The belief that being in or around water provides adequate protection from the sun is a common misconception that can lead to significant ultraviolet (UV) exposure. UV radiation, which includes UVA and UVB rays, causes damage to skin cells and is the primary cause of sunburn and long-term skin damage. While water interacts with these rays, it does not function as a reliable shield. Anyone spending time on the water or submerged must understand the scientific behavior of UV light in this environment to ensure proper sun safety.
The Science of UV Absorption in Water
Water acts as an absorber of UV radiation, but this process is highly dependent on the wavelength of the light and the clarity of the water body. Both UVA and UVB rays penetrate the surface, though their intensity diminishes rapidly as depth increases. In clear, open ocean water, about half of the solar radiation that enters the water column is attenuated within the first meter, but significant amounts of UV still travel much deeper.
UVB, the wavelength most responsible for sunburn, is absorbed more efficiently than UVA. Even so, approximately 40% of the surface UVB intensity can still reach a depth of half a meter in clear water. UVA, which penetrates deeper into the skin and is associated with aging, is less attenuated by water and can be detected at depths greater than 30 meters in very clear marine environments. The presence of suspended particles, dissolved organic matter, and phytoplankton determines the rate of absorption. Turbid coastal waters or lakes, for instance, block UV more effectively than the clear water of a swimming pool or open ocean.
Reflection and Refraction Risks
The water’s surface introduces additional exposure risks that go beyond the light penetrating the water column. When sunlight hits the water, a portion of the UV radiation is reflected back up, which significantly increases the total exposure for anyone partially submerged or standing nearby. This reflection off the surface acts like a mirror, delivering a double dose of UV radiation.
While the amount of UV reflected by the water itself is relatively low, around 5% at midday when the sun is high, this small percentage still contributes to overall exposure. The reflection becomes much more pronounced when the sun is lower in the sky, such as in the early morning or late afternoon. The surrounding environment compounds this issue, as surfaces like white beach sand can reflect up to 15% of UV light, and sea foam can reflect up to 25%. This scattered and reflected radiation hits the body from multiple angles, making exposure near water often more intense than away from it.
Practical Sun Safety for Water Activities
Understanding the limited protection water offers necessitates specific, proactive safety measures during water activities. Having wet skin can increase the risk of sunburn, as the water makes the skin more susceptible to UV damage. The cooling effect of water also masks the sensation of burning, leading people to stay exposed for longer periods without realizing the damage occurring.
Proper use of broad-spectrum, water-resistant sunscreen is essential and must be applied generously before entering the water. Reapplication is necessary because water-resistant formulas are designed to remain effective for only 40 to 80 minutes while swimming or sweating, after which they must be reapplied immediately. Protective clothing with a high Ultraviolet Protection Factor (UPF), such as rash guards, offers a consistent defense against both direct and reflected UV rays, even when wet. Seeking shade and avoiding peak UV hours, typically between 10 AM and 4 PM, remains the most effective strategy to minimize the risk of damage.