 | | SPF Sun Protection Factor
In the United States the Sun Protection Factor of a product tells how long you may stay in the sun without burning from UVB light. SPF is only a sunburn meter allowing tanning with enough sun exposure. UVA light has not been formally tested to acquire a SPF rating but manufactures commonly include them in a broad screen sunblock.
SPF means Sun Protection Factor using this equation.
- Take the time you would normally burn in the sun without protection. 20 minutes would normally produce redness on a light skinned individual.
- Multiply that number by the SPF factor of your product. Example: with an SPF 15 X’s 20 minutes of sunburn time = 15 x 20 = 300 tells how many minutes you may stay in the sun without burning. 300 minutes divided by a 1 hour of 60 minutes = 5 hours of sun protection without sunburn.
How SPF is determined
In a controlled indoor laboratory eliminating any effects of environmental change, i.e. wind, heat, and cold, untanned test subjects are put through a two day test.
- On the first day the lower back is protected except for the test site and exposed to UVB light until mildly red.
- On the second day sunblock is applied to a new test site and exposed to UVB light until the same mild redness occurs.
The amount of time to achieve redness with the sunblock applied determines the SPF. As people vary so will the sun protection product results vary on individuals.
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