Hydrogen cyanide is volatile, and the DOT lists it as a 6.1 poisonous liquid with an inhalation hazard from the vapor. The NFPA 704 designation for AC is health 4, flammability 4, reactivity 2, and special 0, Vapors from hydrogen cyanide are highly toxic. The four-digit UN identification number is 1613 for less than 20% hydrogen cyanide and 1051 for greater than 20% hydrogen cyanide. The Chemical Abstract Service (CAS) number is 74-90-8. Hydrogen cyanide is thought to act on the body by blending with cytochrome oxidase (an enzyme essential for oxidative processes of the tissue) and blocking the electron carrier system. This results in loss of cellular oxygen use. The central nervous system, and particularly the respiratory system, are notably sensitive to this effect, and respiratory failure is the usual cause of death.