What Does SPF Mean?
SPF (Sun Protection Factor) measures how well a sunscreen shields your skin from UVB rays — the primary cause of sunburn and a major contributor to skin cancer. It is expressed as a multiplier of your skin's natural protection time. If your unprotected skin would start to redden after 20 minutes of sun exposure, SPF 30 means it would take approximately 600 minutes (20 × 30) before the same reddening occurs under the same UV conditions.
SPF Numbers and Actual Protection
A common misconception is that doubling the SPF number doubles the protection. In reality, SPF 30 blocks about 97% of UVB, SPF 50 blocks 98%, and SPF 100 blocks 99%. The difference between SPF 50 and SPF 100 is just 1 percentage point. What makes a much bigger practical difference is applying enough sunscreen (roughly 1/4 teaspoon or 2 mg per cm² for the face alone) and reapplying it on schedule.
Reapplication Is Non-Negotiable
Regardless of the theoretical protection time, dermatologists universally advise reapplying sunscreen every 2 hours when outdoors. Sweat, humidity, towel-drying, and clothing friction all remove sunscreen from the skin. Water-resistant sunscreens maintain effectiveness for 40–80 minutes in water and should be reapplied immediately after getting out. No sunscreen is truly "waterproof."
Frequently Asked Questions
A. SPF indicates how much longer protected skin takes to redden compared to bare skin. SPF 30 blocks ~97% of UVB, SPF 50 blocks ~98%.
A. Every 2 hours when outdoors, or immediately after swimming or heavy sweating, regardless of SPF.
A. The difference is just 1% more UVB blocked. Applying enough product and reapplying on time matters far more.