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Quite simple. This number tells us how many Sun rays are blocked by a certain sunscreen.
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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.
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Example:
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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.
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2. SPF 30
100:30= ~3% (not blocked and reach the skin);
100-3=97% blocked.
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3. SPF 50
100:50=2% not blocked (2% is enough to cause redness);
100-2=98% blocked.
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SPF 70, SPF 100 are misleading and incorrect- it is impossible to block all the rays.
