SPF, or Sun Protection Factor, is a measure of how well a sunscreen will protect skin from Ultraviolet B rays. UVB causes sunburn, damages skin, and can contribute to skin cancer. 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. SPF 15 blocks approximately 93 percent of UVB rays. SPF 30 keeps out 97 percent and SPF 50 keeps out 98 percent.
For best protection, a minimum of SPF 15 is recommended. Ratings higher than SPF 30 offer little additional protection. Sunscreen should be applied about one ounce for full body coverage and reapplied every 2 hours. SPF does not measure how well a sunscreen will protect from UVA rays, which can also damage the skin and exacerbate the effects of UVB. Some sunscreens are broad spectrum and offer some protection against UVA.