CHAPTER 2-81: HYDROGEN FLUORIDE AND HYDROFLUORIC ACID

INTRODUCTION

Hydrogen fluoride (HF) is an irritant gas that liquefies at 19.5°C; in an aqueous solution, it produces hydrofluoric acid. HF gas is used in chemical manufacturing. In addition, it may be released from fluorosilicates, fluorocarbons, or Teflon when heated to over 350°C. Hydrofluoric acid (aqueous HF solution) is widely used as a rust remover, in glass etching, and in the manufacture of silicon semiconductor chips. Hydrofluoric acid events at the workplace were shown to be two times more likely to involve injuries compared with other acids. Poisoning usually occurs after dermal exposure, usually on the hands, although ingestions and inhalations occasionally occur. There has been one case report of chemical colitis due to a hydrofluoric acid enema. Similar toxicity can result from exposure to ammonium bifluoride and sodium fluoride.

MECHANISM OF TOXICITY

HF is a dermal and respiratory irritant. Hydrofluoric acid is a relatively weak acid (the dissociation constant is about 1,000 times less than that of hydrochloric acid), and toxic effects result primarily from the highly reactive fluoride ion.

1. HF is able to penetrate tissues deeply before dissociating into hydrogen and fluoride ions. The highly cytotoxic fluoride ion is released and cellular destruction occurs.

2. Fluoride ion binds strongly to calcium and magnesium, resulting in their systemic depletion; this may cause systemic hypocalcemia, hypomagnesemia, and local bone demineralization.

TOXIC DOSE

Toxicity depends on the air levels and duration of exposure to HF gas or the concentration and extent of exposure to aqueous HF solutions.

1. HF gas. The recommended workplace ceiling limit (ACGIH TLV-C) for HF gas is 3 ppm (2.5 mg/m³); 30 ppm is considered immediately dangerous to life or health (IDLH). A 5-minute exposure to air concentrations of 50–250 ppm is likely to be lethal.

2. Aqueous solutions. Solutions of 50–70% are highly toxic and produce immediate pain. Concomitant inhalation exposure may occur with exposure to higher concentrations caused by the release of HF gas. Intermediate concentrations (20–40%) may cause little pain initially but result in deep injury after a delay of 1–8 hours. Weak solutions (5–15%) cause almost no pain on contact but may cause serious delayed injury after 12–24 hours. Most household products containing aqueous HF contain 5–8% or less.

CLINICAL PRESENTATION

Symptoms and signs depend on the type of exposure (gas or liquid), concentration, duration, and extent of exposure.

1. Inhalation of HF gas produces ocular and nasopharyngeal irritation, coughing, and bronchospasm. After a delay of up to several hours, chemical pneumonitis and noncardiogenic pulmonary edema may occur. Corneal injury may result from ocular exposure.

2. Skin exposure. After acute exposure to weak (5–15%) or intermediate (20–40%) solutions, there may be no symptoms because the pH effect is not pronounced. Concentrated (50–70%) solutions have better warning properties because of immediate pain. After a delay of ...