Terephthalylidene dicamphor sulfonic acid (TDSA) is an organic compound and it is a benzylidene camphor derivative.  Terephthalylidene dicamphor sulfonic acid is an FDA-approved sunscreen chemical with UV-filtering and absorbing capacities with an approved usage level of up to 10 percent in the European Union [1]. Ecamsule is the trade name of TDSA and contains in L'Oréal and its brands [2].

Why is it used in cosmetics and personal care products? In Europe, TDSA is an approved UV filter, which means that this ingredient can be added to sunscreen products that are designed to protect the skin from the sun. In Europe, sunscreen products are considered cosmetic products. In the United States, sunscreen products are OTC drugs. Several OTC sunscreen drug products containing TDSA, which has been approved by the FDA via the NDA process. When used in OTC sunscreen drug products in the United States, this ingredient must be called Ecamsule. For cosmetics and personal care products in the United States, TDSA may be used as an ultraviolet light absorber [3].

Exposed to UV, Ecamsule undergoes reversible photoisomerization, followed by photoexcitation and the absorbed UV is then released as thermal energy, without penetrating the skin. As we all know, UVB rays cause short-term sunburn and skin cancer; UVA rays cause wrinkling and may have some role in the development of skin cancer. There is no official rating for UVA protection in the US. In Europe, there are several different rating systems that are used to measure effectiveness in blocking UVA rays, including the IPD (immediate pigment darkening assay) and the PPD (persistent pigment darkening assay). The UVB range is 280 to 320 nanometers, and the UVA range is 320 to 400. Ecamsule protects against UV wavelengths in the 290–400 nanometer range, with peak protection at 345 nm. Since ecamsule doesn't cover the entire UV spectrum, it should 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. This is in contrast to the widely used UVA absorber avobenzone that is not intrinsically photostable and requires photostabilizers to prevent significant degradation in light.

Reference

[1] http://m.is0513.com/ChemicalProductProperty_EN_CB5965160.htm
[2] https://en.wikipedia.org/wiki/Ecamsule
[3] https://cosmeticsinfo.org/ingredient/terephthalylidene-dicamphor-sulfonic-acid-0
[4] https://pubchem.ncbi.nlm.nih.gov/compound/146382
[5] https://pubs.acs.org/doi/10.1021/cen-09219-govpol