Sunburn at the seaside.

Background/purpose: The purpose of this paper is to estimate the contribution to our erythemal exposure at the coast of solar ultraviolet (UV) both reflected from, and transmitted into, the ocean.

Methods: The reflection of solar UV radiation from, and transmitted into, seawater was calculated using a numerical model under a number of atmospheric conditions to estimate erythemal exposure on the skin of supine/prone and ambulant people.

Results: The results were expressed as UV Indices. Even under the most extreme insolation with the sun directly overhead, where the ambient UV Index may be around 14, reflected UV from the ocean contributes an erythemal exposure to the skin equivalent to a UV Index of about 0.7. For typical ocean waters, with the sun high in the sky, the UV index within the water is about 7 at a depth of 2 m.

Conclusion: Whilst our eyes often sense a high level of reflected sunlight from the ocean, especially when the sun is low in the sky, our skin does not share that experience. The reason people get sunburnt at the seaside has more to do with the absence of shade than with reflectance by the water surface or even beach sand.