Understanding SPF: What sunscreen numbers really mean for skin protection
As summer sets in, many consumers are reaching for sunscreen, but experts caution that the Sun Protection Factor, or SPF, printed on bottles doesn’t tell the whole story about sun safety.
SPF is designed to measure protection against ultraviolet B, or UVB, radiation — the part of sunlight primarily responsible for sunburns. However, a higher SPF does not automatically mean better protection, nor does it account for all types of sun damage.
Health specialists note that SPF numbers indicate how much longer it would take UVB rays to redden skin compared to using no sunscreen. For example, SPF 30 means it would take 30 times longer to burn than if skin were unprotected. Yet SPF does not measure protection against ultraviolet A, or UVA, rays, which penetrate deeper and contribute to skin aging and long-term damage. Broad-spectrum sunscreens are needed to guard against both UVA and UVB.
Dermatologists add that the difference between high SPF values is also smaller than many expect. SPF 30 blocks about 97% of UVB rays, while SPF 50 blocks roughly 98%. No sunscreen blocks 100%.
Experts recommend looking for “broad spectrum” on the label, reapplying every two hours, and using other measures like shade and protective clothing. Understanding what SPF measures, and what it does not, can help in making more informed decisions about skin protection.
