Editor—The sun protection factor (SPF) of a sunscreen is determined after phototesting in vivo at an internationally agreed application thickness of 2 mg/cm2. Yet studies have shown that consumers apply much less than this—typically between 0.5 and 1.5 mg/cm2.1 This has an appreciable effect on protection, with typical application rates achieving a sun protection factor of perhaps one third of that stated on the product.2 This mismatch may be one contributing factor why sunscreens have been reported to be a risk factor in melanoma.3
No international standard on sunscreens specifies dosage criteria for users, although it is assumed to be 2 mg/cm2.4 So it is not surprising that consumers do not know how to apply sunscreens effectively. We suggest here a method that would allow users to apply a quantity of sunscreen that would result in closer agreement between the expected and delivered protection.
The “rule of nines” is used to assess the extent of a patient's burns as a percentage of the patient's body surface area. The “fingertip unit” is used to measure the amount of cream or ointment to be used in dermatology: it is a strip of product squeezed on to the index finger, from the distal crease to the fingertip.
With the rule of nines, the body's surface area is divided into 11 areas, each representing roughly 9% of the total (box). Sunscreen can be applied to each of these areas at a dose of 2 mg/cm2 if two strips of sunscreen are squeezed out on to 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 sun protection factor. Such a dosage guide is a means of ensuring that users are protected according to their expectations.
Body areas by “rule of nines” for extent of burns
1 Head, neck, and face
2 Left arm
3 Right arm
4 Upper back
5 Lower back
6 Upper front torso
7 Lower front torso
8 Left upper leg and thigh
9 Right upper leg and thigh
10 Left lower leg and foot
11 Right lower leg and foot
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. A less daunting proposition, and the one that we suggest, is therefore to apply one finger of sunscreen, with the corollary that the resultant protection would be only about half that stated on the product. Users should be encouraged to reapply one finger's worth within half an hour of the initial application in order to achieve optimal protection.5