HCN (CAS 74-90-8) is a very toxic gas (ca. 20 times more toxic than carbon monoxide) that already affects human beings at ppm levels in air whereas 1 min exposure to concentrations of ca. 300 ppm is lethal [19]. It is a very flammable gas with a flash point of 17.8 1C. Its lower and higher explosion limits in air are, respectively, 5.6 and 40 vol%. HCN boiling point is 299 K; it has a density relative to air of 0.94 and a vapor pressure of 87.6 kPa at 295 K [20].