Do we tan faster at the sea than at the pool?

When the skin is exposed to the sun's UV rays, it responds by stimulating the synthesis of melanin by the melanocytes. This pigment, once it has migrated to the surface of the epidermis, gives it its colour. The quality and the speed of tanning of the skin therefore primarily depend on the presence of sunlight. This can then vary according to certain environmental factors, such as the presence or absence of water which reflects the sun's rays and promotes tanning. One might then think that there is no difference between the reflective capacity of sea water and that of pool water and that, consequently, one tans just as well at the sea as at an outdoor pool.

In reality, other factors need to be taken into account. Thewater is indeed capable of reflecting about 10 to 30% of the UV rays it receives, but the sea foam also has a reflection capacity of 25%. Moreover, if you are on a sandy beach rather than a pebble one, the speed of tanning will be further increased because the sand is also capable of reflecting UV rays, to the extent of about 17%. These different elements therefore allow us to conclude that we generally tan faster at the sea than at the pool.

It should also be noted that the level of UV ray reflection is also influenced by theangle of incidence of the sun, that is, the angle formed between the sun's incident ray and the water's surface. When the sun is high in the sky, the sun's rays are emitted vertically in relation to the water's surface. In this configuration, only a small portion of the rays is reflected. Conversely, when the sun is lower in the sky, the UV rays have an oblique angle of incidence and are therefore more reflected by the water.

The key when exposing oneself to the sun is always to ensure adequate protection, even if one is aiming to tan more quickly. Unprotected skin will not tan evenly, but is more likely to develop sunburn or peel. Before each exposure or swim, generously apply a broad-spectrum sun care product that protects against both UVA and UVB rays.

Furthermore, we recommend choosing a sunscreen with a high moisturising capacity if you are swimming in seawater. Indeed, seawater tends to dry out the skin, due to the salt it contains. Dehydrated skin is then more susceptible to sun rays. You might want to try our face sunscreens which contain aloe vera, a natural ingredient capable of retaining water in the skin cells. Finally, remember to reapply every two hours and after each swim.

• GONG F. & al. Characteristics of water leaving reflectance at ultraviolet wavelengths: radiative transfer simulations. Optics express (2020).