Sun Protection Factor (SPF)

Sun protection factor (SPF) is a measure of how effectively a sunscreen prevents sun damage to human skin. The term, first coined in 1928, recognizes that SPF products are numerically graded according to the amount of irritation produced from ultraviolet radiation (UVR). SPF is a calculated ratio of the minimum amount of erythema produced with sunscreen versus the minimum amount produced with bare skin when exposed to UVR.

The U.S. Food and Drug Administration recommends an SPF of 15 to reduce the chance of developing precancerous skin lesions. The greater the SPF number, the higher the UVR absorption; however, when comparing SPF values above 15, there is a less dramatic increase in UVR absorption. For instance, SPF 15 absorbs 93 percent of ultraviolet B (UVB) light while an SPF of 30 absorbs 97 percent, yet an SPF of 50 absorbs 98 percent. Most SPF values are specific for the measurement of UVB.

Historically, sunscreens were labeled according to their ability to absorb or reflect UVB. Even so, these labels characterizing a sunscreen as a chemical or physical agent are becoming increasingly less used and out of date. The wide array of chemical agents used in the United States primarily absorb UVB; however, avobenzone (Parsol 1789) has strict UVA absorption. Paraaminobenzoic acid (PABA), a well-known chemical sunscreen, has been known to cause contact and photocontact dermatitis in 4 percent of users.

Physical sunscreens are those agents previously characterized as sunblocks, which work to reflect and scatter UVB. These opaque emollients have fallen out of favor as they are typically heavy and produce an unattractive appearance. Innovative design to physical sunscreens containing zinc and titanium oxide has led to a micronized, small particle formula with an increased absorptive capability, thus dulling the parameters defining chemical and physical sunscreens.

Ultraviolet A (UVA) light and UVB are most responsible for disease development. UVA is responsible for penetrating deep into skin layers, creating a suntan while UVB's shorter wavelength penetrates less deeply, causing a sunburn. The damage from sunburn and suntan are irreversible and repeat exposure builds over a lifetime—skin changes seen in older individuals are a result of total lifelong sun exposure.

A decreased incidence of squamous and basal cell skin lesions with regular sunscreen use is well documented, yet the incidence of melanoma with sunscreens is less understood. A liberal amount of a high SPF product should be applied daily prior to sun exposure. Sunscreen should be reapplied every two hours or sooner if activities include excessive sweating or swimming. Loose, dry garments containing a tight weave, dark color, and either denim or lycra afford [Page 1631]the greatest SPF protection. To obtain the quoted SPF, the quantity and how frequently the product is applied must be performed as recommended, but the best method of avoiding sun damage is to refrain from exposure between 10 a.m. and 3 p.m.

High SPF sunscreen should be reapplied every two hours or sooner if activities include excessive sweating or swimming.