What is the minimum recommended SPF and the percentage of protection does it provide?

• A. SPF 15 — about 90% blocked
• B. SPF 30 — about 70% blocked
• C. SPF 50 — about 60% blocked
• D. SPF 4 — about 20% blocked

Answer: (A)

SPF tells you how much UVB protection a sunscreen provides relative to unprotected skin. It’s based on the idea that the fraction of UVB rays that get through is about 1 divided by the SPF, so the blocked portion is roughly 1 − (1/SPF). For SPF 15, that’s 1 − 1/15 ≈ 0.93, meaning about 93% of UVB rays are blocked. In practice this is described as around 90% protection, which is why SPF 15 is considered the minimum commonly recommended level. Higher SPFs block slightly more UVB, but no sunscreen blocks 100% of UV radiation, and UVA protection depends on whether the product is broad-spectrum. Hence SPF 15 providing about 90% protection matches the standard teaching for the minimum recommendation.