SPF
Meaning of SPF: A number describing how much more ultraviolet skin can take before burning, measured under laboratory conditions almost nobody reproduces.
Definition of SPF
SPF, short for sun protection factor, rates how well a sunscreen protects against sunburn. It compares the amount of ultraviolet needed to redden protected skin with the amount needed on the same skin unprotected. The scale reflects protection against ultraviolet B, the wavelengths chiefly responsible for burning, rather than protection against all ultraviolet.
What does the number mean in practice?
The relationship is not proportional. A factor of 15 filters roughly ninety-three percent of burning ultraviolet, 30 filters about ninety-seven percent and 50 about ninety-eight. Each step up therefore adds less than the previous one, and the gap between the high numbers is small. What differs more usefully at the top of the range is the margin for error: when a product is applied thinly, a higher rated one retains more of its protection.
What does it not cover?
| Wavelength | Covered by the SPF number | Effects |
|---|---|---|
| Ultraviolet B | Yes | Sunburn, and the main driver of DNA damage in the outer skin |
| Ultraviolet A | No | Penetrates deeper, contributes to aging and to skin cancer |
| Visible and infrared | No | Heat, and pigment changes in some skin types |
Coverage of the longer wavelengths is indicated separately by a broad spectrum claim, or by star or letter ratings in some countries.
Why does the laboratory number overstate real protection?
Testing applies a thick, even layer, considerably more than people apply in ordinary use. Because the relationship between the amount applied and the protection achieved is steep, applying a fraction of the test quantity delivers far less than the labeled factor. Missed areas such as the ears, lips, back of the neck and the tops of the feet receive nothing at all, and sweat, water and rubbing remove what was applied.
Where does sunscreen sit among protections?
Ultraviolet exposure is a well established risk factor for skin cancer and for photoaging. Shade, clothing, broad-brimmed hats and the timing of outdoor activity reduce exposure directly rather than filtering it, and they do not depend on being reapplied. Sunscreen is one layer among those rather than a substitute for them.
Does a high number last longer?
No. The rating describes how much ultraviolet is filtered, not how long the product stays in place. Water, sweat and towel drying remove it regardless of the number on the bottle, and water resistance is tested and labeled separately. A product rated 50 that has washed off protects no better than one rated 15 that has washed off.
Also known as
Used in a sentence
The record noted the use of a broad spectrum sunscreen with a high SPF alongside protective clothing.
Good to know
SPF measures protection against burning ultraviolet B only, so a high number on a product that carries no broad spectrum claim says nothing about ultraviolet A.