[n.m] The outer surface of the epidermis is covered by a protective film composed of a mixture of aqueous (or hydro) elements, water resulting from perspiration; and oily (or lipo) elements, essentially sebum. This film is commonly called the hydrolipidic film, the name born of the hydro and lipo duality. It has a twofold function:

• By virtue of its acidity (see skin PH), it plays a regulatory role in balancing the microorganisms (bacteria, microbes) on the skin’s surface and keeps them from penetrating the skin.
• Having low permeability, it also helps maintain an ideal hydration level for the skin.

Numerous factors, including age, poor diet and unsuitable skincare products, can lead to an imbalance in the film, thereby affecting its role as a skin barrier.