Sun protection factor

Sun Protection Factor (often abbreviated as SPF) is a measure of how well a sunscreen protects the skin from UVB rays, the kind of radiation that causes sunburn, damages skin, and can contribute to skin cancer.

Overview

The SPF rating is a measure of the time it would take for a person to sunburn if they were not wearing sunscreen, as opposed to the time it would take with sunscreen on. For example, "SPF 15" means that it would take 15 times longer for the skin to begin to burn with the sunscreen on than without it. However, this is a rough estimate that depends on skin type, intensity of sunlight, and amount of sunscreen used. SPF is actually a measure of protection from amount of UVB exposure and it is not meant to help you determine duration of exposure.

Calculation

The SPF is the ratio of the amount of light that induces redness in sunscreen-protected skin, following a sunburn, to the amount of light that induces redness in unprotected skin. As the SPF value increases, sunburn protection increases.

Limitations

While the SPF rating is a measure of UVB protection, it does not measure how well a sunscreen will protect from UVA rays. UVA rays are slightly less intense than UVB rays but they are 30 to 50 times more prevalent, and they penetrate the skin more deeply than UVB.

See also

• Sunscreen
• Ultraviolet A
• Ultraviolet B
• Skin cancer

Sun protection factor Resources
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