SPF (Sun Protection Factor)

SPF, or Sun Protection Factor, is a number that indicates how well a sunscreen protects the skin from UVB radiation, the wavelengths chiefly responsible for sunburn.

The value reflects the relative amount of UVB energy required to redden protected skin compared with unprotected skin under standardized testing. Higher numbers indicate more UVB filtering, but the relationship is not linear: SPF 30 filters roughly 97 percent of UVB and SPF 50 around 98 percent, so the jump from 30 to 50 adds only a small increment. Importantly, SPF describes UVB burn protection only and says nothing directly about UVA, which is why a broad-spectrum label and, where used, a PA rating are read alongside it.

Laboratory SPF is measured with a thick, even film of product. Real-world protection is usually lower because people apply less than the tested amount and miss areas. Getting close to the labeled value depends on generous, even application.

• Apply enough to cover all exposed skin, including ears, neck, and the back of the hands.
• Reapply periodically during sun exposure, commonly suggested at roughly every two hours and after swimming, sweating, or towel drying.

SPF is one part of sun safety rather than a complete solution. It is best combined with broad-spectrum coverage, shade, protective clothing, and avoiding peak-intensity midday sun. No sunscreen blocks all radiation, so a high SPF should not be treated as permission for unlimited exposure.