<p>Hydrogen (H₂) is increasingly recognized as a viable low-carbon energy carrier. It can be produced by electrolysis and only emits water when consumed in fuel cells, making it essentially carbon free at the point of use. To support a large-scale hydrogen economy, however, vast quantities of H₂ must be stored to balance seasonal and diurnal mismatches between intermittent renewable generation and end-use demand. Salt caverns have long been considered one of the most promising options for underground hydrogen storage (UHS), owing to their low permeability, high geomechanical stability, and favorable operational characteristics. Despite these advantages, salt caverns present challenges related to in situ gas impurity generation during H₂ residence. Natural salt formations contain mineral inclusions such as anhydrite, carbonates, clays, and metal sulfides, which may participate in geochemical reactions that introduce contaminants, including hydrogen sulfide (H₂S), carbon dioxide (CO₂), methane (CH₄), and other trace gases, into stored hydrogen. Microbial processes and materials corrosion may further contribute to impurity formation. This review synthesizes the current state of knowledge on impurity generation mechanisms in salt cavern hydrogen storage. It examines geochemical, microbiologic, and corrosion-related pathways that may alter hydrogen purity and evaluates the consequences of these impurities for cavern integrity, infrastructure performance, and hydrogen fuel-cell applications. Understanding the geochemical, microbial, and operational characteristics of individual storage sites is essential for predicting impurity behavior and developing effective mitigation strategies. Further work is needed to quantify reaction rates, constrain uncertainty, and support the design of safe and efficient UHS projects in the future.</p>