SPF - what is it? ⠀ Quite simple. This number tells us how many Sun rays are blocked by a certain sunscreen. ⠀ 1) Start with 100% of rays; 2) Divide by a number on your sunscreen tube; 3) this x% can penetrate into the skin; 4) 100-x%=y% are blocked by sunscreen. ⠀ Example: ⠀ 1. Sunblock with SPF 15 100:15= ~7% will not be blocked by cream (7% is enough to cause damage to the cells); 100-7=93% of Sunrays will be blocked. ⠀ 2. SPF 30 100:30= ~3% (not blocked and reach the skin); 100-3=97% blocked. ⠀ 3. SPF 50 100:50=2% not blocked (2% is enough to cause redness); 100-2=98% blocked. ⠀ SPF 70, SPF 100 are misleading and incorrect- it is impossible to block all the rays.