[paul.fr]

Just how does sunblock stop those harmful rays? Also when it say spf15, does that mean you can stop out 15 times longer than without the cream?

[Bored chemist]

The sunblock/ sunscreens contain chemicals that absorb the UV light and turn it into heat which does a lot less damage.
In principle SPF 15 means that only 1/15 of the UV gets through but the idea that you can stay out 15 times longer is complicated. You tend to sweat and wash the stuff off and also the sun's intensity varies over the course of the day.
I'm noted for my pale colour (I spend all my time indoors typing on a computer) so If I go somewhere sunny on holiday I can burn in the first hour on the beach without any sunblock.
Say I put on some factor 15- that ought to mean I can stay out for 15 hours before I burn; but there's a glitch. At noon the sun is a lot stronger than first thing in the morning; even factor 15 wouldn't be up to the job.

[chris]

Does sunblock "go off"? You often see a use by date on the bottle, implying that the UV-blocking chemicals cease to function after a while. Fact, or marketing ploy?

[kdlynn]

i would imagine that its effectivenes would decrease over time

[paul.fr]

Does sunblock "go off"? You often see a use by date on the bottle, implying that the UV-blocking chemicals cease to function after a while. Fact, or marketing ploy?

i'm inclined to think that the active ingredients will not go off. Now, i don't want to be too harsh on the marketers  [] as i don't know what products have to have a use by date, but i want to think that it is just a ploy to make sure we keep buying more.