Is the number on suncreen an indicator of how long it lasts, how much sun it can take or the percentage of rays it blocks? Or is it something else entirely?
In: Chemistry
Assuming you mean the SPF number, it’s a measure of how much UV light is blocked by a fixed layer of cream. A cream with an SPF of X will let through 1/X of the UV light hitting it and therefore let you stay out X times longer before getting a given dose of UV.
SPF means sun protection factor. The higher it is, the better you are protected. It is recommended that you use at least SPF 30, but SPF 50 and above are preferred.
The SPF number estimates roughly how long you can stay exposed to the sun while protected before absorbing the same dose of UV you’d get in an unprotected minute. So if a sunscreen is SPF 30 then in theory 30 minutes of sun with that protection is equivalent to one minute of sun without.
SPF is sun protection factor, and its the multiplier of how long you can be in the sun without burning. So if it normally takes you 20 minutes to burn, SPF 30 gives you 600 minutes or 10 hours until you burn. Your sunscreen will need to be re-applied before this point obviously.
You can get the percentage blocked from the equation 100-(100/SPF)
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