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This is an excerpt of the Cosmetics & Toiletries magazine feature "Enhancing Sunscreen Efficacy for Realistic Application," originally printed in C&T's July 2010 issue . All references are cited in the C&T feature.
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). 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.
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). 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.
Guidelines for evaluating a sunscreen’s water resistance were devised by Coloipa, the European cosmetics association, in December 2005. Sunscreen is applied to the backs of volunteers at a dosage of 2 mg/cm, 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 brand owner to claim extra water resistant, the test is conducted for four periods of 20 min. No toweling is allowed during the procedure.