SPF

zinka

The sun protection factor of a sunscreen is a laboratory measure of the effectiveness of sunscreen — the higher the SPF, the more protection a sunscreen offers against UV-B (the ultraviolet radiation that causes sunburn); it does not measure UV-A (the radiation that causes cancer).

The SPF is the amount of UV radiation required to cause sunburn on skin with the sunscreen on. Sunscreen in general is proven to slow the aging of skin, but not prevent skin cancer.

There is a popular oversimplification of how SPF determines how long one can stay in the sun. For example, many users believe that, if they normally get sunburn in one hour, then an SPF 15 sunscreen allows them to stay in the sun fifteen hours (i.e. fifteen times longer) without getting sunburn. This would be true if the intensity of UV radiation were the same for the whole fifteen hours as in the one hour, but this is not normally the case. Intensity of solar radiation varies considerably with time of day, for one thing. During early morning and late afternoon, the sun’s radiation must pass through more of the Earth’s atmosphere before it gets to you.

In practice, the protection from a particular sunscreen depends, besides on SPF, on factors such as: the skin type of the user; the amount applied and frequency of re-application; the activities in which one engages (for example, swimming leads to a loss of sunscreen from the skin); and the amount of sunscreen the skin has absorbed.

The SPF is an imperfect measure of skin damage because invisible damage and skin aging are also caused by ultraviolet type A, which does not cause reddening or pain. Conventional sunscreen blocks very little UVA radiation relative to the nominal SPF; broad spectrum sunscreens are designed to protect against both UVB and UVA. The best UVA protection is provided by products that contain zinc oxide, avobenzone, and ecamsule.

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