Outdoors 9 facts to help you avoid sunburn By Jackie Davis Published: July 10, 2013 Updated: June 4, 2022 share Share on Facebook Share on Twitter Share on Pinterest Copy Link Cancel Apply about two tablespoons of sunscreen to cover your entire body; reapply every two hours, and after sweating or swimming. Wear a broad-spectrum sunscreen and lip balm with a minimum SPF of 15. Apply sunscreen at least 20 minutes before you go outside, to allow time for your skin to absorb it. Sand can reflect up to 25 per cent of UV rays, and water, up to 100 per cent, depending on the sun’s angle. Water-resistant sunscreens are effective for up to 40 minutes in the water; waterproof products last twice as long. Shade can reduce UV by 50 per cent. Up to 80 per cent of UV rays can reach you through clouds, mist, or fog. Ordinary car windows can block about 97 per cent of UVB rays (but most UVA rays still get through). UV strength increases by four per cent for every 300-metre increase in altitude (higher altitude = thinner atmosphere = more UV penetration = don’t suntan on Mount Everest).