SPF (Sun Protection Factor)
SPF (Sun Protection Factor) — a measure of how well a sunscreen protects skin from UVB radiation, the wavelengths chiefly responsible for sunburn. The number indicates relative protection under controlled testing, though real-world results depend heavily on how much product is applied and how often it is reapplied.
Key points
- SPF primarily reflects UVB protection and is measured by how much longer protected skin takes to redden compared with unprotected skin under laboratory conditions.
- SPF 30 blocks roughly 97 percent of UVB rays and SPF 50 about 98 percent, so higher numbers add smaller incremental gains.
- SPF does not directly indicate UVA protection, which is why broad-spectrum labeling is also important.
- Most people apply less sunscreen than the amount used in testing, so generous application and regular reapplication are essential to achieve labeled protection.
Frequently asked
Is SPF 100 twice as protective as SPF 50?
No, the difference is small at high levels; SPF 50 blocks about 98 percent of UVB and SPF 100 only slightly more, so proper application matters far more than a very high number.
How often should sunscreen be reapplied?
General guidance is to reapply about every two hours and after swimming, sweating, or towel drying, since SPF protection diminishes with time and activity.
Related topics
This is a foundational entry in the SYNC Skin Encyclopedia and is expanded over time. Educational information only — not medical advice.
