You can absolutely get sunburned while submerged in water. A sunburn is the result of damage to the skin caused by overexposure to ultraviolet (UV) radiation from the sun. While water offers a layer of protection, it does not instantly block all harmful UV rays, which is a common misconception. The risk of sun damage underwater is a complex interaction of the water itself and the physics of light transmission.
The Science of Underwater UV Penetration
Sunlight is composed of different wavelengths, including ultraviolet A (UVA) and ultraviolet B (UVB). Water acts as a natural filter, but its ability to absorb these rays varies significantly by wavelength.
UVB, the primary cause of sunburn, is attenuated quickly near the surface. In clear water, most UVB radiation is absorbed within the first few feet, though some can still reach a few meters in depth.
Conversely, UVA radiation penetrates deeper and is associated with long-term skin aging and cancer risk. In pristine ocean water, a detectable amount of UVA can reach depths up to 50 meters or more.
This means that even a few feet underwater, your skin is still exposed to UVA rays. The light intensity decreases exponentially as it travels deeper due to absorption and scattering by water molecules and dissolved materials.
Environmental Variables Affecting UV Depth
The actual depth that UV light reaches depends heavily on the specific water environment. Water clarity, or turbidity, is determined by the concentration of substances like dissolved organic carbon (DOC), sediment, and plankton.
Coastal waters, rivers, and murky lakes, which are high in DOC, rapidly absorb UV light, often attenuating 99% of UVB within the first 5 to 20 centimeters. Clear ocean water, which is low in these absorbing materials, allows UV rays to penetrate much deeper.
The time of day is a powerful variable; UV intensity peaks between 10 a.m. and 4 p.m. The sun’s angle remains a factor in the amount of radiation exposure, even underwater.
The water’s surface can also increase your overall exposure. The surface reflects a portion of the UV rays (up to 10%) and scatters the light that passes through.
This reflection combines with the direct sun’s rays, effectively increasing radiation exposure near the surface. Water also acts as a lens, refracting the light, which can concentrate UV exposure just below the surface.
Essential Strategies for Water Sun Protection
Protecting your skin while in the water requires a multi-layered approach that accounts for the constant presence of UV rays. Start by using a broad-spectrum, water-resistant sunscreen with an SPF of 30 or higher.
The “water-resistant” label means the product remains effective for 40 or 80 minutes while swimming or sweating, as indicated on the packaging. Reapply sunscreen every two hours, or immediately after towel-drying or after the duration listed on the product’s water-resistance claim.
For maximum coverage, physical barriers offer the most reliable protection. Rash guards and swimwear with an Ultraviolet Protection Factor (UPF) rating of 50+ block over 98% of UV radiation.
A standard cotton t-shirt is a poor substitute, as its protection factor is significantly reduced when wet. You should also avoid the sun’s peak hours between 10 a.m. and 4 p.m.
Wet skin tends to increase the skin’s absorption of UV rays after you emerge, making consistent protection necessary both in and out of the water.