SUN PROTECTION FACTOR (SPF)

SPF or Sun Protection Factor: is a measure of how well a sunscreen will protect skin from UVB rays, the kind of radiation that causes sunburn, damages skin, and can contribute to skin cancer.
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SPF = sunburn radiation dose with sunscreen / sunburn radiation dose without sunscreen.
If your skin would normally burn after 10 minutes in the sun, applying an SPF 15 sunscreen would allow you to stay in the sun without burning for approximately 150 minutes (a factor of 15 times longer). 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.
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The SPF (Sun Protection Factor) scale is not linear: .
• SPF 15 blocks 93% of UVB rays.
• SPF 30 blocks 97% of UVB rays.
• SPF 50 blocks 98% of UVB rays.
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So, one way of looking at this is that SPF 30 sunscreen only gives you 4% more protection than SPF 15 sunscreen. .
Or, another way of looking at it is: .
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• SPF 15 (93% protection) allows 7 out of 100 photons through.
• SPF 30 (97% protection) allows 3 out of 100 photons through. .
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So, while you may not be doubling your level of protection, an SPF 30 will block half the radiation that an SPF 15 would let through to your skin.