A simple measure for applying sunscreen while on holidays
Dermatology Department. Hospital Central. Universidad Autonoma de San Luis Potosi. Mexico. email@example.com
To the editor:
The use of topical sunscreens is considered to be an important intervention to reduce skin damage induced by ultraviolet radiation from sunlight. The sun protection factor (SPF) of a sunscreen is based on a uniform application thickness of 2 mg/cm2.  However, there is no known linear relationship between an effective SPF and the amount of sunscreen applied. The efficacy of a sunscreen therefore is highly dependent upon its correct application.  The amount of sunscreen that most people apply usually never exceeds more than 60 percent of the quantity needed to achieve the SPF on the label product. [1, 3] One reason that sunscreens are insufficiently applied is the absence of conventional parameters to assess the amount of sunscreen necessary to cover the exposed surfaces. Although the "teaspoon rule" and the "fingertip unit" have been proposed as a dosage guide users do not find these standards easy to use in practice.[4, 5] Under real-life conditions, a suitable teaspoon is not readily available, when the different sizes, shapes and uses of teaspoons world-wide is taken into account. The semiquantitative "fingertip unit" guide is a good approach but has its disadvantages, especially its lack of utility when lotions or gels are being applied. In addition, the explanation of how to use it is complicated and the time required to carry out the process in such large areas as the chest, back, or legs makes the process unlikely to be widely adopted.
Most sunburns develop while persons are outdoors during their leisure time. At these times, they are often drinking alcoholic or non-alcoholic beverages from bottles with caps. The crown tin lid is a metal bottle cap which is widely used, and which has an almost uniform size no matter where in the world it is manufactured (Fig. 1). Taking this into account, we suggest a practical method that would make it easy to apply a standard quantity of sunscreen with disposable metal bottle caps. Crown tin lids are frequently available at the time when people are about to expose themselves to sunlight. Although the lids may differ slightly in their size, we think they serve as a less variable reference than a spoon.
A crown tin lid filled with a regular SPF-30 sunscreen contains an average of 3.3 grams (3,300 mg) of cream. This amount is enough to cover at least a 1,500 cm2 area. To ensure a proper dosage, some authors have suggested the use of the well-known "rule of nines" in the same way that burned areas are calculated. [4, 5] The total body surface is divided into three 9 percent regions (head and neck, left arm, and right arm), and four 18 percent regions (back, torso, left leg, and right leg). The head and neck segment includes the scalp, so that the face and neck to which sunscreen is applied is over-represented by the "rule of nines". When morphometry was used to calculate the face and neck area in a volunteer who was not bald, we found that the surface area approachs 685 cm 2. Therefore, approximately 1,370 mg of sunscreen would be needed to cover the face and neck ( 685 cm2 x 2 mg = 1,370 mg/cm2). The amount required corresponds to one half of the crown tin lid. If we use the "rule of nines" for the rest of the body, and assume that the average 1.73 m2 (17,300 cm2) adult surface area is divided into segments, we obtain the figures shown in Table 1. Half of a lid is required for the face and neck, one lid for each upper limb, and two lids each for the back, the torso and for the two lower limbs.
In conclusion, we suggest an easy, alternative method to apply suncreens. This system would allow users to apply a reliable quantity of sunscreen in a uniform and practical manner. It would also offer protection which is closer to the protection promised by the bottle label with whatever product vehicle is used (i.e., liquid, gel, or cream).
References1. Stokes RP, Diffey BL. How well are sunscreen users protected? Photodermatol Photoimmunol Photomed 1997;13:186-188.
2. Brown S, Diffey BL. The effect of applied thickness on sunscreen protection: in vivo and in vitro studies. Photochem Photobiol 1986;44:509-13.
3. Autier P, Boniol M, Severi G, Dore JF, European Organization for Research and Treatment of Cancer Melanoma Co-operative Group. Quantity of sunscreen used by European Students. Br J Dermatol 2001;144:288-291.
4. Schneider J. The teaspoon rule of applying sunscreen. Arch Dermatol 2002;138:838-839.
5. Taylor S, Diffey BL. Simple dosage guide for suncreams will help users. BMJ 2002;324:1526.
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