SPF stands for sun protection factor. Sunscreens are rated or classified by the strength of their SPF. The SPF numbers on the packaging can range from as low as 2 to greater than 50. These numbers refer to the product's ability to deflect the sun's burning rays (UVB).
The sunscreen SPF rating is calculated by comparing the amount of time needed to produce a sunburn on sunscreen-protected skin to the amount of time needed to cause a sunburn on unprotected skin. For example, if a sunscreen is rated SPF 2 and a person who would normally turn red after 10 minutes of exposure in the sun uses it, it would take 20 minutes of exposure for the skin to turn red. A sunscreen with an SPF of 15 would allow that person to multiply that initial burning time by 15, which means it would take 15 times longer to burn, or 150 minutes. Even with this protection, sunscreen photo degrades (breaks down) and rubs off with normal wear, so it needs to be reapplied approximately every two hours.
Dermatologists strongly recommend using a broad-spectrum (UVA and UVB protection) water-resistant sunscreen with an SPF of 30 or greater year-round for all skin types. This will help protect against sunburn, premature aging (e.g., age spots and wrinkles) and skin cancer.