Enhancing Sunscreen Efficacy for Realistic Application

Both UVA and UVB solar radiation can cause skin damage, and while sunscreens contain organic or physical UV filters to protect the skin from sunburn, questions remain as to the role of sunscreens in preventing melanoma.1 However, the association made between lack of sunscreen use and melanoma risk may relate to individuals who remain in the sun for long periods of time without seeking shade and/or wearing sunscreen, protective clothing and hats.

The ability of a sunscreen to protect the skin from erythema is expressed on product labels as the sunburn protection factor (SPF)—i.e., the ratio of the minimum erythema dose (MED) with sunscreen to the MED without protection. Yet in reality, consumers do not apply the same mass/cm2 as is utilized in SPF testing, so maximal protection is not achieved.2 In addition, other factors interfere with sunscreen efficacy, including reapplication, sweat and water resistance, formulation and packaging. These challenges are all discussed in the present article.

Insufficient Dosage
Worldwide, SPF testing utilizes the standard sunscreen dosage of 2 mg/cm2 on skin, and this amount is recommended to provide high levels of photoprotection. As noted, however, surveys examining the sunscreen dosages applied by sunbathers suggest they are not applying the recommended amount. For example, in five European locations, Autier et al.3 determined that students were applying a UV filter at a median of 0.39 mg/cm2.

In relation, a study conducted by Azurdia et al., revealed that photosensitive patients felt that UV filters were of minimal benefit.4 Therefore, using fluorescence spectroscopy, the researchers quantitatively assessed the amount of sunscreen applied by ten photosensitive women. They found an overall sunscreen dosage of just 0.5 mg/cm2; and in fact, on no anatomical site of the ten women studied did the amount of sunscreen applied reach 2 mg/cm2. In addition, sunscreen dosage application showed considerable variation from anatomical site to site. These results indicated that the subjects applied far less sunscreen than recommended, resulting in lower protection, and that photosensitive patients, who are urged to be more careful with sun protection, did not utilize a better sunscreen application technique.

Further, another reason that sunscreen dosing is often insufficient relates to cost, since in order to achieve the SPF labeled for one whole body application, one-third of a 100-mL sunscreen container must to be utilized.1 Faurschou and Wulf 2 calculated the realistic sun protection level achieved when an insufficient sunscreen dosage is applied and related the amount of sunscreen on the skin to the SPF measured. Four sunscreen doses from 0.5 to 4 mg/cm2 of an SPF 4 sunscreen were applied to the backs of 20 volunteers and each volunteer was phototested by exposure to UVB radiation. The researchers showed the relation between the amount of sunscreen applied and the SPF followed an exponential growth—i.e., the SPF falls by a square root when 1 mg/cm2 is applied and by a fourth root when 0.5 mg/cm2 is applied, in relation to the recommended 2 mg/cm2. For example, when 0.5 mg/cm2 of an SPF 4 sunscreen is applied to skin, the protection achieved cannot exceed SPF 3; the same result is obtained with SPF 81.

Uneven Application
Another reason for lack of sunscreen efficacy relates to incomplete coverage. Fifty volunteers applied a lotion containing a fluorescent marker that Loesh et al. examined using Wood’s lamp to identify areas missing sunscreen.5 Thus, where there is no fluorescence, there is no lotion. Each volunteer applied 15g of a lotion and they were told to apply it as if it were a sunscreen. Only the missing spots were studied with fluorescence; the authors did not measure the quantity or applied on the face. The investigation revealed incomplete sunscreen coverage areas such as the eyelids, hairline and ears. Azurdia also showed that some anatomical sites were completely missed, especially the neck and V of the chest.4

Lynfield and Schechter further showed that on average, only 22 g of sunscreen is applied to the entire body, where standard textbooks estimate the amount required to cover the entire body at around 30–60 g.6 To obtain the SPF labeled on the package, 35 mL is required.1

Sunscreen Formulations
The unevenness of application and incorrect dosage of sunscreens could also be the result of the formulations used. Diffey and Grice investigated the use of chemical and physical sunscreens labeled SPF 25.7 The median physical sunscreen amount was 0.94 mg/cm2 and the median chemical sunscreen amount was 1.48 mg/cm2. The physical sunscreen was used in a smaller quantity than the chemical sunscreen since subjects found the physical sunscreen more difficult to spread all over the body.

