Dosage
Probably the greatest drawback with sunscreen is that most people do not apply a sufficient dose. The SPF number is determined by the manufacturer using a “standard” application amount (required by the FDA) of 2 milligrams of product per square centimeter of skin. For the average adult in a bathing suit this would be a little more than one ounce, or approximately a shot glass full. (That’s one quarter of the typical 4 oz. sunscreen bottle.) When a person applies less than the “standard” amount, only a fraction of the labeled SPF will be achieved. Studies have shown that the average person applies less than half the “standard” amount of sunscreen, which results in an SPF of only about one third of that labeled on the bottle. Spray on sunscreens are particularly subject to underdosing. By contrast, if more than the “standard” amount is applied, the SPF achieved will exceed that labeled on the bottle.