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. 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/cm as is utilized in SPF testing, so maximal protection is not achieved. 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.
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. 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. 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. Faurschou and Wulf 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.