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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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
It doesn’t protect for “longer” as in “working longer hours”. It just lets less of the harmful radiation through.
Let’s say your skin has a UV tolerance of 100 until you get sunburn. SPF 5 lets through 50 “UV units” per hour through. SPF 50 only lets 5 per hour through, so you can stay in the sun for longer until you reach the limit.
Note that this doesn’t factor in deterioration of the protecting coating itself, e.g. by sweating, swimming or it rubbing off; or the chemicals in the lotion going bad (which is where the expiration date comes from)
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.
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.
It doesn’t “know” or care how long it’s been on the skin for but the ingredients get absorbed into the skin and either broken down or transported somewhere else. That’s why it stops protecting you; the important ingredient just isn’t there anymore.
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