Also known by the acronym HEPES, hydroxyethylpiperazine ethane sulfonic acid is a buffering ingredient typically used to establish a neutral pH in water-based solutions. There’s research indicating it can generate free radical damage in the presence of oxygen, which means it could be a problem when used in skincare products. Even though research on how this directly impacts skin hasn’t been done, it’s an ingredient we advise avoiding in cosmetic products. There are definitely other buffering agents that are less problematic and have had safe concentration protocols established.
References for this information:
Journal of Inorganic Biochemistry, August 2005, pages 1,653-1,660 and November 2004, issue 11, pages 1696-1702