Skin care

Let’s talk about TEWL

Today I do not want to talk about the threat to the skin of heated indoor enviroments while it’s cold outside (at least in most of the north hemisphere), instead, I want to highlight the importance of the skin’s main function, which we tend to forget.

The skin’s main function is to protect the body from external substances and excessive loss of water.

Water is an essential element for the wellbeing of the skin, therefore cleansing and hydration are essential steps to keep it in good condition.

The water follows a well-defined path within the different skin layers: from the bloodstream reaches the dermis, and then spread regularly and constantly up to the upper layers of the epidermis, where it has the task of preserving the skin hydration.

The aqueous film that covers the epidermis is usually defined as an “acid mantle”, with reference to its acidic pH. Its composition comes from the secretion products of the sweat and sebaceous glands, by peptides derived from the breakdown of the corneocytes (dead cells) and from some key epidermal lipids. In other words, it is a mixture of oils and water-soluble substances, whose task is to protect the skin. This protective film is divided into two parts: the upper film, where the sebum and the epidermal lipids lay; below this we find the NMF (natural moisturising factor) and is composed of water- based molecules present on the epidermal surface.

There is a constant and imperceptible evaporation of water at the outer layer of the epidermis.

This mechanism is called Trans Epidermal Water Loss (TEWL). TEWL reflects the integrity of the skin barrier and is therefore used as a benchmark for assessing the health of the skin.

Water, together with proteins and lipids, gives the upper layer of the skin, softness, flexibility and elasticity.

The state of hydration is regulated by substances, present in the corneocytes (dead cells), able to bind water, and the quality of the lipids present.

The most important proteins here are represented by keratin, involucrin, and filaggrin: they have the ability to bind water molecules (remember when I say please don’t over exfoliate because you are killing all these valuable proteins?). The lipids of the most external layer are essential to retain the right amount of water in the skin and to regulate the TEWL (barrier effect). In particular, linoleic acid (omega -6) plays a key role in the synthesis of barrier lipids

The main factors of dehydration are chemical (for example, the solvent and delipidising action linked to the repeated application of surfactants – cleansing agents-, this is why I always mention that cleansing is the most important step in the beauty routines) or can be linked to environmental aggressions: wind, cold and humidity related to the environment, when they intervene separately or jointly , cause dehydration with formation of dry, rough, peeling, chapped skin.

A dehydrated skin promotes the penetration of foreign substances, and the onset of inflammatory issues such as eczema, dermatitis and causes loss of elasticity and plasticity.

The hydration strategies are basically two:

  • Regulate the Cutaneous Water Content
  • Adjust the Cutaneous Lipid Content

A good moisturiser should act on both levels, in order to restore the cutaneous hydrolipidic film.

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