SPF Rating Scale
Authority: US FDA Sunscreen Monograph / Broad Spectrum Test Method
SPF ratings measure UVB protection on a non-linear scale: SPF 15 blocks about 93% of UVB rays, SPF 30 blocks about 97%, and SPF 50 blocks about 98%. No sunscreen blocks 100% of UVB radiation, and the SPF number does not measure UVA protection or how long protection lasts in sunlight.
| SPF Rating | UVB Rays Blocked | UVB Rays Reaching Skin |
|---|---|---|
| SPF 15 | 93% | 7% |
| SPF 30 | 97% | 3% |
| SPF 50 | 98% | 2% |
| SPF 100 | 99% | 1% |
Origins of the SPF Concept
The sun protection factor concept was introduced in 1974 by Austrian chemist Franz Greiter, who built on earlier ultraviolet-measurement work by researchers Friedrich Ellinger and Rudolf Schulze. Greiter had developed a sunscreen called Gletscher Creme in 1946, later estimated to have an SPF of about 2, which became the basis for the Piz Buin sunscreen brand.
SPF gave manufacturers and consumers a standardized way to compare how much UVB protection different products offered, and it remains the global reference measurement used on sunscreen labels today.
What SPF Doesn't Measure
The SPF number reflects UVB filtering only; it says nothing on its own about protection from UVA radiation, which penetrates more deeply into skin and contributes to premature aging and long-term skin damage. In the United States, a sunscreen can only be labeled 'broad spectrum' if it passes a separate FDA test, under rules that took effect in 2012, showing its UVA protection is proportional to its UVB protection.
A higher SPF also does not mean protection lasts longer on the skin. FDA guidance notes that sunscreens of any SPF value need to be reapplied on the same general schedule, typically about every two hours and more often after swimming or sweating, since the number describes filtering strength rather than duration of effectiveness.