SPF stands for Sun Protection Factor and measures the level of protection offered by the product. The higher the number, the greater the protection against UVB rays.
SPF 15 sunscreen may absorb more than 92% of UVB radiation
SPF 30 sunscreen may absorb 96.7% of UVB radiation
SPF 40 sunscreen may absorb 97.5% of UVB radiation
If you know how much time it takes for your unprotected skin to burn, you can use the SPF to estimate how long you'll be protected for when you apply a sunscreen. A SPF of 15 means your skin would take 15 times longer to burn with the sunscreen than it would without. For example, if you normally burn in 10 minutes without a sunscreen, a SPF15 product would protect your skin for about 150 minutes (10 x 15 = 150).
The level of protection depends on your skin type and how well you apply the sun block. Other factors such as the time of day, time of year and weather will also affect how long a sunscreen will protect you.
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