Hydrogen fluoride (HF) is a colorless gas with a pungent odor . It is a highly toxic gas by inhalation (R26), ingestion (R28) or skin contact (R27). It reacts with water to form hydrofluoric acid, which is highly corrosive and extremely dangerous for the environment (changes the pH of the water). Although non-flammable , it reacts in the presence of moisture with most metals, forming hydrogen (H2), a highly flammable gas.