How Do Sunscreens Work? What Is the Sun Protection Factor (SPF)?

          The Sun Protection Factor (SPF) is a numerical value that indicates the effectiveness of a sunscreen in shielding the skin from UVB rays, which are responsible for sunburn, skin damage, and the potential development of skin cancer. As per FDA regulations, all sunscreens are required to display a Sun Protection Factor (SPF) label. 

           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.
        For best protection, experts recommend using a minimum SPF sunscreen of 15, applying the proper amount (2mg/cm2 of skin, or about one ounce for full body coverage), and reapplying every 2 hours. Most people under-apply sunscreens, using ¼ to ½ the amount required. Using half the required amount of sunscreen only provides the square root of the SPF. So, a half application of an SPF 30 sunscreen only provides an effective SPF of 5.5!