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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You’ve got your units messed up. SPF only means “allows a fraction of the blocked sun rays through”, so SPF 50 rated for UVa and UVb light will cut the amount of UVa and UVb light you are exposed to to 1/50th of the original amount, while similar but SPF 10 would let through 1/10th. So with this comparison, it would take 5 times as much exposure for the 50 to have let through the same amount as the 10. The difference comes from the type of UV blocker used in the different sunscreens and how they are formulated into the lotion; some lotions have more blocking chemicals, while others sacrifice the protection to instead have some other benefit (usually cost, lotion endurance, or lotion feel).
But it only works at full strength in factory ideal conditions.
What are those conditions? No water/sweat washing it off (even ‘water proof’ will still be washed off, just slower), no movement wiping some off, and no earth’s rotation changing the intensity of the light source over time.
Edit: And no actual UV involved, because the most common method sunscreen uses involves the active ingredient becoming inactive as it absorbs the UV. The longer the sunscreen is used for its purpose the less effective it is.
Because the physical (and chemically planned) removal of the protection is far more of an issue once you get past a certain SPF (around 30 SPF works for most people), the instructions for using *any* sunscreen will include reapplying at certain intervals (the 2 hours you’ve discovered) while limiting your total sun exposure over time based on the SPF. So if you need to reapply sunscreen every 2 hours, that’s because that’s how long the sunscreen usually lasts in direct light. Meanwhile, if you used a lower SPF, you’d need to apply at least as often but be more likely to be affected anyway over the course of the day.
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