Terephthalylidene Dicamphor Sulfonic Acid

Terephthalylidene Dicamphor Sulfonic Acid (aka ecamsule or Mexoryl SX) is a water-soluble organic UV filter developed and patented by L'Oreal back in 1982.

It's one of the better UVA protectors out there and shields skin across a broad UVA range (~290-390 nm) with peak absorption right around 345 nm.

This peak absorption happens to be the exact type of UVA light that sinks deepest into skin and does the most to cause aging and dark spots.

Ecamsule works by soaking up UV rays and turning them into a tiny bit of harmless heat. The molecule basically flips, releases that energy as heat, then snaps back to its original shape and does it all over again. This is also why it's so stable in sunlight and doesn't break down or wear out the way some filters do.

The clinical backing for this ingredient is solid as well:

A large 2008 review by Fourtanier & colleagues (2008) pulled together human trials showing ecamsule-containing sunscreens prevented UV-induced pigmentation, DNA damage, and signs of aging.

That built on earlier work by Seite et al. (1998) that found it protected skin from repeated low-level UVA exposure. And a controlled study by DeLeo et al. (2009) showed that even an SPF 40 cream with ecamsule helped prevent sun-triggered rash even under the real sun.

On concentrations:

• 10% is the common global ceiling in the EU, UK, Canada, Australia, and most of the Asia-Pacific
• In the US, it's only available through specific New Drug Application-approved products at around 3% (usually La-Roche Posay formulas)

Because this ingredient is an acid, it has to be neutralized so it doesn't tank the product's pH.

Safety-wise, it's pretty well tolerated. There are some rare cases of mild irritation mostly in people with sensitivities towards camphor derivatives.

A 2019 FDA study found that volunteers who slathered on sunscreen heavily for several days had tiny amounts of ecamsule show up in their blood. This was slightly above the level at which the FDA asks for extra safety testing; this just means the FDA wants more safety studies done and not evidence that anything harmful actually happens. No problems were found and dermatologists still recommend using sunscreen.