Extended exposure to UV rays can lead to an increased risk of photoaging, photodermatoses, pigmentation spots and skin cancers. Therefore, it is crucial to protect your skin against the sun's UV rays, particularly UVA rays. The daily and regular application of sun protection products containing sun filters is one of the effective methods to block UVA and UVB rays to the greatest extent possible, thereby protecting the skin from the harmful effects of the sun.
Selecting the correct protection factor alone is not sufficient for adequate sun protection, it is also recommended to reapply the sunscreen at least every two hours for outdoor activities (beach, skiing, etc...), immediately after swimming or excessive sweating. For those working indoors, it is not necessary to reapply sun protection, provided that the recommended amount applied in the morning (2 mg/cm2) has been adhered to.
But why is this so? In order to function, a sunscreen cream must form a uniform film on the skin. However, after a certain period of time, this protective film can "move" or interact with the skin's natural lipid layer. Sweat, water (during swimming)... can also alter it. Furthermore, under the effect of sunlight, certain sun filters (avobenzone, etc...) contained in the sunscreen cream tend to degrade (photo-stability) and their effectiveness is reduced, thus leading to an alteration of the product's protective properties.