Comparatively Speaking: SPF vs. FEF

By: Anthony J. O'Lenick Jr., Siltech LLC; and Thomas O'Lenick, PhD, SurfaTech Corp.

Posted: August 30, 2011

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Fair-skinned consumers are likely to absorb more solar energy than dark-skinned consumers under the same conditions. The amount of sunscreen applied also impacts the amount of solar radiation absorbed, because more sunscreen results in less solar energy absorption.

Because sunscreens wear off and become less effective with time, the frequency with which they are reapplied is critical to limiting absorption of solar radiation. The reapplication frequency is also impacted by a consumer's activities. For example, consumers who swim while wearing sunscreen need to reapply the sunscreen more frequently because water may wash the sunscreen from the body. In addition, high levels of physical activity require more frequent reapplication because the activity may physically rub off the sunscreen and heavy sweating may wash off the sunscreen.

In general, more frequent reapplication is associated with decreased absorption of solar radiation. Because of the various factors that impact the amount of solar radiation, SPF does not reflect time in the sun. In other words, SPF does not inform consumers about the time that can be spent in the sun without getting sunburn. Rather, SPF is a relative measure of the amount of sunburn protection provided by sunscreens. It allows consumers to compare the level of sunburn protection provided by different sunscreens. For example, consumers know that SPF 30 sunscreens provide more sunburn protection than  sunscreens with SPF 8.

Formulation Efficiency Factor (FEF)

Formulation Efficiency Factor (FEF) is a new concept in the sunscreen field. This new factor relates the amount of active sunscreens in a formulation to the overall SPF of the finished product. The formula for formulation efficiency factor (ε) is shown in Figure 1.

The FEF method can be applied to common commercial sunscreen formulations. It is a quick and effective way for a formulation chemist to test the efficiency  of their formula and the effectiveness of their modifications. Commercial sunscreens can be broken down into three major categories based on the formulation active ingredients: organic, inorganic/organic and inorganic actives. Three ranges of FEF factors for the actives were determined by a study of commercial sunscreens in the US market, and these ranges are shown in Table 1.