What Does SPF Mean?

Sun protection factor, or SPF, is a measure of a sunscreen’s ability to protect the skin from ultraviolet B (UVB) radiation, the portion of sunlight primarily responsible for sunburn and a major contributor to skin cancer. The Sun also emits ultraviolet A (UVA) radiation, which penetrates deeper into the skin and contributes to skin aging, tanning, and skin damage. Broad-spectrum sunscreens are formulated to protect against both UVA and UVB radiation.

SPF indicates how effectively a sunscreen reduces exposure to UVB rays when applied as directed. For example, an SPF 15 sunscreen allows only a fraction of UVB radiation to reach the skin compared with unprotected skin. Although SPF is sometimes described as extending the amount of time a person can remain in the sun without burning, actual protection varies depending on factors such as the intensity of sunlight, skin type, sweating, swimming, and the amount of sunscreen applied.

The percentage of UVB radiation blocked by sunscreen increases with SPF value, although the gains become progressively smaller at higher levels. SPF 15 blocks approximately 93 percent of UVB rays, SPF 30 blocks about 97 percent, and SPF 50 blocks about 98 percent. Even high-SPF products do not provide complete protection, and health authorities recommend reapplying sunscreen at least every two hours, as well as after swimming, sweating, or towel drying.