Sun-protection-factor meaning

A measure of the degree to which a sunscreen protects the skin from ultraviolet radiation, expressed as a number representing the ratio of the minimal ultraviolet dose required to produce erythema with and without sunscreen.
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A number indicating the relative effectiveness of a sunblock in preventing certain ultraviolet rays from burning the skin: a higher number indicates a greater degree of protection.
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A measure of the degree to which a sunscreen protects the skin from ultraviolet radiation, expressed as a number representing the ratio of the minimal ultraviolet dose required to produce erythema with and without sunscreen.
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A measure, expressed numerically, of the degree to which a preparation containing sunscreen protects the skin from ultraviolet rays. The higher the value, the greater the level of protection from sun damage to the skin.
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An indication of how long a sunscreen remains effective when applied to the skin.

If someone normally burns in 10 minutes with no sunscreen, then a sunscreen with sun protection factor 20 will provide protection for up to 20x10 = 200 minutes.

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