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?
In: 1
Sunscreen filters out some of the ultraviolet light. It doesn’t completely filter it all out so some is leaking through. That’s where the strength of it comes in.
It also tends to wear off. If you’re just sitting out the sun and not sweating it’s going to last longer. But if you’re sweating, moving around, rubbing it, or in the water it’s going to wear thinner and we can less effective so you need to reapply it.
Some sunscreen can deteriorate over time. Zinc oxide is less likely to. You’re just putting a reflective coat of temporary paint on your face
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