Several recent publications have shown that most persons apply only 20% to 60% of the amount of sunscreen needed to achieve the sun protection factor (SPF) rating of their applied product.1-3 One reason that sunscreens are insufficiently applied is lack of a simple rule for the amount needed for each region of the body. The SPF is tested at an applied thickness of 2 mg/cm2 (or 2 µL/cm2, assuming a specific gravity of 1). To cover the average 1.73-m2 adult body requires about 35 mL.2
Schneider J. The Teaspoon Rule of Applying Sunscreen. Arch Dermatol. 2002;138(6):838–839. doi:10.1001/archderm.138.6.838
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