UVB rays are the famous ones; they cause your skin to burn, and they are what SPF protects you from. They’re are often filtered out, for example by glass (you don’t get sunburnt if you’re sitting next to a window on a sunny day), and to a certain extent by clouds (but not totally – you can definitely still get badly burnt on a cloudy day; I tell you this from painful experience.) “In theory, SPF15 absorbs about 93% of UVB rays, SPF30 absorbs 97% and SPF50 absorbs 98%,” explains Williams.