How Much Sunscreen Is Enough?

The sun protection factor (SPF) is used by the U.S. Food and Drug Administration (FDA) to quantify the relative efficacy of a sunscreen in preventing sunburn. To achieve the stated SPF value requires even application of sunscreen to the skin at the FDA’s standard dose of 2 mg/cm² prior to UV exposure. Previous studies have shown that in real life, consumers use only about 0.5 mg/cm², or 25% of the standard dose. Investigators in Denmark examined the degree to which application of amounts other than the FDA standard would affect photoprotection.

Nineteen volunteers (skin types I–III) were treated with 0.5, 1.0, 2.0, and 4.0 mg/cm² of SPF 4 sunscreen before receiving a minimal erythema dose of UV light. All the sunscreen concentrations safegua…