Hydrogen fluoride (HF) is an extremely toxic substance that is commonly used in many industries including textiles, pesticides, and semiconductors. When combined with water, it is known as hydrofluoric acid. For practical purposes, HF and hydrofluoric acid are considered the same chemical. In addition to severe cutaneous manifestations, HF can have devastating systemic effects. Systemic toxicity can occur by inhalation of gas, fumes, or aerosol, or by absorption through eyes, mucous membranes, and skin.

Hydrofluoric acid is unique among strong acids in that the small size of the HF molecule allows dilute hydrofluoric acid to pass through the epidermis and dermis into the underlying subcutaneous tissue as the undissociated acid. Once in the subcutaneous tissue, the hydrogen and fluoride ions will go along concentration gradient and dissociate to form hydrogen (H+) ions and fluoride (F-) ions. The high electronegativity of the fluoride ions causes interaction with biological macromolecules to cause tissue necrosis and destruction. This process continues unabated until the fluoride complexes with calcium ions to precipitate as calcium fluoride. Thus, calcium gluconate gel, which also penetrates through the epidermis and dermis into subcutaneous tissue, is the preferable treatment for dermal exposure to hydrofluoric acid. Pain is generally a good end point for this process in that when there are free fluoride ions in the subcutaneous tissue there is ongoing pain, but when the calcium ions form a calcium fluoride sink, then the pain and macromolecular destruction ceases.

Dermal contact produces immediate onset of redness, pain, and blistering that progress to deep chemical burns and skin necrosis. At concentrations less than 20%, erythema and pain may be delayed for 24 hours and may not be reported until significant tissue injury has occurred. At concentrations between 20% and 50%, erythema and pain may be delayed for 8 hours and may not be reported until tissue injury has occurred. Concentrations greater than 50% may result in immediate pain and erythema, rapid destruction of tissues, and acute systemic toxicity.

Respiratory symptoms may be immediate or delayed, up to 35 hours, and include sore throat, nasal irritation, sneezing, rhinorrhea, and cough. Severe exposures also produce dyspnea, wheezing, chest tightness, cyanosis, laryngeal swelling, chemical pneumonitis, pulmonary edema, and collapsed lung.

Systemic effects of HF toxicity may take days to appear and include nausea, vomiting, abdominal pain, and cardiac arrhythmias. Absorption of HF interferes with calcium metabolism, resulting in hypocalcemia with resultant tetany, seizures, and cardiovascular collapse. It may also produce hypomagnesemia and hyperkalemia with resultant paresthesias, weakness, paralysis, and cardiac arrest.

Victims at high risk for systemic effects include those with burns over 1% of body surface with a HF solution > 50% concentration, or those with burns to 5% of body surface at any concentration. Deaths have been reported with 2.5% of body surface exposure.

The Immediately Dangerous to Life or Health (IDLH) air concentration of HF is 30 ppm.