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 tumors), 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 increased mutagenesis of keratinocytes and melanocytes in the skin. Therefore, the best approach is to consistently go through 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 sunscreen 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, which 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 sunscreen over the entire body for an adult of standard height and weight, which is two 200 mL bottles of sunscreen per person per week.

Indeed, studies have shown that consumers only use about a quarter of this amount, or 0.5 mg per cm², in real life. However, the SPF indicated on the label is related to the amount of product applied. This means that the SPF spread on the consumer's skin is lower than expected. Therefore, to avoid getting lost in calculations and 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 an amount equivalent to 7 teaspoons over all parts of the body ;

• Apply between 12 to 15 sprays with the sunscreen creams in spray form.

Thus, the application of a generous amount of sunscreen 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 absorption spectrum specific to the filter used.

The application of your sunscreen should be done approximately every two hours. Indeed, the value of the SPF decreases with the impact of environmental factors, such as perspiration, swimming, skin rubbing against the beach towel or sand, and photodegradation. For better protection of your skin, you should also apply sunscreen to the eyelids, nose, ears, lips, etc... that is, on all areas that are sometimes neglected and yet can catch a sunburn. Finally, we advise you to choose a sunscreen suitable for your skin type : fair skins need a sun product with a high SPF, while a lower UV protection index is suitable for dark and tanned skins.

• LIPOZENCIC J. & others. Sunscreens: the ultimate cosmetic. Acta Dermatovenerologica Croatica (2003).

• WULF H. C. & al. The correlation between sun protection factor and quantity of sunscreen applied in vivo. British Journal of Dermatology (2007).

• MOHAMMAD T. F. & et al. Sunscreens and photoaging: a review of the current literature. American Journal of Clinical Dermatology (2021).