How do these sun blocks work? I don't understand what the numbers mean.

The spf or sun protection factor ...for example 15spf...this means you can stay in the sun 15 times longer than if you went in the sun without any...so if you start burning in 10 min. the protection for you woulld be 15 times 10 min....and so on

SPF stands for Sun protection factors

I remember doing a report in college about SPF's.
The SPF number helps you determine how long you can stay in the sun without burning. It gives you enough to protect you based on how sensitive you are to the suns rays (how long it takes you to burn). The higher the SPF number, the more protection it gives you.

A SPF of 15 gives you 15 times your normal protection. A 15 SPF or higher was the recommended protection of any sunscreen.


I just found this:

quote:

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SPF (sun protection factor): measures the length of time a product protects against skin reddening from UVB, compared to how long the skin takes to redden without protection. If it takes 20 minutes without protection to begin reddening, using an SPF 15 sunscreen theoretically prevents reddening 15 times longer -- about 5 hours. (Actually, it may take up to 24 hours after sun exposure for redness to become visible.) To maintain the SPF, reapply sunscreen every two hours and right after swimming.

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http://jaxmed.com/articles/wellness/spf.htm

This site is from 1998 so a few things may have changed but this should let you know the basics. I read the info and its close to what I researched in school so I think the site is pretty accurate.