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
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)
Latest Answers