Ivens et al. evaluated skin coverage using a fluorescent cream, solution and ointment formulations.8 While these formulations were not sunscreens, they demonstrated the skin coverage of different types of basic formulations. The fluorescent marker helped to localize the product on the body under Wood’s lamp; therefore, the researchers were able to determine which formulation spread best on the body. Results indicated that only the ointment was equally distributed on the skin. Other formulations were spread sparsely, compared with the application center. This is because ointment does not immediately dry and become tacky. Rather, it remains in a flowing state with the active substance dissolved in its carrier ingredient. In comparison, creams and solutions rapidly increase in viscosity after application due to rapid alcohol/water evaporation.

In addition, Barr studied the efficacy of sunscreens in a spray form, observing that they provide protection only in the area where the mist is applied.9 Since spray is transparent and fast-drying, users reported difficulty in determining whether all exposed skin was protected.

Sunscreen Packaging
Lynfield and Schechter compared the application amount of four preparations: an o/w emulsion, an ointment, a liquid with an alcoholic phase (sunscreen), and an o/w suspension (sunscreen).7 The researchers used 29 volunteers, 15 men and 14 women, who applied each formulation as if it was a cream outside the bathing suit and scalp area. The researchers found that even when 30% of the body was skipped with the alcoholic solution, there was no difference in amount applied with the various formulations. The cream was given to the volunteers both in jar and tube packaging. When the sunscreens were dispensed in a small tube or a large-mouthed jar, the amount applied differed. Specifically, application of the emulsion from a jar was “wasteful,” whereas sunscreen in a tube was applied sparingly. When the sunscreen was applied from a jar, the amount approximated 24 g, which is closer to the amount required to cover the entire body (35 g), whereas only 10 g of the same formulation was applied from the tube.1, 6

Water, Sweat Resistance
Guidelines for evaluating a sunscreen’s water resistance were devised by Coloipa, the European cosmetics association, in December 2005.10 Sunscreen is applied to the backs of volunteers at a dosage of 2 mg/cm2, the sunscreen is dried for 15 min to 30 min, and the SPF is measured. The volunteer’s back is then immersed in water for two periods of 20 min and dried for 15 min after each immersion. The SPF is measured again 15 min after the last water immersion. To claim water resistant, the SPF measured must be equal or greater than 50% of the SPF level measured before water immersion. For a manufacturer to claim extra water resistant, the test is conducted for four periods of 20 min. No toweling is allowed during the procedure.

In the 1999 US Food and Drug Administration (FDA) monograph, the same time periods were required for a sunscreen to be labeled as water resistant or very water resistant;11 however, the final SPF level indicated on the label is the SPF measured after the last immersion and there is no comparison with the SPF level before water cycles. Therefore, water resistance tests are different in the United States and Europe and a manufacturer must conduct different tests to claim water resistance in both countries. As a result, the same product may show different claims on its label depending upon where it is being sold.

With regard to the eccrine sweat resistance of sunscreens, no protocol has yet been published to test for it. However, the FDA allows water resistant sunscreen to be labeled as sweat resistant, which is most likely a marketing strategy.

