The "rule of nines" divides the body's surface area into 11 areas, each representing roughly 9% of the total. Sunscreen can be applied to the recommended thickness if two strips of sunscreen are squeezed out onto both the index and middle fingers from the palmar crease to the fingertips. The application of this "two fingers" of sunscreen will provide a dose of the product that approximates to that used during the laboratory determination of the SPF.
Users in fact are unlikely to be willing to cover themselves or their families with such a copious layer of sunscreen and would prefer to apply half this amount, say the authors.
They therefore suggest applying one finger of sunscreen, with the corollary that the resultant protection would only be about half that stated on the product. Users should then be encouraged to reapply one finger's worth within half an hour of the initial application in order to achieve optimum protection, they conclude.