Do you tan more quickly on beach sand or in pool water?

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

Exposure can then vary depending on certain environmental factors, such as the presence or absence of water capable of reflecting some of the sun's rays and thereby increasing the total UV dose received by the skin, even indirectly. One might then assume that there is no difference between the reflective capacity of seawater and that of pool water and that, consequently, you tan just as well at the sea as at an outdoor pool.

In reality, other factors must be considered. Water can reflect about 10 to 30% of incident UV rays depending on its motion, clarity, and the angle of sunlight. At sea, foam also has a significant reflective power, reaching approximately 25%. Moreover, on a sandy beach—unlike pebbles, which absorb more radiation—the tanning rate can be enhanced, as sand is capable of reflecting nearly 17% of UV rays.

The addition of these various reflective surfaces increases the overall irradiance received by the skin and thus helps explain why people generally tan faster at the sea than at the pool.

Finally, it should be noted 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 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 oblique, which increases UV reflection by the water and can increase skin exposure, even late in the day.

Even if your goal is to attain a tanned complexion, the most important consideration when exposing your skin to the sun is to protect it with sunscreen.

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

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

Discover here Typology’s sun care products.

Important : Although it is often perceived as aesthetic, the tanning is, first and foremost, a skin defense response in response to an assault by UV rays. The increased production of melanin aims to limit damage to the DNA of skin cells, without fully preventing the deleterious effects of UV. We therefore recommend limiting your sun exposure, even if it means giving up 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).