Does one tan more quickly on beach sand or in swimming pool water?

When the skin is exposed to the sun’s UV rays, it responds by stimulating the synthesis of melanin by the melanocytes, specialised cells located in the basal layer of the epidermis. This pigment, subsequently transferred to keratinocytes and gradually conveyed to the skin’s surface at the pace of cell renewal, is responsible for the skin’s varying shades of colour. The quality, intensity and rate of tanning depend primarily on the amount of ultraviolet radiation received, but also on phototype, stratum corneum thickness and individual melanin production capacity.

Exposure can then vary according to certain environmental factors, such as the presence or absence of water capable of reflecting a portion of the sun’s rays and thus increasing the total UV dose received by the skin, including indirectly. One might then assume that there is no difference between the reflective capacity of seawater and that of swimming-pool water and that, consequently, one tans just as well at the seaside as at an outdoor pool.

In reality, other parameters must be considered. Water can reflect roughly 10 to 30% of incident UV radiation depending on its turbulence, clarity and the angle of sunlight. At sea, foam also has a notable reflective capacity, reaching about 25%. Moreover, on a sandy beach — unlike pebbles, which absorb more radiation — the speed of tanning can be enhanced, as sand can reflect nearly 17% of UV.

The combined effect of these different reflective surfaces increases the overall irradiation received by the skin and thus explains why people generally tan more quickly at the seaside than in a swimming pool.

Finally, note that the level of UV reflection also depends on the sun’s angle of incidence, that is, the angle formed between the sun’s rays and the water’s surface. When the sun is high in the sky, the rays arrive more vertically and a smaller proportion is reflected. Conversely, when the sun is lower on the horizon, the incidence becomes more oblique, increasing UV reflection off the water and potentially enhancing skin exposure, even later in the day.

Even if your objective is to attain a sun-kissed complexion, the most crucial measure when exposing your skin to solar radiation is to protect it with sunscreen.

Indeed, inadequately protected skin does not tan more effectively: it primarily sustains UV-related damage, which can manifest as sunburn, peeling or even premature alteration of dermal structures. UVB rays are mainly responsible for sunburn and contribute to melanin synthesis, while UVA rays penetrate more deeply into the skin and contribute to oxidative stress, to photoageing and to certain alterations of cellular DNA.

Before every sun exposure and after each swim, it is therefore advisable to generously apply a broad-spectrum sunscreen, capable of protecting against both UVA and UVB. The amount applied is crucial: insufficient application dramatically reduces the sun protection factor SPF. In addition, remember to reapply every two hours and after swimming to stay protected throughout the day.

Discover here Typology’s sun care products.

Important : Although often regarded as aesthetic, tanning primarily constitutes a skin defence response to UV radiation exposure. The increased production of melanin aims to limit DNA damage in skin cells, though it does not fully prevent the harmful effects of UV rays. We therefore recommend limiting your sun exposure, even if it means forgoing a tanned complexion.

• PHILPOT W. D. & al. Directional optical transmission through a sand layer: A preliminary laboratory experiment. Remote Sensing for Agriculture, Ecosystems, and Hydrology (2017).

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