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Light Energy and Sunscreens

By: Peter T. Pugliese, MD
Posted: March 26, 2009, from the April 2009 issue of Skin Inc. magazine.

page 7 of 13

Australia uses an in vitro test to assay UVA protection based on the ability of a sunscreen preparation to reduce the transmission of UV radiation between a wavelength of 320–360 nm to less than 10% of the incoming light. Japan uses the PPD method, as well. Another method is the critical wavelength test based on the previously described method, but modified to yield a new measure of the breadth of UVA protection. In this test, the absorbance of the thin film of the sunscreen is scanned from 290 nm across the UV wavelengths until the sum reaches 90% of the total absorbance of the sunscreen in the UV region (290–400 nm). The wavelength at which the summed absorbance reaches 90% of total absorbance is defined as the critical wavelength and is considered to be a measure of the breadth of sunscreen protection.9–12

Photostability. The stability of sunscreens has become a problem in sun protection. When a sunscreen becomes altered by UV absorption and breaks down, it is no longer an effective sunscreen. This has become a serious problem for manufacturers and is known as photostability. Essentially, the task is to have effective sunscreens absorbing high-energy photons and getting rid of the energy without being destroyed. Although this is not an easy task, excellent progress has been made. A new generation of organic molecules has been developed that can absorb this energy and release it quickly and with self-destruction. This process takes advantage of a photochemical reaction known as fluorescence quenching.13

The ingredients used in photostabilizing sunscreens can be added to a formulation or combined with other sunscreens. For example, ecamsule, a broad-spectrum sunscreen (290–400 nm with a peak at 345 nm), doesn’t cover the entire UV spectrum, so it must be combined with other active sunscreen agents to ensure broad-spectrum UV protection. Ecamsule is a photostable organic UVA absorber, meaning it doesn’t degrade significantly when exposed to light. A broad-spectrum sunscreen with ecamsule, avobenzone and octocrylene significantly reduces the skin damage associated with UV exposure in humans. Octocrylene is one of the cinnamate group of chemicals that absorbs light with wavelengths from 250–360 nm and is a photostabilizer.

Choosing a sunscreen

The selection of a sunscreen should be based on a client’s Fitzpatrick Skin Type and their lifestyle. Consider also if they have an associated skin disease of any autoimmune disease. See Fitzpatrick Skin Types for type descriptions, as well as recommendations for sunscreen selection.

Make sure clients have both UVA and UVB protection, and, in addition, that they have photostabilizers in their sunscreens. Help them identify whether there is a critical wavelength listed and if some type of UVA protective factor is noted besides the SPF. Remind them that a SPF of 30 is not twice as protective as a SPF of 15 and that they should wear hats and cotton gloves.