I truly don’t understand how this works. I get that SPF 50 is 5 times as long as SPF 10, but WHY does it do that? Why do I need SPF 50 every 2 hours and then I start turning red?
Same question with SPF that’s a year old, does that still work like when it was bought, or not? Does the “clock” start ticking once it’s squeezed out of a tube?
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There are a lot of confidently wrong answers on here. For starters, SPF has no relationship with how often you have to re apply it, it’s a measure of the amount of UV light being absorbed by the product. There are diminishing returns, and another over SPF 30 is unnecessary (SPF 30 blocks 97% of UVB radiation). Also, the reason why you need to reapply sunscreen is it breaks down in UV light. The difference between sunscreen and sun block is that sub block reflects the UV light back away from you, while sunscreen is a chemical that breaks down when UV light hits it. This is preferable to your DNA breaking down when UV light hits it, so we wear sunscreen. Because the active compound breaks down over time, it needs to be reapplied every so often, depending on sun exposure as well as sweating/water exposure.
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