SPF stands for sun protection factor.
The Food and Drug Administration (FDA) defines SPF as a measure of how much solar energy (UV radiation) is required to produce sunburn on protected skin (i.e., in the presence of sunscreen) relative to the amount of solar energy required to produce sunburn on unprotected skin. As the SPF value increases, sunburn protection increases.”
SPF does not measure the amount of time that can be safely spent in the sun, but should be used as a relative basis for how much protection you will receive from the product. Since many factors dictate the amount of UV radiation you will be exposed to including time of day, reflective surfaces, and skin tone, it is important that you choose your SPF based on these factors.
SPF is the measure of protection for blocking UVB rays. There is currently no rating system in place for protection from UVA rays, which penetrate the skin more deeply. Since the number on the label isn’t helpful in disclosing the amount of UVA protection a sunscreen will provide, look for the word ‘broad-spectrum’ on the label, which implies that there is at least some amount of protection for these harmful rays.
What the SPF Numbers Mean
Theoretically, the number on the label is the multiple to the time your untreated skin can endure the sun without damage. As an example if your skin is normally affected by the sun in 10 minutes an SPF of 15 will allow you 15 times that protection or 150 minutes.
Because there are a variety of factors that dictate the effectiveness of sunscreen including higher elevations, how much and how often you reapply protection, and natural skin tone the SPF numbers on your sunscreen label should be used as a basis for a comparative measure and not as a guide to the time you can stay out in the sun.
When deciding which sunscreen is best for you, look at the SPF in terms of percent of coverage.
SPF 15: Blocks 93% of UVB rays
SPF 30: Blocks 97% UVB rays
No sunscreen can block out 100% of UVB rays
Water Resistant Sunscreen
Waterproof, water resistant, all day protection-with so many variations for a product claim that are seemingly equivalent, which should you choose? The FDA has designated two distinct and straightforward categories for water resistant sunscreen: water resistant and very water resistant. Water resistant formulas are those that maintain their SPF effectiveness after being worn in water for forty minutes. Very water resistant formulas are designed to last up to eight minutes in water.
No sunscreen is waterproof.
“Sunburn Protection Factor.” Food and Drug Administration. N.p., n.d. Web. 15 Sept. 2009. <www.fda.gov/AboutFDA/CentersOffices/CDER/ucm106351.htm >.
“How to Select Sunscreen.” Skin Cancer Foundation. N.p., n.d. Web. 15 Sept. 2009. <www.skincarephysicians.com/skincancernet/selecting_sunscreen.html. >.
Janssen, Jennifer. “Sunscreen.” Texas Corporate Extension. 9 Aug. 1926 http://fcs.tamu.edu/money/your_money/cdm/2003_2004/sunscreen_03_04.pdf.