Sunscreen: How much should you apply?

Theregular use of sun protection products is recommended as a means to reduce skin damage caused by the sun's UV rays. Indeed, excessive sun exposure is known to induce sunburn, photocarcinogenesis (development of cancerous tumours) and photoaging, a process where the skin undergoes changes/damage: increase in the thickness of the epidermis, pigment heterogeneity and degeneration of the elastic tissue of the dermis, degradation of collagen in the dermis, visible dilation of blood vessels under the skin and increase in the mutagenesis of keratinocytes and melanocytes in the skin. Therefore, the best approach is to consistently ensure therigorous application of effective photoprotection , which should be adapted according to the intensity of the radiation and one's phototype, to prevent skin photoaging. Indeed, sunscreen products are designed to filter UVA and UVB rays.

Choosing a sun cream with a high protection factor is not enough to shield you from the harmful effects of the sun's rays. You must apply it in sufficient quantity, that is 2 mg per cm² of skin, to achieve the protection factor value indicated on the label. This amount equates to approximately 35 g of sun protection all over the body for an adult of standard height and weight, which is two 200 mL bottles of sun cream per person per week.

However, studies have shown that consumers only use about a quarter of this amount, equating to 0.5 mg per cm², in real life. Yet, the SPF indicated on the label is linked to the amount of product applied. This means that the SPF spread on the consumer's skin is less than expected. Therefore, to avoid getting lost in calculations and to apply the correct layer of sun protection, you can proceed in several ways:

• Apply the sunscreen along your knuckle before spreading it over different areas of the body, such as 2.5 fingers on the face and neck, 8 fingers on the chest and back, 4 fingers on each arm, 1 finger on each hand, 6 fingers on each leg and 2 fingers on each foot ;

• Spread the equivalent of 7 teaspoons over all parts of the body ;

• Apply between 12 to 15 sprays with the spray-on sun creams.

Thus, the application of a generous amount of sun protection has proven to be by far the most important factor for achieving good product efficacy, as well as a good distribution of the application and a good specific absorption spectrum for the filter used.