SUNSCREENS

Unquestionably, the safety and efficacy of sunscreen products is of paramount importance. To this end, the function, UV filters, and product design will be discussed.

Function

As stated, the function of sunscreen products is to absorb/scatter/reflect solar UV, thereby reducing the dose of such harmful radiation to the skin. This is accomplished through the use of a combination of UV filters (Table 2) and an appropriate film-forming vehicle. Whereas for most products, such as cosmetics or over-the-counter (OTC) drugs, it might be enough to simply include ingredients that have an established effect, i. e., cough/cold preparations with antihistamines, decongestants, etc., for sunscreens, the protective effectiveness is communicated directly to consumers as the sun protection factor (SPF). Recently, it has been recognized that SPF is incomplete and some additional measure of protection against long wavelength UV, i. e., UVA-I (340-400 nm), is needed. None­theless, the SPF is meaningful to consumers and the single most important in vivo measure of sunscreen product efficacy.

For consumers, the most recognized and understood skin response to sunlight exposure is erythema or “sunburn.” This can occur in most Fitzpatrick Skin Types and can

be produced in a clinical setting following exposure to an artificial light source. Most important, the erythema action spectrum, i. e., erythemal response as a function of UV wavelength, is nearly identical to the action spectrum for DNA damage (20-22) and nonmelanoma skin cancer as evaluated in hairless mice and predicted for humans (23). Thus, the current in vivo test used to evaluate the functional efficacy of sunscreens is based on an endpoint that is meaningful to consumers, e. g., sunburn protection, and a surrogate for clinically relevant acute and longer term skin damage.

The SPF is a ratio of the response to solar-simulated UV exposure in protected skin versus unprotected skin. Specifically, the minimum erythema dose (MED) is determined for each panelist in an SPF test. This is the time/dose of solar-simulated UV needed to produce a uniform, barely perceptible redness in the skin. The MED will vary depending on Fitzpatrick Skin Type (24,25). To determine the SPF, a product is applied at a fixed dose of 2 mg/cm2 over a 50-100 cm2 area of the lower back. Five to seven “spots” are exposed to varying doses of solar-simulated UV, two/three above, two/three below, and one at the “expected” product SPF. The “expected” SPF is a predicted value from in vitro estimates or the experience of the sunscreen product formulator. At 16-24 hours after UV exposure, the sites are evaluated and the one receiving the lowest UV dose in which a uniform, barely perceptible redness was produced is recorded (26). The “UV-dose/time” is used to calculate the SPF using the following equation: SPF = MED protected skin/MED unprotected skin. According to the methods stipulated by FDA, the SPF is determined in 20 panelists.

There has been much effort to make the SPF test reproducible and reliable, most recently with the introduction of an International SPF Test coordinated by the European Cosmetic Toiletry and Perfumery Association (COLIPA). However, there are several shortcomings of the SPF test and resulting label which should be pointed out. First, it must be clearly understood that the SPF test measures a biological effect using an artificial light source, fixed dose, and an endpoint, i. e., erythema, that is weighted for short wavelengths of UV, namely 290-340 nm (27). Unfortunately, it is overly convenient to refer to SPF as a measure of UVB protection even though it is determined using full spectrum, 290-400 nm, solar – simulated UV and that the UVA-II region (320-340 nm) contributes significantly to high SPF

values. Moreover, because it is a “number,” even knowledgeable individuals overemphasize this quantitative index of what is most certainly a qualitative response. For example, an SPF12 is quantitatively different than an SPF 17, yet from a biological standpoint, the protection afforded by proper use of an SPF 12 or 17 is indistinguishable. As well, the SPF determined under controlled laboratory conditions is dependent on the light source and may be different if the light source changes, e. g., solar-simulated light versus natural sunlight. Finally, the SPF ratio is “nonlinear” since an SPF 15 is not half of an SPF 30 based on the ability to reduce erythemally-weighted UV. That is, the SPF is determined by the erythema action spectrum which, as stated previously, is weighted for short wavelengths of UV. The SPF can be represented as a percent of erythemally-weighted UV transmitted, i. e., 1/SPF X 100, or blocked, i. e., [1 – (1/SPF) X 100]. Thus, an SPF 15 blocks 93.3% and SPF 30, 96.7% of the erythemally-weighted UV or a mere 3% difference. Finally and perhaps the most significant limitation of SPF is the failure to provide assurance of protection against long wavelengths of UV, 340-400 nm, or the so-called UVA-I.

Whereas it is now established that protection of long wavelength UVA is essential (28-30), as of December 2005 there is no agreed to, regulatory-mandated means of measuring or communicating UVA protection of sunscreen products to consumers in the United States. To further complicate the situation, there are currently no known surrogates for long wavelength UVA damage that can be measured in the skin following acute exposure to filtered solar-simulated UV. This fact has profound implications for any in vivo human study measuring a UVA protection factor, i. e., the ratio of response to a filtered, artificial light source at a fixed dose in protected versus unprotected skin. The most prominent concern is that any such UVA protection factor based on a response which has no direct relationship to a human health concern, e. g., photoaging or skin cancer, is nothing more than misleading at best. Further, the resulting UVA protection factor is meaningless to consumers since the skin response, e. g., persistent pigment darkening or “color change,” is not a response to which protection is considered when purchasing a sunscreen product. Finally, it is possible to measure a UVA protection factor without protecting against the breadth of UVA. Nonetheless, there are numerous proponents of such an approach and tests including persistent pigment darkening (PPD) (31) and protection factor UVA (PFA) (32,33).

Since any in vivo test is without merit, there have been several in vitro methods proposed (34,35). In general, the in vitro approach is based on measurement of absorbance/ transmittance of UV through a sunscreen product applied to a substrate (36). The resulting data can be used to calculate a “metric” from which some labeling designation can be derived (37-39). This general approach has been used successfully in several countries including Australia (AS/NZS 2604 Sunscreen products, evaluation, and classification), U. K. (Boots Star Rating), and Germany (DIN draft standard 67502 whereas the UVA-protection is now calculated as UVA-balance). The stated objection to such in vitro approaches is that they are not done on human skin and, as such, cannot provide quantitative information regarding “protection.” Despite this concern, substrate spectrophotometric measures of absorption have several advantages including cost, reproducibility, and human subjects are not intentionally exposed to an artificial filtered light source the health consequences of which are unknown. Moreover, the results from in vitro substrate spectrophotometric studies can provide complimentary information to the in vivo derived SPF.

In sum, the function of sunscreen products is to reduce the dose of solar UV thereby mitigating or reducing damage to the skin. The SPF test provides meaningful, in vivo information regarding the protection against solar UV which is weighted for short wavelengths based on the erythema action spectrum. The SPF “number” is recognized by consumers as the measure of sunscreen product efficacy. In vitro substrate

spectrophotometric measures of sunscreen product absorbance and calculation of a metric such as Critical Wavelength can serve as an independent, complementary measure of long wavelength, UVA, sunscreen product efficacy. The American Academy of Dermatology has provided a recommendation (40) which may serve as the basis for a regulatory – mandated testing and labeling for UVA efficacy of sunscreen products.