Ultraviolet (UV) radiation is a form of electromagnetic energy that exists beyond the visible light spectrum, typically ranging from 10 to 400 nanometers in wavelength. This invisible energy from the sun is known for its effects on skin, yet many believe being submerged in water offers complete protection. The scientific reality of how water interacts with UV radiation is more nuanced, making understanding this interaction important for sun safety.
How UV Rays Interact with Water
Water does not completely block ultraviolet radiation. While water molecules absorb some UV light, a notable amount still penetrates the surface. This interaction involves attenuation, a process where light intensity gradually diminishes as it travels through the water column.
Different types of UV rays interact distinctly. Ultraviolet A (UVA) rays, with longer wavelengths, penetrate more deeply than Ultraviolet B (UVB) rays, which are largely absorbed closer to the surface. For example, 80% of UVA rays can reach a meter below the surface, while 50% of UVB rays can penetrate to the same depth. Thus, even when submerged, skin remains exposed to the sun’s damaging UV energy.
Variables Influencing UV Penetration
Several external factors influence the amount of UV radiation that penetrates water. Water depth plays a role, as UV intensity decreases exponentially with increasing depth. While significant reductions occur within the first few feet, some UVA radiation can still be present much deeper, reaching up to 50 meters in clear ocean water.
Water clarity, or turbidity, also directly affects UV penetration. Water with suspended particles like sand, algae, or pollutants, and dissolved organic matter, absorbs and scatters UV light more effectively, reducing its penetration. Clearer water allows more UV radiation to pass through, meaning pristine swimming conditions offer less natural protection. The water’s surface can also reflect UV rays, with some studies suggesting up to 10% reflection. This reflection can increase overall UV exposure for individuals on or near the water, amplifying the sun’s intensity.
Essential Sun Protection When Wet
Since water does not offer complete UV protection, sun safety measures are important during aquatic activities. Applying water-resistant or waterproof sunscreen is a primary defense. Apply it generously at least 15 to 20 minutes before entering the water to allow it to set. Reapply every two hours, or more frequently (typically every 40 to 80 minutes) after swimming or sweating, as towel drying can remove sunscreen.
Other protective measures include seeking shade, such as under an umbrella or tree, which provides respite from direct sun exposure, especially when not actively in the water. Wearing protective clothing, like rash guards or long-sleeved swimsuits with a UPF 50+ rating, offers a physical barrier against UV radiation, blocking over 98% of harmful rays. Limiting time outdoors during peak UV hours, generally between 10 AM and 4 PM, can significantly reduce exposure to the strongest rays.