Because UV rays harm the skin in ways that aren’t always immediately visible, and the damage accumulates over time. Beyond sunburn, the most visible sign of damage, UV rays penetrate deep into the dermis, where they alter the DNA of the skin cells to speed up skin aging. Over time, this means dark patches, lines and loss of elasticity, not to mention the risk of developing skin cancer, which is especially serious for paler skins. Minimize that risk by using sun protection even when you’re in the shade, re-applying every two hours and after each dip. Cover up with a T-shirt and hat during the hottest part of the day and be aware that UV rays can be intensified around reflective surfaces like sand, snow and water, and so at altitude. It’s often when you might think you’re at less of a risk that you need to be especially careful – UV rays can penetrate water, your skin can still be damaged through cloud and that gentle breeze that cools the air is doing nothing to make the sun any less likely to burn your skin.