SPF
Definition
SPF stands for Sun Protection Factor, a numerical rating that indicates how much longer a sunscreen allows you to stay in the sun before UVB rays cause sunburn compared to unprotected skin. For example, SPF 30 theoretically allows you to stay in the sun 30 times longer than without protection. However, this is somewhat misleading — SPF is better understood in terms of UVB filtration: SPF 15 blocks about 93% of UVB rays, SPF 30 blocks about 97%, and SPF 50 blocks about 98%. No sunscreen blocks 100%. SPF only measures UVB protection, not UVA protection, which is why broad-spectrum coverage is equally important. Real-world protection is significantly lower than lab conditions because people typically apply far less sunscreen than the tested amount (2 mg/cm²) and fail to reapply every two hours. Dermatologists recommend SPF 30 or higher, applied generously (about a nickel-sized amount for the face) and reapplied every two hours during sun exposure.
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