Background: The rule of thumb "Fill up a handful of sunscreen and spread it all over your body" has been used in several sun safety campaigns. The intention was to increase the applied sunscreen to obtain a quantity of 2 mg/cm2 to all accessible skin. The present study is the first to investigate how this advice works in practice, evaluated by quantity of sunscreen applied and amount of covered skin.
Methods: Seventeen volunteers wearing swimwear were asked to "Fill up a handful and spread it all over your body." Before and after sunscreen application, the volunteers were photographed in black light. As sunscreen absorbs black light, the darkness of the skin increases with increasing amounts of applied sunscreen, making it possible to identify skin left without coverage. The sunscreen container was weighed before and after to quantify the amount of sunscreen applied.
Results: A median of 21% of the accessible skin was left completely without coverage. The 79% covered area was covered with a median of 1.12 mg/cm2, not the expected 2 mg/cm2.
Conclusion: In practice, the advice "Fill up a handful of sunscreen and spread it all over your body" led to a better but still modest protection, compared to the intended effect.
Keywords: Sun protection factor; Sunscreen; Sunscreen recommendations.