genetic predisposition (e.g. neurodermatitis (atopic eczema))
(2) low sebaceous gland activity
(3) increasing skin ageing
(4) incorrect skin care
(5) much contact with water
(6) häufige Anwendungen von Reinigungsprodukten (enthalten sogenannte Tenside)
(7) dry indoor air (favoured by poorly ventilated and overheated rooms)
(8) dry outdoor air (favoured e.g. by cold weather)
(9) sun exposure
(10) low fluid intake (insufficient amounts of drink)
(11) inflammations (e.g. eczema)

Healthy skin protects itself from dehydration by a compaction of dead cells in the outermost layer, which stick together relatively tightly here and are more or less compact (cornea (stratum corneum)). We have a cornea on every part of our skin, even on the eyelids. The cornea is particularly thick on certain mechanically stressed regions such as the palms of the hands and the soles of the feet. This cornea you have to imagine like a stone wall (the stones are the dead skin cells (corneocytes) and the mortar between the stones represents all the substances and molecules that keep the dead cells together for a while). The cornea represents the actual barrier. Together with the right pH value (slightly acidic), a mixture of lipids (fats (ceramides, free fatty acids and cholesterol)) and many small water-binding molecules (natural moisturising factor (NMF)), the skin builds up a mixture of water and lipids (fats) here in its outermost layer, the cornea, i.e. a hydro-lipid emulsion. Water and lipids can normally only combine in the presence of emulsifiers. Here, too, evolutionary biology has done something wonderful, namely 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 that a fat (oily) skin is usually also moist and that a low-fat skin is usually also dry.

If one suffers from dry skin, for whatever reason (see above), both lipids (fats) and water should be supplied. This is best done with water-in-oil emulsions, i.e. relatively fat-containing creams that have an ointment character, i.e. contain a lot of fat and little water (a real ointment in the pharmaceutical sense is water-free). So-called oil-in-water emulsions (creams with low fat but high water content) are less suitable for really dry skin, but are still better than no care at all.

Dry skin should be cared for regularly with emulsions. Ideas about how high the fat content of a nourishing emulsion should be vary considerably. Some insist on a very greasy care, others prefer a somewhat lighter or less "heavy" care, i.e. a care that contains less grease and more water. The important thing is to care. Less than ideal care is still better than no care at all. Dry skin should be creamed 2 times a day, whereby it is also true that once is better than 'not at all'. Ideally, the skin should be creamed after bathing or showering. The cornea is then fully saturated with water due to the water contact and this hydration favours the absorption of a more or less greasy emulsion, which can now also be better absorbed.

How dry a skin is can be determined with a so-called corneometry. The corresponding device is also called a corneometer.

Dry skin is caused by complex interrelationships, which have been described here. Since dry skin often provokes secondary problems (e.g. itching, feeling of tightness, irritation and possibly also infections) and drying processes also promote premature ageing, dry skin should always be treated.