Extended exposure to UV rays can lead to an increased risk of photoaging, photodermatoses, pigment spots and skin cancers. Therefore, it is crucial to protect your skin from the sun's UV rays, particularly UVA rays. The daily and regular application of sunscreen products containing sun filters is one of the effective methods to block as much UVA and UVB rays as possible, thereby protecting the skin from the harmful effects of the sun.
Selecting the right sun 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 individuals working indoors, it is not necessary to reapply sunscreen, provided that the recommended amount applied in the morning (2 mg/cm2) has been adhered to.
But why is this so? For a sunscreen to work, it must form a uniform film on the skin. However, after a certain period, 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 (like avobenzone, etc...) contained in the sunscreen tend to degrade (photo-stability) and their effectiveness is reduced, leading to an alteration of the product's protective properties.