DermUV
SPF
SPF (Sun Protection Factor) measures how much UVB a sunscreen blocks: SPF 30 = ~97% of UVB blocked; SPF 50 = ~98%.
SPF explained
SPF is a multiplier on burn time, but only when 2 mg/cm² is applied — most users apply 25–50% of that, so real-world SPF is much lower than labeled. AAD recommends a minimum of SPF 30 broad-spectrum daily, applied 15 minutes before exposure and reapplied every 2 hours.
Related terms
Terms related to SPF connect the UV and sunscreen topic cluster.
Related DermUV pages
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