Sun Protection Factor / SPF Number / PA Rating
SPF (Sun Protection Factor) is a measure of a sunscreen's ability to protect against UVB radiation — the primary cause of sunburn and a significant driver of skin cancer and aging. SPF 15 filters ~93% UVB; SPF 30 ~97%; SPF 50 ~98%; SPF 100 ~99%. The PA rating system (PA+, PA++, PA+++) used in Asian markets measures UVA protection. "Broad-spectrum" certification in the US/EU indicates meaningful UVA protection alongside UVB. The effectiveness of any SPF in real-world use depends critically on the amount applied — most people apply 20–50% of the tested amount.
The practical difference between SPF 30 and SPF 50 is small (~1% more UVB filtered) but meaningful over a lifetime of daily use. More important is applying enough (1/4 teaspoon for face), applying it every day regardless of weather, and reapplying every 2 hours in direct sunlight.
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