Summary: A standardized measure of a sunscreen’s ability to protect skin from UVB radiation–the type that causes sunburn.
SPF, or Sun Protection Factor, quantifies how much UVB radiation is filtered by a sunscreen when properly applied. SPF values are determined through standardized in vivo testing, where sunscreen is applied at a dose of 2 mg/cm² to human skin. The skin is then exposed to controlled UVB light to determine the minimal erythema dose (MED)—the smallest amount of UVB that causes visible redness. The SPF is calculated as the ratio of the MED on protected skin to the MED on unprotected skin. For example, SPF 30 means it took 30 times more UVB exposure to cause redness compared to unprotected skin. This value reflects a protection factor against dose, not time.
Since SPF quantifies the fraction of UVB radiation that reaches the skin when sunscreen is properly applied, SPF 30 allows approximately 1⁄30th of UVB rays to reach the skin, meaning it blocks about 97% of UVB exposure. SPF 50 blocks about 98%, and SPF 15 blocks around 93%.
SPF does not measure protection against UVA radiation unless the product is also labeled “broad-spectrum.”