Also called ecamsule (technical name terephthalylidine dicamphor sulfonic acid), Mexoryl SX is a synthetic sunscreen agent developed and patented by L’Oreal and used in the company’s sunscreen products sold outside the United States since 1993 (first approved for use in Europe in 1991). In July 2006, the FDA approved the use of Mexoryl SX in the United States, but only in a single sunscreen product, La Roche-Posay’s Anthelios SX SPF 15 (L’Oreal owns La Roche-Posay). The FDA did not approve Mexoryl SX for use in any other sunscreen; only in that one specific product. Anthelios SX will list Mexoryl SX as ecamsule on the label along with the other actives avobenzone and octocrylene (both of these sunscreen ingredients have been approved for use in the United States for years).
L’Oreal blitzed the media with press releases about this approval, touting Mexoryl SX’s improved stability when compared with avobenzone, or intimating that it is the best UVA sunscreen available. According to sunscreen expert Ken Klein, president of Cosmetech Labs, who also teaches sunscreen formulation classes for the Society of Cosmetic Chemists, although Mexoryl SX does not degrade after hours of sun exposure at the same rate as avobenzone, it does indeed break down, losing 40% of its protective properties. Studies have shown that after controlled doses of UV exposure, avobenzone breaks down at a rate of 65%, so Mexoryl SX does have a slight stability edge. However, avobenzone can be made more stable by combining it with other active ingredients, specifically octocrylene (Source: International Journal of Pharmaceutics, January 13, 2006, pages 123–128). Outside the United States, Tinosorb (another sunscreen active) is often used to enhance the stability of avobenzone (Source: Photochemistry and Photobiology, September 2001, pages 401–406). It is also important to note that all sunscreen ingredients break down to some extent when exposed to sunlight, which is why reapplication of sunscreen is critical to maintaining protection.
Interestingly, the press releases touting Mexoryl SX’s superiority as a UVA-protecting sunscreen ingredient don’t mention the active ingredients titanium dioxide and zinc oxide, which have long been available worldwide in sunscreen formulations, and that offer protection across a greater range of wavelengths with almost no possible risk of irritation, which is a pervasive problem with synthetic sunscreen agents such as Mexoryl SX. Regarding protection, UVA rays have a range of 320–400 nanometers. Although Mexoryl SX protects within this range, titanium dioxide and zinc oxide protect across the entire UVA and UVB spectrum, from 230 to 700 nanometers. Mexoryl SX is an effective UVA sunscreen agent, but it is by no means the only or absolute best one to look for.