Belleza Official — Sunscreen SPF and Dosage | bellezaofficial.com
SPF 15 means 1/15 of the burning radiation reaches the skin at the tested dose of 2 mg per square centimetre.
SPF 15 means 1/15 of the burning radiation reaches the skin when sunscreen is applied evenly at a dose of 2 mg per square centimetre. The number is a fraction of the burning radiation that gets through, measured at that fixed dose, not a general strength rating.
The dose used in FDA sunscreen testing is 2 mg per square centimetre of exposed skin, so every printed SPF value assumes you apply that amount. Apply less and you move to a different, untested point on the curve.
On narrow screens, swipe or scroll the plate sideways.
There is an exponential relation between SPF and the amount of sunscreen applied in vivo, which means protection falls off faster than the dose does. Half the dose does not give you half the SPF.
Higher SPF products do not stay effective longer on the skin than lower SPF ones. The number describes the fraction of burning radiation let through at the tested dose; it does not extend the reapplication interval, which stays about every two hours.
Covering the whole body at the tested dose needs about 35 mL of product, roughly one ounce or six teaspoonfuls. That volume is the practical way to check whether you are applying near 2 mg per square centimetre.
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