special subjects
Hydrofluoric Acid Exposure
Introduction
Hydrofluoric acid is commonly used in industry and dermal exposure causes deeply-penetrating painful soft tissue injury and also local and sometimes systemic fluoride toxicity. Treatment includes local burn care and topical and/or parenteral calcium. Hydrofluoric acid (HF) is commonly used in numerous industrial processes, including oil refining, silicon and glass etching, refrigerant production, fluoropolymer production, and aluminum metal production. HF is also used as a brick and stone cleaner, rust remover, and wheel cleaner. Although most HF exposure is occupational, some products containing HF are available for retail purchase, and nonoccupational exposure does occur. HF is a weak acid (pKa = 3.17) in dilute aqueous solution and exists primarily in the non-ionized form. Compared with strong acids that exist primarily in the ionized form, HF in dilute solutions is better able to penetrate the skin barrier and diffuse into local soft tissues, where it causes injury. Strong acids cause only direct caustic injury (coagulative necrosis, which typically spares the underlying soft tissues), but HF causes caustic skin injury due to its acidity and also injures underlying soft-tissue due to penetration of the fluoride ion. In aqueous intracellular and extracellular spaces, fluoride complexes with divalent cations (calcium, magnesium), forming insoluble calcium fluoride and magnesium fluoride. Formation of these insoluble salts promotes the release of more fluoride ion from HF (per Le Chatelier’s principle). The removal of calcium and magnesium from solution results in local Hypocalcemia and Time to symptom-onset from time of exposure varies and is inversely related to the concentration of HF. Exposure to concentrated preparations (> 50% HF) results in immediate pain and tissue damage due to the direct caustic effect on the skin or mucous membranes. This effect is partly due to a marked increase in the acidity of HF at high concentrations and HF homoassociation resulting in an increase in disassociated protons. Conversely, exposure to dilute preparations (< 20% HF) may not result in pain or clinical findings for several hours after the exposure because cellular dysfunction and death caused by local hypocalcemia and hypomagnesemia take time to develop. HF exposure may be dermal, ocular, oral, or inhalational. The most common route of occupational exposure to HF is dermal. 