Ask the Chatbot Games & Quizzes History & Society Science & Tech Biographies Animals & Nature Geography & Travel Arts & Culture ProCon Money Videos
What Does SPF Mean?
Introduction References & Edit History Related Topics
Images
Sunburn prevention
Quick Facts
Britannica AI Icon
Adding us as a Preferred Source tells Google you trust our content, so our articles show up more often in your Google Search, Discover feed, and AI Overviews.
verifiedCite
While every effort has been made to follow citation style rules, there may be some discrepancies. Please refer to the appropriate style manual or other sources if you have any questions.
Select Citation Style
Feedback
Corrections? Updates? Omissions? Let us know if you have suggestions to improve this article (requires login).
Thank you for your feedback

Our editors will review what you’ve submitted and determine whether to revise the article.

External Websites
Sunburn prevention
Sunburn prevention Sunscreen helps protect the skin from ultraviolet radiation in sunlight.

What Does SPF Mean?

Written and fact-checked by
Britannica Editors
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
Britannica Editors
History

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.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Kara Rogers.