Sports Activities
Sunscreens can be less efficient during outdoor sports due to water exposure, sweat and friction, so participants of outdoors sports require high levels of sun protection. Even tan athletes who use sunscreen get sunburn. Ambros-Rudolph et al. have shown that melanoma risk increases in marathon runners due to UV exposure and immunosuppression.12

UV exposure was assessed on the backs of three athletes involved in an Iron Man competition. The athletes wore a UV spore detector and an SPF 25 water resistant sunscreen. Despite the UV filter, however, erythema was visible at the race’s end. It is possible that the sunscreen was washed off by the sea water and/or sweating; it also was not indicated whether the athletes reapplied sunscreen during the race.13

Sunscreen Reapplication
Sunscreen reapplication during sun exposure is recommended every 2 hr by the FDA to maintain total protection. Bodekaer et al. examined the persistence of a single sunscreen application during an 8-hr day of sun exposure, physical activity and water bathing.14 Physical and organic sunscreens at 2 mg/cm2 were tested on the backs of 24 volunteers. During the 8 hr, subjects wore tee shirts, exercised on bicycles, then entered a room held at approximately 30°C and took a 20-min bath. As a result, the SPF level after 4 hr was reduced by approximately 40%, and 56% after 8 hr for organic and inorganic sunscreens. This demonstrates the necessity of reapplying sunscreen to keep total photoprotection under real life conditions.

Although some sunbathers reapply sunscreen, less is known about the optimal application time. Diffey developed a mathematical model to estimate how the time of sunscreen reapplication affects the UV exposure of skin.15 His work mathematically showed the benefit of early reapplication of sunscreen; a single reapplication of an SPF 15 sunscreen after 20 min during 6 hr of sun exposure was found to be better than reapplication 2 hr or 3 hr later. The reapplication was independent of the SPF rating or UV intensity.

Since studies have shown that consumers apply far less sunscreen than that used in SPF evaluations, the SPF achieved is often half of what is labeled.2 Sunscreens are also spread onto small areas and many anatomic sites are missed.4, 5 Thus, it is recommended that consumers be educated to understand how to appropriately apply sunscreens.16

In addition, it was found that consumers typically apply more sunscreen when it is packaged in a large-mouthed jar.6 It is therefore recommended that manufacturers observe consumer behavior when designing sun product packaging. Offering sunscreen in a jar could permit consumers to better judge the amount applied.

Furthermore, Galenic formulations should be considered when developing formulations, as ointments lead to better application than alcoholic solutions and sprays.8, 9 Because high SPF sunscreens tend to be sticky, dermatologists should instead advise patients to use lower SPF level sunscreens but in greater quantity. There is only a 4% difference in UVB protection between an SPF 15 and an SPF 30 sunscreen. Therefore, using a lower SPF would not only be more aesthetically pleasing for the consumer, it would encourage subjects to apply more sunscreen.17

Some experts have recommended changing the amount of sunscreen used for SPF testing in order to adapt to real life usage conditions. While the FDA considered public comments and data/information brought to its attention and proposed a rule to amend the Final Monograph for OTC sunscreens in August 2007,18 the rule did not include a proposed method change. This was not only because the group believes that lowering the sunscreen density is not necessary to more accurately test for sunburn protection, but also because changing the amount of sunscreen used for SPF testing implies international coordination toward a globalized method and labeling requirements.

Instead, the FDA prefers that product labels encourage consumers to apply a thicker layer. Manufacturers can select one or more of the following terms: liberally, generously, smoothly or evenly. In addition, it would be beneficial for sunscreen labels to direct consumers to: apply to all skin exposed to the sun, serving as a reminder that all uncovered anatomic sites are exposed to UV. No sweat resistance testing was proposed in the new FDA monograph either, which could be important for individuals practicing outdoor exercise.

Finally, the efficacy of day care and makeup products containing UV filters should also be investigated. Foundations are applied in thinner layers and the protection achieved is likely far from the SPF level indicated on the label. A special SPF test should exist for such makeup and moisturizers that incorporate UV filters.

Sunscreen application and use have become highly efficient but educational messages to the consumer require further development. Besides learning to apply the right amount, consumers must also understand that wearing sunscreen does not mean they should spend longer periods of time in the sun. In addition, sunscreen should not be used as the primary prevention but as a complement to shade, clothing and broad-brimmed hats.

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