Sun Protection Factor, a measure of how well a sunscreen protects against ultraviolet B radiation. Higher SPF numbers indicate greater protection from sunburn.
Acronym coined in 1962 by Franz Greiter, an Austrian chemist who developed the first commercial sunscreen. The term became standardized in the cosmetics industry in the 1970s.
Most people don't realize that SPF 30 blocks about 97% of UV rays while SPF 50 blocks 98% - the difference is much smaller than the numbers suggest. The SPF scale is logarithmic, not linear, which explains why dermatologists often say SPF 30 is sufficient for most people.
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