The acid blend
Official distributor information specifies a 37% four-acid blend of mandelic, salicylic, glycolic and phytic acids. Other listed actives include low- and medium-molecular-weight hyaluronic acids, succinic acid, ferulic acid, phytosphingosine and hydrolyzed glycosaminoglycans.
The percentage describes the combined acid blend, not a single acid. Strength cannot be compared accurately using percentage alone because pH, buffering and formulation also affect professional activity.