It is widely known that prolonged exposure to UV radiation can cause severe skin damage.1 This radiation can be divided into three subregions: UV-A, B and C. UV-A rays, 320 nm to 400 nm, induce skin tanning. Long-term exposure to UV-A causes the loss of skin elasticity and the appearance of wrinkles. UV-B rays, 290 nm to 320 nm, can cause erythema and sunburn. UV-C rays, 200 nm to 290 nm, are mostly blocked by the ozone layer. To reduce skin damage, an effective sunscreen therefore must absorb in the UV-A and UV-B regions.2 Additionally, one that works on wet skin would be advantageous since outdoor activities such as swimming as well as perspiration can reduce sunscreen retention on the skin.
The composition of anhydrous sunscreen sprays typically includes ethanol, UV filters, esters and a polymer to increase water resistance. These formulations are usually clear and appear clear when applied to dry skin. However, when the skin is wet, these products appear milky. This is a result of the emulsification process. The UV filter oils and the water on the skin are not miscible; therefore, one gets dispersed into the other, forming many oil and water interfaces that scatter light, thus appearing white.
This problem can be addressed by considering the polarity of each ingredient. Figure 1 shows a schematic of the polarity of the major components in a typical anhydrous sunscreen spray formulation. Water is on the opposite side of the polarity spectrum from the sunscreen and esters. The addition of alcohol to water makes the mixture less polar, i.e., less hydrophobic, whereas adding alcohol to esters or sunscreens makes the mixture more polar, i.e., more hydrophilic.