In the preliminary studies with hydrogen fluoride (HF) described in HLR 281-88, the inhalation toxicity of HF increased approximately 3-fold as the relative humidity (RH) of the dilution air was increased from approximately 10 to approximately 60%. Although the basis for this difference could not be determined, it was believed to have resulted from the formation of aerosols which escaped collection within the sampling train. To determine whether the apparent increase in HF toxicity was real or related to the presence of aerosols or due to a sampling artifact, additional analytical trials and animal experiments were conducted. In contrast to the previous findings presented in HLR 281-88, no difference in toxicity was associated with exposure to HF at high humidity. The observed 1-hour LC50 values for HF at low or high RH were 2240 and 2340 ppm, respectively and were not significantly different. Most deaths due to HF exposure occurred 1-7 days post exposure following prolonged weight losses; clinical signs indicative of a respiratory tract irritation (lung noise, labored breathing, gasping, and nasal discharges), ocular irritation (discharges and opacity) and dermal corrosion (necrotic lesions of the nose, face and ears) were commonly seen. Pathologic examination of rats killed 1 day after exposure showed that compound-related microscopic lesions occurred exclusively in the nose. Nasal lesions were characterized by extensive acute necrosis and inflammation of the respiratory epithelium in the anterior nose; minimal necrosis of olfactory epithelium was also found in 1 rat from the low humidity group. In all rats, evidence of epithelial regeneration/repair was noted 14 days after exposure. These data indicate that the 1 hour LC50 for head-only exposure to anhydrous HF is approximately 2300 ppm and is not dependent upon relative humidity. The previously described increase in HF toxicity with humidity could not be repeated and may have been due to the comparatively low
recovery of HF using glass impingers and other analytical difficulties associated with HF measurement. No evidence for the role of HF aerosols was found.